UBC Theses and Dissertations

UBC Theses Logo

UBC Theses and Dissertations

Artificial environments for plant research Gibson, Jonathan Stephen 1972

Your browser doesn't seem to have a PDF viewer, please download the PDF to view this item.

Item Metadata

Download

Media
831-UBC_1972_A6_7 G52.pdf [ 4.04MB ]
Metadata
JSON: 831-1.0101902.json
JSON-LD: 831-1.0101902-ld.json
RDF/XML (Pretty): 831-1.0101902-rdf.xml
RDF/JSON: 831-1.0101902-rdf.json
Turtle: 831-1.0101902-turtle.txt
N-Triples: 831-1.0101902-rdf-ntriples.txt
Original Record: 831-1.0101902-source.json
Full Text
831-1.0101902-fulltext.txt
Citation
831-1.0101902.ris

Full Text

ARTIFICIAL ENVIRONMENTS FOR PLANT RESEARCH  by  JONATHAN STEPHEN GIBSON B.Sc,  Washington S t a t e U n i v e r s i t y , P u l l m a n , Washington, 1967  A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE  i n t h e Department o f Plant  Science  We a c c e p t t h i s t h e s i s as c o n f o r m i n g t o the r e q u i r e d  THE UNIVERSITY OF BRITISH COLUMBIA J a n u a r y , 1972  standard  In presenting  t h i s t h e s i s i n p a r t i a l f u l f i l m e n t of the requirements f o r  an advanced degree at the U n i v e r s i t y of B r i t i s h Columbia, I agree that the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r reference and study. I f u r t h e r agree that permission f o r extensive copying of t h i s t h e s i s for s c h o l a r l y purposes may by h i s representatives.  be granted by the Head of my Department or I t i s understood that copying or p u b l i c a t i o n  of t h i s t h e s i s f o r f i n a n c i a l gain s h a l l not be allowed without my w r i t t e n permission.  Department The U n i v e r s i t y of B r i t i s h Columbia Vancouver 8, Canada  ii ABSTRACT  A r e v i e w was made o f e n v i r o n m e n t a l  t e c h n o l o g y as a p p l i e d t o t h e  e n g i n e e r i n g and c o n s t r u c t i o n o f a r t i f i c i a l  environments f o r p l a n t  physiology  research.  The r e s u l t s o f t h i s s t u d y were u t i l i z e d i n the development o f an  artificial  environment which i n c o r p o r a t e d the n u t r i e n t m i s t t e c h n i q u e  of  growing p l a n t s . The q u a l i t y o f the environment i n p l a n t growth chambers i s dependent on the type o f c o n t r o l control  i n s t r u m e n t s used.  Solid state  partly electronic  devices o f f e r many advantages, p a r t i c u l a r l y w i t h r e s p e c t t o  r e s p o n s i v e n e s s , r e l i a b i l i t y and remote  control.  A t r a v e l l i n g sensor was developed t o d e t e c t t h e e n v i r o n m e n t a l within a r t i f i c i a l  accuracy,  environments by remote c o n t r o l .  This sensor  conditions  greatly  i n c r e a s e d the r a p i d i t y and convenience o f measurement w i t h minimum d i s t u r b a n c e o f the  environment.  The c o n d i t i o n s o f l i g h t i n t e n s i t y ,  t e m p e r a t u r e , wind speed and h u m i d i t y  w i t h i n a commercial growth chamber, the P e r c i v a l  Model PGC-78, were a n a l y s e d .  The r e s u l t s i n d i c a t e d t h a t the chamber's performance was q u i t e for all  the v a r i a b l e s t e s t e d .  nonuniform  The m a n u f a c t u r e r ' s s p e c i f i c a t i o n s f o r  the  chamber were c o n s i d e r e d t o be l i m i t e d i n e x t e n t and t o some degree misleading. • The design o f t h e a r t i f i c i a l project is described.  environment system c o n s t r u c t e d f o r  With t h i s s y s t e m , t e m p e r a t u r e c o n t r o l  achieved w i t h i n the p l a n t growth a r e a .  this  o f ± h°C was  In a d d i t i o n , the u n i f o r m i t y  of  l i g h t i n t e n s i t y and a i r f l o w i n t h e c o n s t r u c t e d chambers was s u p e r i o r the PGC-78.  to  iii TABLE OF  CONTENTS  PREFACE  ix  LITERATURE CITED  CHAPTER I .  Artificial  Environment  Design  INTRODUCTION  1  (A)  Light  2  (B)  Temperature  (C)  Humidity  System Control  .  Control  12 20  (D) A i r C i r c u l a t i o n  30  (E)  31  R e l i a b i l i t y and S e r v i c e  CHAPTER I I .  Instruments f o r C o n t r o l , I n d i c a t i o n , and Measurement o f E n v i r o n m e n t a l V a r i a b l e s  (A)  Control  (B)  Indication  (C)  Measurement  CHAPTER I I I .  .  Jo  33 .37 39 •  Commercial Growth  Chamber  Performance  INTRODUCTION  42  (A)  Light  42  (B)  Temperature  (C)  Air Circulation  51  (D)  Humidity  56  (E)  Maintenance  57  CHAPTER I V .  System Control  Laboratory  (A) W a l l  49  System  Temperature  Division  Control  (B)  Temperature  (C)  A i r C o n d i t i o n i n g System  (D) P l a n t  Control  System  System  Platform A i r Diffusing Manifold  59 63 . 65 67  (E) Nutrient Mist System  70  (1) Nutrient Solution  70  (2) Nutrient Mist Control System  72  (F) A r t i f i c i a l  L i g h t i n g System  •  (1) Photoperiod Control CHAPTER V.  Laboratory A r t i f i c i a l  73 73  Environment Performance  (A) Light System  76  (B) Temperature  82  (C) A i r C i r c u l a t i o n  83  (D) Humidity  85  (E) Summary of Laboratory System Performance  87  SUMMARY  88  LIST OF TABLES  Psychrometric  Terminology  vi LIST OF FIGURES Fi gures 1-1  1-2  Spectral Energy D i s t r i b u t i o n o f Direct Solar Radiation I n t e n s i t y at Normal Incidence f o r the Upper Limit o f the Atmosphere and at the Earth's surface f o r Clear Days . . . .  3  Action Spectra o f Phototropism, Phytochrome and Photosynthesis  4  1-3  Emission Spectrum of a Cool White Fluorescent Lamp  5  1-4  M o r t a l i t y of Fluorescent Lamps as a Percent of Rated Lamp L i f e  5  1-5  Typical Range of Fluorescent Lamp Depreciation with Time . .  7  1-6  Energy D i s t r i b u t i o n of a Typical Cool White Fluorescent Lamp  7  1-7  Energy D i s t r i b u t i o n of a Typical 40 Watt Incandescent Lamp .  8  1-8  Spectral Energy D i s t r i b u t i o n o f a Typical Incandescent Lamp Horizontal Light I n t e n s i t y D i s t r i b u t i o n 24" Below an Uncurtained Light Canopy o f Fluorescent Lamps . . . . . . . Horizontal Light I n t e n s i t y D i s t r i b u t i o n 24" Below a Curtained Light Canopy of Fluorescent Lamps Processes Occuring When Cooling A i r in a Conditioning System  1-9 1-10 1-11 1-12 1-13 1-14 1-15  8 11 11 13  Temperature Variations About a Control Point Caused by D i f f e r e n t Rates o f Heating and Cooling  17  The r e f r i g e r a t i o n System of the Percival Model PGC-78 Growth Chamber U t i l i z i n g Two Evaporator Coils  18  A Typical R e f r i g e r a t i o n System U t i l i z i n g the Hot-Gas Bypass Technique of Evaporator Coil Temperature Regulation . 18 - Processes Occuring When Air i s Cooled and Condensation Results  20  1-16  Processes Occuring When A i r is Heated and Humidified . . . .  21  1-17  Processes Occuring When Two A i r Streams are Mixed  23  1-18  Humidity and Temperature Changes Occurring in a Stream of A i r Passing Through a Cooling Coil  24  vii Fi gures 1-19  Dual Damper-controlled Cooling Coils  1 - 20  The E f f e c t of Varying Heat Load on Cooling Coil Performance  2- 1  Apparatus f o r the Remote P o s i t i o n i n g of Environmental Sensors  40  The Spacing and Arrangement of Fluorescent and Incandescent Lamps in the Percival Model PGC-78  44  The Horizontal Light I n t e n s i t y D i s t r i b u t i o n in the Percival Model PGC-78 Growth Chamber at Distances Between 12 and 40 inches Below the Light Canopy B a r r i e r  46 & 47  Horizontal A i r Temperature P r o f i l e 14" Below the Light Canopy B a r r i e r  50  Horizontal A i r Temperature P r o f i l e s Through Center of Chamber f o r 2 2 " , 30", and 38" from Liqht Canopy B a r r i e r in the Percival Model PGC-78  50  3- 1 3-2 (a-h) 3-3 3-4  3-5  28 . 29  Horizontal A i r V e l o c i t y P r o f i l e s at 1 4 " , 2 2 " , 30" and 38" Below the Light Canopy B a r r i e r in the Percival Model PGC-78  . 53  3- 6  General Design of the Percival Model PGC-78 Growth Chamber . . 55  4- 1  Design of the A i r Flow System in the Laboratory A r t i f i c i a l Environment  60  4-2  Perspective View of Lower Chamber and Base  62  4-3  Wall Temperature Control System  4-4  Diagram o f the A i r Temperature Conditioning System . . . . . .  4-5  Top View and Perspective of the A i r D i s t r i b u t i o n Assembly  4-6  N u t r i e n t Mist Spray System  4- 7  Laboratory Light Canopy Consisting of 20 48T12 VH0 Cool White Fluorescent Lamps Spaced 1 5/8" on centers with 15 40 Watt Incandescent Lamps Spaced 10" Apart in Three Rows  5- 1 (a-j) 5-2  . . 64  •  Horizontal Light I n t e n s i t y P r o f i l e s Above the A i r * D i s t r i b u t i o n Manifold in the Laboratory System Upper Chamber Lamp Canopy and Chamber Location Relative to Laboratory Walls  66  . . 69 71  . . 74  78 & 79 81  vi i i Figures 5-3 5-4  O r i g i n a l Non-uniform A i r Flow Pattern in Upper Chamber of Laboratory System  84  Asymmetrical Duct from Cooling Coil to Upper Chamber of Laboratory System  86  ix PREFACE This p r o j e c t served as a stage in a continuing research programme aimed at developing an enclosed, accurately c o n t r o l l e d , a r t i f i c i a l ment f o r plant growth.  environ-  The ultimate goal i s a system which provides a wide  range of uniform l i g h t i n t e n s i t i e s , a i r c i r c u l a t i o n v e l o c i t i e s , temperatures and h u m i d i t i e s , any combination o f which i s programmable by the operator. This work represents an attempt to reduce environmental conditions as uncontrolled variables in experiments on the physiology o f plant growth. To approach t h i s o b j e c t i v e , a review of the components in an a r t i f i c i a l environment system was made and is presented in Chapter I .  This review  provides fundamental information concerning the design and s t r u c t u r e o f plant growth chambers.  In Chapter I I , some instruments f o r c o n t r o l l i n g ,  i n d i c a t i n g and recording environmental variables in p l a n t growth chambers are described.  A special technique f o r measuring the physical components  of the i n t e r n a l environment o f growth chambers i s also described. A d e t a i l e d study o f the performance of a commercially manufactured growth chamber (Percival Model PGC-78) was c a r r i e d out and i s reviewed in Chapter I I I .  The Model PGC-78 chamber approached the performance levels  s p e c i f i e d by the manufacturers.  Nevertheless, several major d e f i c i e n c i e s  in environmental q u a l i t y were e v i d e n t , and the chambers were considered to be unsatisfactory f o r c r i t i c a l of plant growth.  research on many aspects of the physiology  For t h i s reason, i t was decided to design and construct  a new laboratory plant growth chamber. The design of the laboratory growth chamber i s described in Chapter IV. This system was intended to provide improved u n i f o r m i t y and control o f environmental  conditions.  X  In developing the laboratory system, i t was necessary at the outset to define the q u a l i t y of environmental control which the system should possess.  The requirements f o r environmental control and u n i f o r m i t y are  l a r g e l y dependent on the types o f f u t u r e research contemplated.  I t was  expected that the system would mostly be applied to studies o f the e f f e c t s of environmental conditions on the growth and gas exchange o f shoots and roots o f small p l a n t s .  Growth chamber performance c r i t e r i a was therefore  developed in the f o l l o w i n g way from known information on the e f f e c t s o f environmental conditions on the components o f growth and gas exchange. Studies with many d i f f e r e n t p l a n t species have shown that the rates o f growth and net photosynthesis can change by as much as 5% of the maximum rate i f the l i g h t i n t e n s i t y i s varied by 1000 lux or i f the temperature is varied by 1°C (Cooper and Tainton, 1968).  Very l i t t l e information is available on  the d i r e c t e f f e c t s of wind speed and humidity on growth.  At low a i r  v e l o c i t i e s ( i . e . between 10 and 80 fpm), a 10 fpm change in a i r v e l o c i t y may cause as much as a 10% change in the rate o f t r a n s p i r a t i o n of sunflower (Martin and Clements, 1936).  A l s o , psychrometric tables i n d i c a t e t h a t , f o r  example, at i n i t i a l l y 60% R.H., 26°C dry b u l b , a 1% change in r e l a t i v e humidity could change the atmospheric water vapour pressure surrounding the l e a f by 10%.  While these examples omit the i n t e r a c t i o n s between  d i f f e r e n t environmental v a r i a b l e s , they provide an estimate of the scale of environmental change which may cause s i g n i f i c a n t e f f e c t s on growth. I t was therefore decided t h a t s a t i s f a c t o r y environmental control would be provided by a chamber w i t h the f o l l o w i n g c a p a b i l i t i e s : temperature control to w i t h i n ± 1°C and a temperature range of 5°C to 35°C; l i g h t i n t e n s i t y range from 0 to 40,000 lux with less than 1000 lux nonuniformity; average a i r  xi v e l o c i t y v a r i a b l e from 20 to 100 feet per minute with a uniformity of ± 10 fpm; and humidity control and s t a b i l i t y to ± 1% r e l a t i v e humidity, v a r i a b l e from 20 to 95% r e l a t i v e humidity. The system described above would permit the study of i n t a c t , whole plants where l e a f chambers and excised t i s s u e are presently u t i l i z e d . The a d d i t i o n of the n u t r i e n t mist technique provides a system e a s i l y adapted for n u t r i e n t studies and f o r studies of the differences in root and shoot r e s p i r a t i o n and development.  Such a system would provide environ-  mental control adequate f o r a wide range of studies on the physiology of plant growth.  1 Chapter I ARTIFICIAL ENVIRONMENT DESIGN  INTRODUCTION The basic purpose o f a plant growth chamber i s to provide e n v i r o n mental conditions s i m i l a r to natural conditions which are r e p r o d u c i b l e , and which can be varied at w i l l .  For t h i s , a l i g h t system o f adequate i n t e n s i t y  and spectral q u a l i t y , as well as mechanisms f o r the control o f root and shoot temperature and humidity are r e q u i r e d .  The type of research contem-  p l a t e d , the size and number o f plants i n v o l v e d , and the l i m i t a t i o n s o f laboratory f a c i l i t i e s must be considered in the s e l e c t i o n o f the physical components o f a p l a n t growth chamber. The size o f c o n t r o l l e d environmental f a c i l i t i e s b u i l t f o r the study of plants has ranged from m u l t i s t o r y buildings to small desk top chambers and l e a f chambers.  I n t e r e s t here w i l l be centered on what are usually r e f e r r e d  to as plant growth chambers, which have an i n t e r i o r usable plant platform from seven to twenty square f e e t , and a distance of about four f e e t between the plant p l a t f o r m and the l i g h t canopy. The environmental conditions which can be produced by a r t i f i c i a l systems are l i m i t e d and d i f f e r from natural c o n d i t i o n s .  This difference  l i e s in the complex i n t e r a c t i o n s of natural environmental v a r i a b l e s , and in the physical size of the natural environment r e l a t i v e to a closed chamber. The complexities of the natural environment are f a i r l y well understood, but the equally complex i n t e r a c t i o n s o f the components of an a r t i f i c i a l ment must be known before desirable performance can be achieved.  environ-  The  f o l l o w i n g discussion w i l l consider some factors involved in incorporating mechanical and e l e c t r i c a l devices i n t o a plant growth chamber.  2  (A) Light System A l i g h t system in a plant growth chamber must possess adequate and uniform l i g h t i n t e n s i t y and spectral q u a l i t y on a horizontal plane, and as small an i n t e n s i t y gradient v e r t i c a l l y as i s possible.  I t must have  photoperiod c o n t r o l , and should not emit excessive heat or harmful r a d i a t i o n . The natural source o f radiant energy f o r plant growth is the sun. However, the atmosphere f i l t e r s out certain wavelengths to y i e l d a spectrum as in Figure 1 - 1 .  Also shown f o r comparison are the spectra o f incandescent  and fluorescent lamps (Figures 1-3, 1-8) and the action spectra of photosynthesis, phototropism, and phytochrome (Figure 1-2).  Usually, a combination  o f incandescent and fluorescent lamps are used as a source o f l i g h t f o r plant growth.  This combination most p r a c t i c a l l y s a t i s f i e s the l i g h t q u a l i t y  and i n t e n s i t y requirements o f plants as represented by the above action spectra.  The f o l l o w i n g discussion of a r t i f i c i a l  l i g h t sources w i l l  demonstrate t h e i r difference from s u n l i g h t and w i l l i n d i c a t e some o f the problems o f l i g h t system design in a plant growth chamber. The spectral energy d i s t r i b u t i o n of a Cool-White fluorescent tube shows a peak emission at 580 nanometers with a secondary maximum at 475 nanometers. 1960).  The output i s very low at 700 nm (Figure 1-3) (General' E l e c t r i c ,  As w i t h a l l fluorescent lamps, i n t e r m i t t e n t operation shortens lamp  l i f e appreciably.  Figure 1-4 shows percent o f burnouts o f fluorescent  lamps p l o t t e d against percent rated lamp l i f e (General E l e c t r i c , 1960). The i n i t i a l  l i g h t output i s variable from lamp t o lamp, and t h i s value may  decrease r a p i d l y during the f i r s t one hundred hours o f o p e r a t i o n .  The  lumen depreciation may amount to as much as ten percent in t h i s p e r i o d .  For  commercial r a t i n g purposes, the one hundred hour value i s used as the i n i t i a l  3  WAVELENGTH -  Fig. 1-1  MICRONS  Spectral energy of d i r e c t s o l a r r a d i a t i o n i n t e n s i t y at normal incidence f o r the upper l i m i t of the atmosphere and at the e a r t h ' s surface during c l e a r days. THRELKELD, 1970.  \  4  Fig. 1-2  Action spectra of phototropism ( a ) , photochrome ( b ) , and photosynthesis ( c ) . ( ( a ) , (b) S a l i s b u r y & Ross, 1969; (c) Bui l e y et a l , 1969  ).  Cool  White  WAVELENGTH  F i g . 1-3  -  NANOMETERS  Emission spectrum of a cool white fluorescent lamp. (General E l e c t r i c , 1960)  o1  25I/I  t— O  2; m  50  z  tu u  or UJ o-  7 5  ,  1004  — i  -  20  F i g . 1-4  1 1 i0 60 PER CENT RATED L A M P L I F E  1— 80  100  —i  M o r t a l i t y of fluorescent lamps as a percent of rated lamp l i f e . (General E l e c t r i c , 1960)  120  6 value.  Figure 1-5 shows t y p i c a l ranges of fluorescent lamp depreciation  in l i g h t output with respect t o time (General E l e c t r i c , 1960).  A typical  Cool-White fluorescent lamp w i l l emit twenty-one percent o f the input e l e c t r i c a l energy as v i s i b l e l i g h t , t h i r t y - f o u r percent as i n f r a r e d r a d i a t i o n , and f o r t y - f i v e percent of the input as dissipated heat (Figure 1-6)  (General E l e c t r i c , 1961).  The luminous e f f i c i e n c y o f a Cool -  White fluorescent lamp ranges from seventy-five to eighty lumens per watt.  By comparison, general service incandescent lamps may range from  twelve to twenty-two lumens per watt (Sylvania, 1962).  The optimum  pressure f o r maximum l i g h t output f o r most fluorescent lamps occurs when the coolest spot on the bulb surface i s about f o r t y - f i v e degrees Centigrade. The bulb wall temperature i s affected by lamp wattage and bulb diameter, by the design o f the bulb i t s e l f , and by the ambient temperature and d r a f t v e n t i l a t i o n conditions (General E l e c t r i c , 1960). A f o r t y watt incandescent lamp requires about .34 amperes, gives an i n i t i a l lumen output o f approximately 460, and has a rated average l i f e of 1000 hours.  For such a f o r t y watt incandescent b u l b , 7.4 percent o f  the input energy is radiated as v i s i b l e l i g h t , 63.9 percent as i n f r a r e d , g i v i n g a t o t a l of 71.3 percent.  A twenty percent loss o f input energy is  caused by convectional flow of the f i l l i n g gas in a stream past the filament (Figure 1-7) (General E l e c t r i c , 1960).  Conduction of heat through the  bulb and base of a f o r t y watt bulb represents an 8.7 percent loss o f input energy.  Of a l l standard incandescent lamps, the f o r t y watt bulb  is the l e a s t e f f i c i e n t e m i t t e r of v i s i b l e energy, but also emits the l e a s t amount o f i n f r a r e d r a d i a t i o n .  The spectral d i s t r i b u t i o n o f a t y p i c a l  incandescent bulb is given in Figure 1-8 (General E l e c t r i c , 1961).  Reflector  — I  1  2000  F i g . 1-5 .  4000  —  —  —  —  I  I—————\  1  6000 BURNING TIME  8000 (hours)  10000  12000  Typical range of f l u o r e s c e n t lamp d e p r e c i a t i o n with time. (General E l e c t r i c , 1960)  INPUT 100  ENERGY %  60 V .  EXCITING  ULTRAVIOLET  60 V . 38 V .  AO V .  I'  I HEAT  20 V .  78  1  2 V. 36 V .  f  V.  I  LIGHT 22 V .  Fig. 1-6  1  INFRARED 36 V .  42'/.  DISSIPATED 42 V .  HEAT  Energy d i s t r i b u t i o n of a t y p i c a l cool-white f l u o r e s c e n t lamp. (General E l e c t r i c , 1960)  8 INPUT 100  FILAMENT  RADIATION 71.3  V.  BEYOND BULB  GAS  %  LOSS  CONDUCTION  8.7  LOSS  20 V .  TOTAL HEAT LOSS 92.6 V .  7.4 V.  r  ENERGY  UGH  F i g . 1-7  Energy d i s t r i b u t i o n of a t y p i c a l 40 watt incandescent lamp. (General E l e c t r i c , 1960)  100  75  IS  or  LiJ  50  LU >  UJ  25  — i 300  Fig. 1-8  1 400 WAVELENGTH -  i 500 NANOMETERS  1— 600  700  Spectral energy d i s t r i b u t i o n of a t y p i c a l incandescent lamp. (General E l e c t r i c , 1960)  9  bulbs which tend to focus the l i g h t and create uneven i n t e n s i t y are not as s u i t a b l e f o r plant growth chamber applications as n o n - r e f l e c t o r bulbs. In natural c o n d i t i o n s , the n e g l i g i b l e change o f l i g h t  intensity  from the s o i l surface t o the top of a plant (assuming no absorption by the p l a n t ) i s a function of the distance from the earth t o the sun, assuming clear atmospheric c o n d i t i o n s .  In an a r t i f i c i a l environment, the  l i g h t source may i n i t i a l l y be about four f e e t from p l a n t s .  As the plants  grow towards the l i g h t system, they are exposed to gradually higher and higher l i g h t i n t e n s i t i e s .  V e r t i c a l l i g h t i n t e n s i t y gradients can be  p a r t l y reduced by making a l l the wall panels and l i g h t support surfaces as r e f l e c t i v e as possible.  Clear baked enamel f i n i s h e s (high gloss) can  approach 95 - 98% r e f l e c t i v i t y (General E l e c t r i c , 1960).  With the use  of such r e f l e c t i v e m a t e r i a l s , a more uniform v e r t i c a l l i g h t  intensity  pattern can be obtained in the plant growth area ( K a l b f l e i s c h , 1963). I t i s usually expensive to design a plant growth chamber with an extremely low v e r t i c a l l i g h t i n t e n s i t y g r a d i e n t .  Any objects in the chamber, i n c l u d i n g  p l a n t s , shelves, p o t s , and instruments, d i s t u r b the u n i f o r m i t y of l i g h t intensity.  Special designs are a v a i l a b l e i f unfform v e r t i c a l  light  i n t e n s i t y is important (Controlled Environments, 1970). The u n i f o r m i t y of l i g h t i n t e n s i t y on any horizontal plane between the p l a n t p l a t f o r m and the l i g h t canopy i s determined l a r g e l y by the pos'ition, dimensions and wattage o f the lamps used.  Reflective walls may  influence horizontal u n i f o r m i t y , depending on how the chamber is constructed. K a l b f l e i s h (1963) has published extensive measurements on a v a r i e t y o f lamp canopy arrangements using various lengths o f fluorescent lamps spaced from 1/8 inch to several inches a p a r t .  A t y p i c a l chart (Figure 1-9)  indicates  10  t h a t the center o f the horizontal plane 24 inches below the l i g h t canopy had the highest l i g h t i n t e n s i t y which decreased in l i g h t i n t e n s i t y towards the sides o f the canopy.  Figure 1-10 demonstrates the influence o f a  24 inch long highly r e f l e c t i v e c u r t a i n mounted around the perimeter of the same lamp canopy used in Figure 1-9. the canopy was again 24 inches.  Measurement distance from  I t is c l e a r l y evident t h a t the r e f l e c t i v e  c u r t a i n produces more uniform i n t e n s i t y . The l i g h t i n t e n s i t y to which a plant is exposed in a small plant growth chamber can be varied by moving the plant closer to the lamp canopy by means of movable shelves.  A l t e r n a t i v e l y the lamp canopy may be mounted  on a pulley system, allowing the l i g h t i n t e n s i t y to be adjusted without moving the p l a n t s .  Moving the lamp canopy or plants may change the a i r  c i r c u l a t i o n and heat t r a n s f e r in the chamber and thus may require a change in temperature and humidity control s e t t i n g s .  Moving the lamps or  plants are the most commonly used techniques f o r varying l i g h t  intensity  in commercial plant growth chambers. Another method i s to vary the i n t e n s i t y of the lamps themselves. A special b a l l a s t i s required to vary the i n t e n s i t y o f fluorescent lamps. This b a l l a s t i s expensive and l i m i t e d to lower wattage lamps seldom used in plant growth chambers.  The i n t e n s i t y of incandescent lamps can be  varied by a series potentiometer.  This method i s less s a t i s f a c t o r y since  several a d d i t i o n a l e l e c t r i c a l components are required and uniform spectral q u a l i t y cannot be maintained. Photoperiod control i s generally provided by o n / o f f switches driven by 24-hour clocks.  To simulate natural c o n d i t i o n s , several clock-switches  may be used to switch on and o f f both the incandescent and fluorescent lamps.  11  Fig. 1-9 Horizontal light intensity distribution 24" below an uncurtained light canopy of fluorescent lamps. (Kalbfleisch, 1063.)  Fig. 1-10 Horizontal light intensity distribution 24" below a curtained light canopy of fluorescent lamps. (Kalbfleisch, 1963.) . >  12  Lamp canopies f o r plant growth chambers can consume several thousand watts o f power.  The influence o f the heat input from the lamp canopy  on other.environmental variables in a chamber i s important and w i l l be discussed l a t e r . New l i g h t i n g systems and lamps now being developed may g r e a t l y improve lamp canopy performance f o r plant growth chambers.  Newly developed  high i n t e n s i t y lamps using mixtures o f sodium, mercury and other gases produce s u n - l i k e emission spectra ( P h i l l i p s , 1969).  Any lamp o f s u i t a b l e  i n t e n s i t y and spectrum f o r plant growth t h a t reduces i n f r a r e d output would considerably s i m p l i f y plant growth chamber design.  (B) Temperature Control The temperature control equipment of a plant growth chamber must provide reproducible c o n t r o l , and must not cause e r r a t i c changes in the other environmental v a r i a b l e s .  The temperature range o f general purpose  plant growth chambers usually extends well below and above the optimum growth temperatures f o r p l a n t s , t y p i c a l l y -10° to 50°C.  The precision and  r e p r o d u c i b i l i t y of temperature control in the chamber depends on the s e n s i t i v i t y and l o c a t i o n o f the control device, the selection of heating and cooling elements, the rate of flow and volume of a i r c i r c u l a t i o n , and the v a r i a t i o n s o f the heat load w i t h i n the chamber.  The u n i f o r m i t y o f  a i r temperature depends on the a i r c i r c u l a t i o n patterns w i t h i n the plant growth area and the e f f e c t s o f the l i g h t canopy. Three types of temperature control are commonly encountered.  The  simplest i s the constant temperature t y p e , which maintains temperature close to a single control p o i n t .  Second is the diurnal type, in which the  13  day temperature is different from the night temperature.  Thirdly, a  temperature programmer may be utilized to simulate natural temperature cycles.  A discussion of instruments to implement the three types of  temperature control is contained  in Chapter II.  In order to control temperature in a plant growth chamber, moist air must be heated or cooled to a predetermined control point.  Some  aspects of these processes can be visualized with the aid of a diagram such as Figure 1-11  which shows the various processes occuring when chamber  air flows through a conditioning system.  In this case, only a cooling  element is in the conditioning system; ma-j is the mass of air in pounds entering the conditioning section which is equal to mag,  the mass of air  leaving, and re-entering the plant growth area.  /////////////////////////// ma  ma 2  W  W,  2  h  7T777777T77T7777T7777777777  Fig. 1-11  Processes occuring when cooling air in a conditioning system.  14  A i r enters the c o n d i t i o n i n g section at temperature t-j and leaves at tr>.  The entering and leaving humidity r a t i o s ( l b . o f water/ l b . o f a i r )  and enthalpies (BTU/lb. o f a i r ) were W-j, W and h-j, h 2  2  respectively.  The heat added to or subtracted from the a i r stream is represented by q (BTU).  Unless the control point is changing, entering a i r w i l l be  warmer than a i r at the exhaust o f the conditioning section due t o the p o s i t i v e heat load of the l i g h t s and other equipment in the chamber. The humidity r a t i o W-j w i l l be equal to W i f no water condenses on the 2  cooling c o i l or i s added to the a i r stream.  The enthalpy change between  the i n l e t and exhaust depends on the change in temperature and humidity ratio.  For the sensible heating or cooling of moist a i r (not i n v o l v i n g  the addition or subtraction o f water) the f o l l o w i n g heat t r a n s f e r equations summarize the possible conditions of Figure 1-11 ( T h r e l k e l d , 1970). ma-| = ma  (1)  2  ma-jh-j + q = ma h 2  2  (2)  ma-jW^ = ma W  (3)  q = ma(h -h )  (4)  2  2  and i f  2  1  W-j = W  2  q = ma(0.24 + 0.45W) ( t ^ )  (5)  A pound o f dry a i r at 50% r e l a t i v e humidity and a barometric pressure of 14.696 Psia w i l l have a volume o f 12 cubic feet at 16°F and a volume o f 14.5 c u . f t . at 98°F (General E l e c t r i c , 1957).  Therefore, i f a fan  c i r c u l a t e d 1000 cfm at a l l temperatures and h u m i d i t i e s , a greater mass o f a i r would be c i r c u l a t e d at lower temperatures than at higher temperatures. Changes in a i r mass w i l l cause changes in fan power requirements.  One  15 wonders whether t h i s e f f e c t should be considered in plant growth chamber research.  At any temperature, and i f W-j = Wg, the amount of head added  to or subtracted from a i r by the c o n d i t i o n i n g process equals the product of the mass of a i r c i r c u l a t e d per u n i t time and the change in enthalpy of the a i r as i t passes through the c o n d i t i o n i n g elements. Also, i f W-j = Wg, q w i l l equal the product of the mass of a i r c i r c u l a t e d per minute times a f i x e d c o e f f i c i e n t r e l a t e d to humidity r a t i o times the temperature d i f f e r e n c e across the c o i l  (Equation 5 ) .  For the t r a n s f e r  of s e n s i b l e heat, Equation 5 allows the c a l c u l a t i o n of the heat t r a n s f e r required to balance the heat load of the chamber.  The value c a l c u l a t e d  w i l l be the t h e o r e t i c a l quantity of heat exchanged in a given c o n d i t i o n i n g process.  In p r a c t i c e , however, the heat required w i l l d i f f e r from q  since a c o o l i n g c o i l i s not a perfect heat t r a n s f e r system.  An ideal  cooling or heating element would have low thermal r e s i s t a n c e , extensive contact with the a i r moving through i t , uniform temperature across the e n t i r e surface area of the element, and large face area r e l a t i v e to the volume of a i r c i r c u l a t e d .  The properties of a real cooling c o i l w i l l be  discussed l a t e r (Page 2 0 ) , since humidity changes almost always accompany the operation of a c o o l i n g c o i l . Two heating and c o o l i n g methods are commonly used on plant growth chambers.  The f i r s t system involves a r e f r i g e r a t e d cooling c o i l and a  separate heating element.  When cooling i s demanded, the compressor of  the r e f r i g e r a t i o n system i s a c t i v a t e d and the heating element i s turned o f f . The reverse occurs during the heating c y c l e .  I f the heating and cooling  elements are c l o s e l y matched in t h e i r rate of heat exchange, s a t i s f a c t o r y operation can r e s u l t .  However, there i s a tendency f o r the thermal resistance  16 o f the heating and cooling elements to cause overshooting of a control point.  I f the overshoot and undershoot v a r i a t i o n s about a control  are s u f f i c i e n t l y s m a l l , the problem can be ignored.  point  Figure 1-12 shows  how heating and cooling rates might vary with time in respect to a given control point and a desired ± 1/4°C c o n t r o l .  The e f f i c i e n c y of heat  t r a n s f e r of a heating or cooling element varies with temperature. Consequently, over the f u l l temperature range in a p l a n t growth chamber, the r e l a t i o n of the rates o f heating and cooling may vary.  I t i s possible  to c a l c u l a t e whether the rate o f e i t h e r heating or cooling may be too great and cause overshoot ( T h r e l k e l d , 1970).  Direct experimentation,  however, is usually r e l i e d upon in prototype stages to discover undesirable performance.  In p r a c t i c e , the a i r temperature is measured at a l o c a t i o n  s u f f i c i e n t l y past the heating and cooling elements so t h a t remixing o f the c o n d i t i o n e d - a i r stream is nearly complete and before the influence o f the chamber heat loads. in Figure 1-13.  A t y p i c a l heating and cooling system is shown  In Chapter I I , the importance o f s e n s i t i v i t y and response  time of the device c o n t r o l l i n g the heating and cooling elements w i l l be discussed. A c o n d i t i o n i n g technique more recently incorporated in plant growth chambers is the hot-gas bypass system (Figure 1-14).  The differences  from the preceding system are t h a t the compressor operates continuously and the hot or vaporized gas in the r e f r i g e r a t i o n system i s shunted past the condensor d i r e c t l y i n t o the evaporator c o i l thus making the evaporator c o i l heat rather than c o o l .  This technique i s very e f f i c i e n t in reducing  over- and undershoot problems.  The e f f e c t i v e thermal resistance o f the  17  OVERSHOOT  +W c 5  lu a « " =o UJ O p z  2 o y —i  a cc ui U J D »- •  UNDERSHOOT •  Equal heating and cooling rates  —COOLINS  — -  HEATING - — COOLING  --  HEATING  -  Cooling rate > heating rate  -  Heating rate > cooling rate  /  COOLING  -  HEATING -  F1g. 1-12 Temperature variations about a control point caused by different rates of heating and cooling.  EVAPORATOR  PRESSURE  PRESSURE REGULATING VALVE  EVAPORATOR  C  (  )  REGULATING VALVE  )  RECEIVER  CONDENSOR  F1g. 1-13  DRYER  The refrigeration system of the Percival Model PGC-78 growth chamber u t i l i z i n g two evaporator c o i l s . (Percival, 1963. •)  EVAPORATOR  ( ACCMULATOR  H E A T EXCHANGER CONDENSOR  DRYER  HEATING SOLENOID  Lb  PRESSURE RECUL.ATIN6 VALVE  F1g. 1-14  ^ Q  COMPRESSOR  •o  COOLING SOLENOID  A typical refrigeration system u t i l i z i n g the hot-gas by pass technique of evaporator c o i l temperature regulation. (Controlled environments, 1967. |  T9  evaporator c o i l is reduced by the i n t r o d u c t i o n o f hot gas.  I f a heating  element i s r e q u i r e d , i t s wattage can be s i g n i f i c a n t l y decreased since the hot gas acts as a heating source.  Temperature control w i t h i n ± 1/8°C  can be r e a l i z e d w i t h the hot gas bypass system. Before proper conditioning capacity can be selected, the magnitude of the plant growth chamber's heat load must be determined f o r a l l p r a c t i c a l ambient and chamber c o n d i t i o n s . is the l a r g e s t heat source.  Generally, the l i g h t system  Each watt of fluorescent and incandescent  l i g h t i n g y i e l d s 4.09 and 3.41 BTU/hour, r e s p e c t i v e l y .  These conversion  figures allow the heat input of the l i g h t system to be calculated d i r e c t l y . Fan motors and other e l e c t r i c a l and e l e c t r o n i c devices in the a i r system w i l l add heat r e l a t i v e t o t h e i r duty cycles and wattage.  circulation Heat t r a n s f e r  through the chamber's w a l l , r o o t , and f l o o r panels i s the product of the surface area's involved ( A ) , the heat t r a n s f e r c o e f f i c i e n t of the panels ( K ) , and the temperature d i f f e r e n c e across the panels ( t g - t ^ ) ( K r e i t h , 1964): q = kA ( t  2  - t^  (6).  The i n f i l t r a t i o n o f a i r i n t o a plant growth chamber can provide a heat load i f the exchange volume is s i g n i f i c a n t and the inside and outside conditions are d i f f e r e n t .  O r d i n a r i l y , chamber leakage becomes important  only when extreme chamber conditions are attempted. Fresh a i r make-up, which i s the addition o f a predetermined volume of a i r to a plant growth chamber to maintain normal C0 concentration l e v e l s , 2  also acts as a heat load. Once the general heat loads of a chamber are known, the e f f e c t of varying heat loads, such as diurnal l i g h t c o n t r o l , can be estimated. Discussion of such e f f e c t s are presented in the f o l l o w i n g s e c t i o n .  20  (C) Humidity Control Knowledge of the humidity in the natural environment i s important to the study of plant growth.  For example, the gradient o f p a r t i a l  pressure  of water vapor across the boundary layer of a l e a f i s the d r i v i n g force of t r a n s p i r a t i o n , the loss of water by evaporation from p l a n t s . A plant growth chamber should provide some degree o f humidity control to remove humidity f l u c t u a t i o n s caused by the c o n d i t i o n i n g equipment. Humidity is a very complex variable of the environment, and therefore careful analysis o f a chamber's proposed operating ranges i s necessary to provide adequate humidity c o n t r o l .  The i n c l u s i o n of humidity control can  double the cost of a plant growth chamber. I t i s convenient to introduce additional equations which summarize the physical processes involved and the h u m i d i f i c a t i o n and dehumidification of moist a i r .  Table 1 gives the common terms used in psychrometric  r e l a t i o n s along with an example of how each o f the terms varies with temperature. An analysis of Figure 1-15 provides the f o l l o w i n g r e l a t i o n s describing the cooling o f a i r which r e s u l t s in condensation o f water.  ////////////\/\/////////////, m,.  W  l  F i g . 1-15  Processes occuring when a i r is cooled and condensation results.  21  m-j =  (7)  = m  IU,  q = m(h - h-j) - m h 2  c  (8)  (  m = m(W - W ) c  1  (9)  2  In Equation 8, assuming ideal heat t r a n s f e r , the amount of heat subtracted from the a i r equals m (the mass of a i r passing through per minute) times the enthalpy change minus the amount of heat contained in the water that was condensed.  The amount of water that condenses  equals m times the change in humidity r a t i o . The h u m i d i f i c a t i o n and heating of moist a i r i s i l l u s t r a t e d in Figure 1-16.  The amount of heat added to the a i r equals m times the  enthalpy change minus the heat contained in the added water.  The amount  of water added equals m times the change in humidity r a t i o (Equation 11),  ////////\//////////////////  TTTTTTTTTT F i g . 1-16  Processes occuring when a i r i s heated and humidified.  q = m(h - h ^ - m h 2  w  w  (10) (11)  TABLE 1  Psychrometric  Terminology r  TERM  SYMBOL  Temperature Humidity  .  Ratio  Relative  t  °C  lb, w'  W  Humidity  UNITS  a  .  • »  R.H.  10° c  21° C  32° C  .0038 •  .0080  .0152  50%  50%  50%  25.4  35.7  Enthalpy  h  BTU/lb d r y a i r  16.1  Volume  V  f t 3 / l b dry a i r  12.9  lb / l b w' a  .0076  .0158  50%  50%  Saturation  potential  Saturation ages  percent-  s  -  u  Thermodynamic wet bulb temp.' Dew p t .  W  temp.  Source:  .  13.55  •  13.9 .0312  49% »  t*  °C  6° C  V  °C  0° C  Threlkeld Psychrometrlc Chart,  1970.  •14°C 10° C  24°C 20° C  ro ro  23  The h u m i d i f i c a t i o n o f m o i s t a i r when no o t h e r energy i s added i s a s p e c i a l case o f t h e system i n F i g u r e 1-16.  The amount o f heat added  by t h e w a t e r t o t h e a i r e q u a l s m times t h e t o t a l e n t h a l p y change.  The  amount o f w a t e r added e q u a l s m times the change i n h u m i d i t y r a t i o . \h  s w  m(h  = m(W  2  - V  (12)  - W.,)  (13)  2  The a d i a b a t i c m i x i n g o f two streams o f m o i s t a i r o c c u r s when f r e s h air  i s added t o chamber a i r ( F i g u r e 1-17).  the  volume o f f r e s h a i r make-up i s u s u a l l y s m a l l , b u t i t s e f f e c t upon  chamber a i r can be c a l c u l a t e d .  F o r p l a n t growth chambers,  The make-up a i r may r e p r e s e n t a s i g n i f i c a n t  w a t e r vapor l o a d which s h o u l d be c o n s i d e r e d when h u m i d i f i c a t i o n is designed. the  equipment  The e f f e c t s o f f r e s h a i r i n f i l t r a t i o n can be determined i n  f o l l o w i n g manner,  m^, t h e mass o f a i r r e t u r n i n g from the chamber, mixes  w i t h m^, the mass o f make-up a i r , p r o d u c i n g the c o n d i t i o n s a t p o i n t 3 i n F i g u r e 1-17 ( E q u a t i o n 1 4 ) .  Fig.  1-17  P r o c e s s e s o c c u r i n g when two a i r s t r e a m s a r e mixed.  24 (14) (15) m  l 1 W  +  m  22 W  =  0 6 )  3^3  m  The airiat point 3)then enters the conditioning equipment! It is important to mention that a mass of air equal to tr^ must be exhausted from the chamber after conditioning and before entry to the plant growth area to compensate for the mass of made-up air introduced. The above heat transfer relations assume that the heating, cooling, humidifying and dehumidifying processes are ideal.  The non-ideal qualities  of real conditioning elements are an important aspect of the overall performance of a plant growth chamber. The following discussion will develop problems and conditions that can occur in plant growth chambers, and will consider those chamber designs which provide the most practical solutions. The hypothetical situation in Figure 1-18 illustrates the general affect of a cooling coil on moist air. 2^0^ 50% R.H.  Enterinq P-  A i r  tem  18.5°C  Cooling Coil (side view) Exhaust Air temp.  21 C 47% R.H.  r Flow Coil surface temp. 12.5°C_ Entering L7ew~P~bTnt~  Fig. 1-18  10°C ExhaustTTew Point  Humidity and temperature changes occurring in a stream of air passing through a cooling coil.  25  Assume t h a t the desired control point is 21°C and 47% r e l a t i v e humidity, and t h a t such a i r leaves the c o i l in Figure 1-18.  After circulating  through the plant growth area, suppose t h a t the a i r returns to the cooling c o i l a t 24°C and 50% r e l a t i v e humidity.  The heat and moisture loads of  the chamber w i l l determine the state o f the a i r r e t u r n i n g to the c o i l . Further assume t h a t , in the c o i l of Figure 1-18, the temperature o f the surface o f the f i n s and tubes ranges from 18°C at the leading edge where the a i r enters the c o i l , to 7°C at the end where the a i r leaves the c o i l . The actual temperature gradient w i l l be a function o f the face area of the c o i l , the t o t a l outside surface area of the f i n s and tubes of the c o i l , the temperature and rate of c i r c u l a t i o n o f r e f r i g e r a n t , the i n l e t face v e l o c i t y , and the number of rows of tubes.  A l i n e a r gradient i s shown  in Figure 1-18 f o r convenience. The dewpoint o f the a i r entering the c o i l in Figure 1-18 is 13°C and o f the a i r l e a v i n g , 10°C.  This implies t h a t condensation has occurred  since a change in dewpoint i s possible only when the humidity r a t i o changes (at constant atmospheric pressure).  Furthermore, part of the surface o f  the c o i l is above 13°C, and w i l l remain d r y .  Condensation w i l l occur on  the p o r t i o n of the c o i l which is below the dewpoint. should be provided f o r such condensation.  A drainage system  The surface area of the c o i l  which becomes wet i s mostly determined by the surface area of the c o i l below the dewpoint o f the entering a i r .  When the c o i l surface temperature  is l o w e r t h a n in Figure 1-18, even more condensation w i l l occur from each pound of a i r c i r c u l a t e d , since more c o i l surface area w i l l be below the dewpoint o f 13°C.  The mass of water that condenses is given by Equation 9.  26  Some of the water t h a t is condensed may also be re-evaporated. Since dehumidification is an inherent consequence o f cooling c o i l operation, control o f humidity w i t h i n a chamber requires a source o f water vapor to replenish the mass of water l o s t from the c i r c u l a t e d a i r due to condensation.  The capacity o f equipment selected f o r h u m i d i f i c a t i o n  should be determined in r e l a t i o n to the highest rate o f h u m i d i f i c a t i o n expected to be required over the operating range o f the chamber.  In general,  a plant growth chamber i s designed to provide a p a r t i c u l a r humidity range over a s p e c i f i e d temperature range.  Close s p e c i f i c a t i o n i s necessary since  some combinations o f temperature and humidity are very d i f f i c u l t to achieve. For example, low humidities at low temperatures are d i f f i c u l t to produce because the surface temperature of a cooling c o i l tends to drop below the freezing point of water.  I f f r o s t forms on the c o i l , heat  t r a n s f e r is g r e a t l y reduced, causing less e f f i c i e n t a i r temperature c o n t r o l . At high temperatures, high humidities are often d i f f i c u l t to maintain since a very high rate of evaporation may be necessary.  Wall temperatures  may be lower than the dewpoint of such a i r , causing undesired condensation. A plant growth chamber may require careful sealing to reduce undesired water vapor i n f i l t r a t i o n or l o s s . I n s t a l l i n g dehumidification equipment is a much more d i f f i c u l t and expensive problem than h u m i d i f i c a t i o n .  F i r s t , the water that i s removed  from the a i r must be drained away from the a i r c i r c u l a t i o n system so t h a t re-evaporation is not s i g n i f i c a n t .  Secondly, the temperature o f the a i r  being dehumidified should not be s i g n i f i c a n t l y changed by the process. Dehumidification capacity should be selected in r e l a t i o n to the lowest humidity desired at the highest temperature at which humidity control specified.  is  This condition w i l l require the highest rate of d e h u m i d i f i c a t i o n ,  in terms of pounds of water removed per pound o f a i r , t h a t w i l l occur in the  27  plant growth chamber.  I t i s important to consider the e f f e c t s of plant  t r a n s p i r a t i o n as a source performed.  of water vapor when dehumidification i s  Several references contain data from which estimates of moisture  load due to t r a n s p i r a t i o n can be obtained (Salisbury and Ross, 1969; Geiger, 1965).  To make an accurate estimate of t r a n s p i r a t i o n such factors  as l i g h t i n t e n s i t y , rate of photosynthesis, temperature, r e l a t i v e humidity, wind v e l o c i t y , time of day, and atmospheric pressure should be considered. As mentioned e a r l i e r , i f f r o s t accumulates on the surface of a cooling c o i l , the heat t r a n s f e r e f f i c i e n c y of the c o i l i s reduced.  Defrost  c y c l e s , usually only a few minutes long, are programmed on c o i l s when f r o s t problems are encountered.  A defrost cycle usually involves the  shut-down of the r e f r i g e r a t i o n compressor and fan, and p o s s i b l y the a c t i v a t i o n of a heater to melt the accumulated ice or f r o s t .  The length  and required number of defrost cycles are usually found by experimentation. During a defrost c y c l e , the a i r temperature of the plant growth area also r i s e s .  This i s an unavoidable problem in chambers with only  one compressor and cooling c o i l .  One method that may be used to avoid  the undesirable a f f e c t s of a defrost cycle on temperature control is the two-compressor system.  With two compressors and c o i l s , which are indepen-  dent, one c o i l can be processing the chamber a i r while the other i s being defrosted  (Figure 1-19).  chamber performance i s improved.  Of course, the expense i s greater but  28  /////////////////////////  7/////////////////////V,  COIL A —  1  COIL A  '  a*  —  COIL B —  COIL B  '////////////////////,  (a)  (b)  COIL A  Defrosting  COIL A  Processing  COIL B  Processing  COIL B  Defrosting  F i g . I-19  Dual damper-controlled cooling c o i l  The e f f e c t of changing heat loads on humidity should also be considered in the design of a plant growth chamber.  A good example of a varying heat  load occurs in the diurnal control of the l i g h t system of a chamber.  When  the l i g h t s are on, some heat i s added to the a i r which must be removed by the c o n d i t i o n i n g equipment before the a i r can be re-introduced to the plant growth area at a given temperature.  With the l i g h t s o f f , no such heat i s  added to the a i r , but the c o n d i t i o n i n g equipment must recondition the a i r to the same temperature.  Figure 1-19 i n d i c a t e s what occurs i n the cooling  29 coil in both lights-on (a) and lights-off conditions (b)  Air Temp  Air Temp  Air F o lw  Air Flow  Dew Point  (b) Lights Off  Fig. 1^20. The effect of varying heat load on cooling coil performance.  Since the refrigerant temperature in the coil is essentially constant, and i f the face velocity is constant, the surface temperature of the fins and tubes of the coil will be determined by the heat load.  The higher  the heat load, the higher will be the average surface temperature of the coil.  The result is that with the lights off, more water may be condensed  since more of the coil may be below the dewpoint of the a i r passing through. It may be concluded that greater humidity fluctuations should be expected in lights-off operation of a coil relative to the lights-on condition.  30  (D) A i r C i r c u l a t i o n The q u a l i t y of the environment of a plant growth chamber w i l l be a function of the uniformity of a i r v e l o c i t y and temperature throughout the chamber.  The provision of a proper quantity of conditioned a i r i s  one problem, while that happens to the conditioned a i r w i t h i n the plant growth area, under intense l i g h t s , with shelves and f o l i a g e blocking a i r flow, depends on the design of the a i r c i r c u l a t i o n system. As mentioned e a r l i e r , a v e r t i c a l a i r flow i s most e f f e c t i v e in chambers of the reach-in type (about 12 sq. feet of growing a r e a ) .  If  the a i r flow i s uniformly d i s t r i b u t e d at the base of the chamber, much more uniform v e r t i c a l a i r flow i s p o s s i b l e .  The a i r w i l l tend to  go up through the plant f o l i a g e , and i f the plants are evenly d i s t r i b u t e d in the chamber, the a i r flow w i l l meet (nearly) the same amount of resistance across the chamber, making i t more probable that a l l of the plants w i t h i n the chamber w i l l experience approximately the same r e s u l t a n t a i r flow.  The development of a v e r t i c a l a i r flow system i s described in  Chapter V. The e f f e c t s of the a i r c i r c u l a t i o n system upon humidity mainly r e f l e c t the e f f e c t s of a temperature gradient from the chamber base to the l i g h t system.  For example, once conditioned a i r has entered the plant growing  area, the absolute humidity i s constant (excluding t r a n s p i r a t i o n and s o i l water evaporation f o r the moment).  Therefore, i f a temperature gradient  does e x i s t v e r t i c a l l y in the chamber (with v e r t i c a l a i r f l o w ) , a lowering of the r e l a t i v e humidity w i l l be noticed along the gradient.  The smaller  the temperature gradient (or d i f f e r e n t i a l ) , the smaller w i l l be the change in humidity.  There w i l l always be a temperature gradient (though i t can  be minimized or compensated for) since high-powered, intense l i g h t systems  31  •are c o n s t a n t l y h e a t i n g the a i r o f the chamber and a l l the chamber, i n c l u d i n g the chamber w a l l s .  the m a t e r i a l  This v e r t i c a l  in  temperature  g r a d i e n t i s a f u n c t i o n o f the l i g h t i n t e n s i t y o f the l i g h t system and the v e l o c i t y o f a i r  circulation.  Because o f the above e f f e c t s , i t  i s u s e f u l t o d e f i n e an e n v i r o n -  mental c o n d i t i o n a t a s p e c i f i c l o c a t i o n i n the chamber. r e f e r e n c e p o i n t , the p o s s i b i l i t y o f r e p r o d u c i n g a s p e c i a l greatly increased.  By u s i n g such a environment  T e m p e r a t u r e , h u m i d i t y , and a i r v e l o c i t y  is  gradients  occur w i d e l y i n n a t u r e , so the problems are n o t unique t o growth chambers.  (E) R e l i a b i l i t y  and S e r v i c e  For a r e l i a b l e p l a n t growth chamber, s o l i d - s t a t e e l e c t r o n i c should be i n c o r p o r a t e d wherever p o s s i b l e .  The use o f  controls  electromechanical  devices s h o u l d be r e d u c e d , as the number o f o p e r a t i o n s i n a c o n t i n u o u s l y r u n n i n g growth chamber becomes enormous.  For example, a r e l a y which  operates once a minute ( n o t uncommon) w i l l in a year.  Such e l e c t r o m e c h a n i c a l  v a l v e s o f the r e f r i g e r a t i o n  e x p e r i e n c e 526,600 o p e r a t i o n s  r e l a y s are used t o c o n t r o l  compressor.  When a s o l e n o i d i s  the  solenoid  energized  o r de-energized, a radio-frequency or h i g h - v o l t a g e pulse t r a v e l s  the  l i n e , and when t h i s p u l s e h i t s t h e c o n t a c t s o f a r e l a y , a spark can occur.  As a r e s u l t , the c o n t a c t s o f a r e l a y can erode u n t i l  c i r c u i t occurs.  A r e l a y r a t e d a t 10  operations  a short  ( m e c h a n i c a l ) may l a s t  o n l y 500,000 o p e r a t i o n s when i t a c t i v a t e s s o l e n o i d s .  The l i f e o f  relay  c o n t a c t s can i n c r e a s e g r e a t l y by f i l t e r i n g the h i g h - v o l t a g e p u l s e o u t o f the l i n e b e f o r e i t a "clipper"  reaches the c o n t a c t s .  ( d i o d e ) accomplishes t h i s  A General E l e c t r i c t h y r e c t o r  easily.  or  32  A schedule of l i g h t bulb replacement should be arranged so that a predetermined average l i g h t i n t e n s i t y and q u a l i t y i s maintained continuously over any period of time.  The average l i g h t i n t e n s i t y that  w i l l be p r a c t i c a l to maintain w i l l depend on the number of l i g h t bulbs in the chamber, the c o s t , and the average l i f e t i m e u n t i l l i g h t output becomes too low.  By f o l l o w i n g a schedule of l i g h t bulb replacement, the  researcher can have greater confidence that short- and long-term v a r i a t i o n s in l i g h t i n t e n s i t y and q u a l i t y w i t h i n a chamber and l i g h t i n t e n s i t y differences between chambers w i l l be minimimal. Most of the mechanical devices and e l e c t r i c motors ( e . g . compressors and fans) are e i t h e r constructed with sealed bearings or are s e l f lubricating.  Thus, s e r v i c e of these devices i s a matter of replacement  at the end of t h e i r l i f e t i m e s .  I t i s good p r a c t i c e to maintain an inven-  tory of c r i t i c a l components that are not r e a d i l y a v a i l a b l e from l o c a l retail outlets.  A c r i t i c a l component i s defined as one such that i f  f a i l e d , the operation of the chamber must stop u n t i l i t i s replaced.  it  33 Chapter  II  INSTRUMENTS FOR THE CONTROL, INDICATION, AND MEASUREMENT OF ENVIRONMENTAL VARIABLES  (A)  Control A t p r e s e n t , most commercial and custom b u i l t  growth chambers p r o v i d e  o n l y a minimum o f l i g h t i n t e n s i t y c o n t r o l and moderate ( e . g . ± 2°C) temperature c o n t r o l .  A i r f l o w i s r a r e l y found t o be a c o n t r o l l a b l e  v a r i a b l e and h u m i d i t y c o n t r o l equipment.  i s so expensive t h a t few chambers have such  A i r f l o w , h u m i d i t y , and a t m o s p h e r i c c o m p o s i t i o n (0  concentration in p a r t i c u l a r )  2  and  C0  are g e n e r a l l y assumed c o n s t a n t , which i s  2  not  usually a safe assumption. C o n t r o l s f o r the l i g h t system u s u a l l y c o n s i s t o f time c l o c k s v a r y i n g the p h o t o p e r i o d , and moveable shelves o r p u l l e y systems i n c r e a s i n g or d e c r e a s i n g t h e l i g h t i n t e n s i t y t o which the p l a n t s  for  for are  exposed. Temperature c o n t r o l total  i n a p l a n t growth chamber i s more dependent on  chamber d e s i g n , w h i l e l i g h t system c o n t r o l  the p r o p e r t i e s o f lamps a l o n e .  i s l a r g e l y determined by  Three b a s i c types o f t e m p e r a t u r e  have been used on p l a n t growth chambers; b i m e t a l l i c s t r i p s , t h e r m o s t a t s , and e l e c t r o n i c  controls  hydrostatic  controls.  B i m e t a l l i c s t r i p s and h y d r o s t a t i c t h e r m o s t a t s were used on e a r l y growth chambers w i t h h y d r o s t a t i c c o n t r o l s s t i l l equipment.  b e i n g used on l e s s  Both methods l a c k a s i m p l e means o f v a r y i n g s e n s i t i v i t y  expensive ( t o the  change i n t e m p e r a t u r e between an on and o f f c y c l e ) , which i s necessary p r o v i d e p r o p e r balance between h e a t i n g and c o o l i n g l o a d s .  to  The b i m e t a l l i c  s t r i p does n o t a l l o w remote s e t p o i n t l o c a t i o n , an i m p o r t a n t  convenience  34  f o r the chamber operator.  Neither method provides a c a l i b r a t i o n  adjustment f o r d r i f t s in c a l i b r a t i o n with time, or a single p o l e , double throw output which i s required f o r c o n t r o l l i n g both heating and cooling loads.  The small d i a l usually provided may or may not be in temperature  units and seldom is i t possible f o r a s p e c i f i c temperature to be set d i r e c t l y on the d i a l .  Several types of hydrostatic controls are not  compensated f o r changes in ambient conditions around the chamber.  The  hydrostatic c a p i l l a r y tube extends from the sensor bulb ( i n the chamber) to the body o f the c o n t r o l , which is usually mounted on an e x t e r i o r chamber wall and is exposed to ambient temperature f l u c t u a t i o n s . Users o f plant growth chambers often require diurnal or programmable temperature c o n t r o l .  To provide such c o n t r o l , the Percival Model  PGC-78, f o r example, uses a time clock t h a t switches between two independent h y d r o s t a t i c controls t h a t are pre-set to day and night temperatures. Programming ( m u l t i - s t e p ) temperature control is not p r a c t i c a l with hydros t a t i c devices, since a separate control would be required f o r each step in the program.  C a l i b r a t i o n o f hydrostatic diurnal controls is more  d i f f i c u l t than f o r a single control and since two controls are i n v o l v e d , there are more devices to f a i l in s e r v i c e . S o l i d - s t a t e e l e c t r o n i c temperature controls are more complicated and- expensive than the above devices, but o f f e r r e l a t i v e l y simple solutions to a l l of the problems associated with growth chamber temperature control in presently e x i s t i n g equipment.  Nearly a l l s o l i d - s t a t e controls use the  Wheatstone bridge technique with a temperature dependent resistance element as the sensor (Doebelin, 1968).  The composition of the resistance  element may be semiconductor ( t h e r m i s t o r s ) , wire o f a l l o y composition (NiFe),  35  or pure metal (nickel or p l a t i n u m ) .  The sensor can be made small so  t h a t i t s time response to a temperature change is very f a s t .  I f the  resistance of the sensor i s large (greater than 1000 ohms) lead r e s i s tance is n e g l i g i b l e and lead lengths can be up to 200 feet with only small errors r e s u l t i n g .  For low resistance sensors (less than 150 ohms),  such as platinum, lead length compensation should be provided.  Remote  l o c a t i o n i s r e l a t i v e l y simple when using a resistance sensor. Remote s e t - p o i n t adjustment i s also e a s i l y provided by s o l i d - s t a t e controls.  A potentiometer with a scale c a l i b r a t e d according to the  temperature c o e f f i c i e n t o f the resistance sensor may be mounted on a panel convenient to the chamber operator.  The length of the scale is  usually about f i v e inches, which allows a large temperature range (-20° to 50°C i s t y p i c a l ) to be covered on one d i a l , without l o s i n g the c a p a b i l i t y of s e t t i n g the temperature to w i t h i n 1/4°C. C a l i b r a t i o n and s e n s i t i v i t y adjustments are also accomplished with potentiometers and can be made a v a i l a b l e on a convenient control panel. C a l i b r a t i o n is required f o r compensation of sensor aging and to balance out the non-uniformity between the o r i g i n a l and replacement sensors. Resistance elements are t y p i c a l l y manufactured to tolerances of 1% or more. S e n s i t i v i t y can be varied in several ways.  A sensor may be wrapped with  cloth or tape to increase i t s mass, which w i l l decrease i t s response time to a given temperature change.  A second technique i s to reduce the voltage  across the Wheatstone b r i d g e , which reduces the signal to the control amplifier.  I t then takes more temperature change to make the a m p l i f i e r  switch from heating to c o o l i n g .  Changing the bridge voltage i s not as  s a t i s f a c t o r y a method o f varying s e n s i t i v i t y since the current flowing through the sensor i s changed, which a l t e r s the s e l f - h e a t i n g e r r o r of the  36 sensor.  Such a change can a f f e c t the o v e r a l l c a l i b r a t i o n of the  controller. The best a v a i l a b l e method of varying c o n t r o l l e r s e n s i t i v i t y accomplished e l e c t r o n i c a l l y .  By using a resistance-capacitance  is circuit  in the feedback o f the a m p l i f i e r , the difference signal o f the wheatstone bridge is i n t e g r a t e d .  By varying e i t h e r the resistance or capacitance,  the slope of the i n t e g r a t i o n can be changed, which d i r e c t l y changes the time response o f the c o n t r o l l e r without i n t e r f e r i n g with the sensing element at a l l . V i r t u a l l y any type and capacity of output device can be provided with a s o l i d - s t a t e temperature c o n t r o l .  The most common i s a single-pole  double-throw electromagnetic relay rated at 10 to 25 amps ( n o n - i n d u c t i v e ) . This form o f relay allows control of heating and cooling elements. recent s o l i d - s t a t e control designs incorporate s o l i d - s t a t e  More  alternating  current switches (Triacs) f o r the control of heating and cooling elements. Triacs eliminate the electromagnetic r e l a y , a mechanical device with contacts t h a t has proven to be the l e a s t r e l i a b l e component of s o l i d - s t a t e controls.  The use o f t r i a c s can extend the mean-time-between-failure  r a t i n g to years instead o f months. Diurnal temperature control is e a s i l y provided by s o l i d - s t a t e control systems.  For diurnal c o n t r o l , a time clock switches between two s e t - p o i n t  potentiometers which are preset  to the desired day and n i g h t temperatures.  Instead of r e q u i r i n g two complete h y d r o s t a t i c c o n t r o l l e r s , a single potentiometer is the only extra p a r t .  The same sensor and a m p l i f i e r control  both day and n i g h t temperatures, increasing accuracy while s i m p l i f y i n g the t o t a l control system.  37  The programming of temperature with a s o l i d - s t a t e control i s an extension of diurnal c o n t r o l .  One method i s to switch a s e r i e s of set  point potentiometers by means of a stepping relay sequenced by a time clock.  Proper switching must be provided (low contact r e s i s t a n c e ) , and  a c i r c u i t f o r c a l i b r a t i o n of the set-points devised. i s required f o r t h i s type of programmer.  Only one c o n t r o l l e r  The more f a m i l i a r cam-type  control is also adaptable to growth chamber temperature programming and gives stepless c o n t r o l . Maintenance of h y d r o s t a t i c temperature controls consists of replacement of the e n t i r e u n i t .  For s o l i d state c o n t r o l s , however, i f a component  f a i l s , i t can be located and replaced.  I f electromagnetic relays are used  as the output of a c o n t r o l , they should be inspected f i r s t in diagnosing failure.  S o l i d - s t a t e components have acquired a great reputation f o r  r e l i a b i l i t y , and should e x h i b i t a mean-time-between-fai1ure of more than 10,000 to 20,000 hours operation. In the f u t u r e , i t w i l l be possible to have such controls on other environmental v a r i a b l e s such as l i g h t i n t e n s i t y , humidity, and a i r speed. Complexity and expense prevents common use of such controls at present.  (B) Indication The design of instruments f o r i n d i c a t i o n of environmental variables is more complex than f o r c o n t r o l s .  Discussion here w i l l be l i m i t e d to  those instruments and devices used in measuring l i g h t i n t e n s i t y , temperature, a i r c i r c u l a t i o n , and humidity f o r the data in Chapters III and V. In the c o l l e c t i o n of l i g h t i n t e n s i t y information in the plant growth area of the P e r c i v a l Model PGC-78, a photometric l i g h t sensor was u t i l i z e d , since the l i g h t q u a l i t y (made up of fluorescents and incandescents) remained  38 e s s e n t i a l l y constant during the measurement p e r i o d , and was f a r simpler to use than the only available radiometric l i g h t sensor.  An I n t e r n a t i o n a l  R e c t i f i e r s i l i c o n photovoltaic photocell (Serial No. S1020E4PL) was c a l i b r a t e d with a Gossen Tri-Lux f o o t candle meter.  The response o f the  photocell was found to be l i n e a r between 300 and 2500 f o o t candles, a useable range f o r t h i s a p p l i c a t i o n .  The s i l i c o n photocell was attached  d i r e c t l y across the input o f a d i g i t a l voltmeter with an input impedence of greater than 1000 megohms.  The various readings were d i g i t a l l y  on command by a Solatron Data Logging system.  printed  The surface area of the  photocell was 1/2 inch square ( 1 / 2 " by 1 " ) , p e r m i t t i n g measurements to be taken at close spacing (four inches) without overlapping o c c u r r i n g . Temperature measurements were made with a copper-constantan thermocouple (wire diameter .032 i n c h ) .  Radiation compensation was not necessary  because of the small surface area o f the thermocouple.  Cold j u n c t i o n com-  pensation was accomplished with a platinum regulated j u n c t i o n box ( S o l a t r o n ) , accurate to ± .15°C.  The thermocouple output was also connected to the  d i g i t a l voltmeter f o r measurement and recording. A i r c i r c u l a t i o n w i t h i n the plant growth area was measured with a Flow Corporation Model 55B1 hot-wire anemometer, which is most accurate in the region below 200 fpm.  For a l l the a i r flow measurements, the probe  was held so t h a t the hot-wire element was always h o r i z o n t a l .  This was done  in an attempt to obtain the v e r t i c a l a i r v e l o c i t y component, as much as p o s s i b l e ; and to improve the r e p r o d u c i b i l i t y o f readings at a given l o c a t i o n . The r e l a t i v e humidity in the plant growth area was measured w i t h a Phys-Chemical Research Corporation Model 11 precision copolymer styrene p l a s t i c sensor. humidity.  The resistance of t h i s device is a function of r e l a t i v e  The sensor was connected in an a l t e r n a t i n g current Wheatstone  39  bridge and c a p a c i t i v e l y coupled to a n u l l detecting a m p l i f i e r .  Accuracy  was ± 1.5% R.H., s e n s i t i v i t y ± 1% R.H., and response time to a 63% change o f R.H. was less than 30 seconds in s t i l l a i r .  Response time in an a i r  flow o f 50 to 100 feet per minute was less than 2 seconds.  The p l a s t i c  sensor was temperature compensated with a thermistor bead mounted very close to the surface o f the wafer.  The p l a s t i c sensor was s e n s i t i v e to  l i g h t i n t e n s i t y and required s h i e l d i n g before measurements could be taken under the l i g h t system in the plant growth area.  (C) Measurement The technique used f o r measuring the environmental variables was made as s i m i l a r f o r each as possible.  To accomplish t h i s , a remote measuring  system was developed to move sensors from one p o s i t i o n t o another without opening the door o f the chamber.  A variable length u-channel was made o f  Plexiglass with rubber f e e t mounted on the ends (Figure 2 - 1 ) .  The u-channel  could be suspended between the end walls o f the chamber by applying a s l i g h t pressure on an adjustable s l i d e , and then t i g h t e n i n g a thumb screw. A miniature battery-operated motor with a gear reduction u n i t was mounted at one end of the u-channel with a small pulley facing the center of the channel.  At the other end o f the u-channel, a second pulley was mounted  also facing the center o f the channel.  A s l i d e t h a t closely f i t the u-channel  was then pulled back and f o r t h by the motor and pulley system, by reversing the b a t t e r y p o l a r i t y .  The various sensors could be attached to the s l i d e  and moved across the chamber.  Wires were run from the motor and sensors  to outside the chamber f o r manual operation.  (C)  > Fig. 2-1  SIDE VIEW  Apparatus for the remote positioning of environmental sensors.  41  A four inch g r i d was placed on both end walls of the chamber being measured (with .100 inch wide tape) so t h a t the ends of the u-channel bar could be moved in sequence, v e r t i c a l l y and h o r i z o n t a l l y , with the four inch g r i d as reference so that one end of the u-channel was in the same r e l a t i v e p o s i t i o n as the other end.  Along the length of the u-channel,  reference marks were made so t h a t the sensor could be stopped every four inches. The procedure f o r operating the device was to p o s i t i o n the u-channel bar at one corner of the f o u r - i n c h grids on the end w a l l s .  The sensor  s l i d e was then run along the u-channel in four inch increments with measurements made every four inches.  When the length of the u-channel  was traversed ( c o n s i s t i n g of t h i r t e e n 4 inch increments), the ends of the u-channel were moved to the next g r i d l o c a t i o n on the end walls and the sensor s l i d e moved back again in four inch increments.  The chamber  door has to be opened to r e p o s i t i o n the ends o f the u-channel b a r , but only once every 13 measurements instead o f once every measurement i f a motorized sensor s l i d e wasn't used.  A f t e r opening and closing the chamber  door, a s u f f i c i e n t amount o f time was allowed f o r the chamber conditions to r e - e q u i l i b r a t e .  42 Chapter  III  COMMERCIAL GROWTH CHAMBER PERFORMANCE  INTRODUCTION A thorough examination of the performance of a Percival PGC-78 w i l l be presented in t h i s chapter.  The information obtained i s valuable f o r  researchers using these growth chambers, as well as to the designer and manufacturer of a r t i f i c i a l environment equipment.  The growth chamber  selected f o r performance analysis was one o f eight of the same model operated by the Department of Plant Science.  The i n t e r i o r of the chamber  was thoroughly cleaned and adjusted to approach o r i g i n a l as closely as possible.  specifications  New fluorescent and incandescent lamps were  i n s t a l l e d and the screens f o r d i f f u s i n g a i r flow were cleaned and straightened. The information gathered w i l l be compared to the advertised p e r f o r mance of the Percival Model PGC-78, and t o the standards of plant growth chamber design presented in Chapter I .  Controlled Environment Limited  r e f r i g e r a t i o n and a i r c i r c u l a t i o n systems w i l l be discussed and compared to the Percival system.  (A) Light System The Percival Company advertises a maximum o f 5000 f o o t candles 4 (5.4 x 10  lux) l i g h t i n t e n s i t y w i t h i n the chamber u t i l i z i n g sixteen VHO  150 watt Cool-White type F72T12 fluorescent lamps, and ten 50 watt incandescent lamps.  The distance from the l i g h t source at which the  above measurement was taken was not s p e c i f i e d .  However, the maximum l i g h t  43  i n t e n s i t y measured in a t o t a l l y empty Model PGC-78 growth chamber in the •4 Plant Science Department was 2500 f o o t candles (2.7 x 10  l u x ) , with the  measurement being taken s i x inches below the center of the l i g h t canopy. This measurement was repeated several times with a Gossen footcandle meter using fluorescent and incandescent lamps with approximately 100 hours operating time ( f o r burn-in purposes) and with chamber temperature at 20°C ± 1°C.  The user o f a plant growth chamber can e a s i l y be  confused by l i g h t i n t e n s i t y s p e c i f i c a t i o n s t h a t do not include the conditions under which the manufacturer measured the performance of his system.  In any case, a maximum i n t e n s i t y r a t i n g i s of l i t t l e  value,  because o f the progressive decrease in output of fluorescent lamps with age (General E l e c t r i c , 1960).  An average l i g h t i n t e n s i t y r a t i n g over a  given period of time would be o f more use to the researcher. Light i n t e n s i t y p r o f i l e s w i t h i n the growth chamber were determined by measuring horizontal planes with the remote sensing apparatus described in Chapter I I (Page 3 9 ) .  Eight horizontal planes, each i n c l u d i n g  91 measurements ( 7 x 1 3 ) taken at four inch i n t e r v a l s , were spaced between 12 inches and 44 inches from the l i g h t system in four inch increments. The shelves and other equipment were removed from the chamber leaving only the l i g h t sensing equipment, which was constructed of P l e x i g l a s s . An empty chamber provides the most reproducible s i t u a t i o n f o r l i g h t measurement, and gives the operator a view of the t o t a l chamber c a p a b i l i t y . Measurements were taken with the chamber door and observation window closed.  The spacing o f the fluorescent and incandescent l i g h t bulbs and  chamber door l o c a t i o n are shown in Figure 3 - 1 .  F i g . 3-1  The spacing and arrangement of fluorescent and i n candescent lamps in the Percival Model PGC-78.  45  Figures 3-2 ( a — * h ) show the l i g h t i n t e n s i t y gradients on horizontal planes in four inch i n t e r v a l s below the l i g h t system.  The lines in  Figures 3-2 ( a — * h ) represent points of equal i n t e n s i t y with the difference between one l i n e and the next being 50 f o o t candles (500 l u x ) .  The  d i f f e r e n c e in l i g h t i n t e n s i t y of horizontal planes progressing down from the l i g h t system in the center o f the chamber i s approximately 150 to 200 f o o t candles (1500 to 2000 l u x ) .  This indicates t h a t the l i g h t  system, i n association with r e f l e c t i v e walls performs s i m i l a r l y to a d i f f u s e - h o r i z o n t a l l i g h t source.  The e f f e c t of distance from the l i g h t  source on i n t e n s i t y is extremely d i f f i c u l t to minimize with t h i s type o f l i g h t system.  The v a r i a t i o n s of l i g h t i n t e n s i t y in any one horizontal  plane r e s u l t from the difference in a d d i t i v e e f f e c t s o f l i g h t  intensity  i n the center of the l i g h t system r e l a t i v e to the sides or ends of the l i g h t system.  Even p e r f e c t l y r e f l e c t i v e walls would not eliminate the  i n t e n s i t y v a r i a t i o n s near the edges of the l i g h t canopy.  One s o l u t i o n ,  not always p o s s i b l e , is to make the size o f the lamp canopy much l a r g e r than the growth area p l a t f o r m , and t h i s technique is often used in open chambers and w a l k - i n rooms. The Percival Company s p e c i f i e s that up to 500 f o o t candles (50,000 lux) are obtainable in t h e i r Model PGC-78.  I t i s c l e a r t h a t at 12 inches  from the b a r r i e r between the l i g h t bulbs and the plant growth area, a distance t h a t i s much higher than would be p r a c t i c a l to use, an i n t e n s i t y of 2000 f o o t candles (20,000 lux) i s a l l t h a t is available under the d i r e c t center o f the l i g h t canopy (Figure 3-2a).  This measurement was taken at  20°C using lamps t h a t had operated 150 hours.  At three thousand hours  operating t i m e , the fluorescents can be expected to give out 75% o f the initial  150 hour value and t h i s would r e s u l t in approximately 1500 f o o t  46  F i g . 3-2 (a-d) Horizontal l i g h t i n t e n s i t y p r o f i l e s at 1 2 " , 1 6 " , 2 0 " , and 24" below the Percival Model PGC-78 Light Canopy (Door Closed) (500 lux i n t e r v a l s )  "  h)  ^ ^ &  n  Z Z W l ° l ^ " '  (500 lux i n t e r v a l s )  1 g h t  Cam  T- " 3 6  ^  W ( °r Closed). Do  48  candles (15,000 lux) at the same l o c a t i o n (General E l e c t r i c , 1960). Because o f these v a r i a t i o n s o f l i g h t i n t e n s i t y in space and time, the s p e c i f i c influence o f l i g h t i n t e n s i t y v a r i a t i o n s on plant growth would be d i f f i c u l t to measure in the Percival chamber.  Such  measurements would require a l l factors of the plant environment except l i g h t to be held at n o n - l i m i t i n g , constant, reproducible l e v e l s , which would be a formidable t a s k . The a d d i t i o n of a few fluorescent lamps across the ends o f the chamber and one placed v e r t i c a l l y in each o f the corners should considerably improve the horizontal u n i f o r m i t y of l i g h t i n t e n s i t y in the chamber. The Percival Company states t h a t the purpose o f the mylar b a r r i e r between the l i g h t system and the plant growth chamber i s t o maintain optimum fluorescent bulb surface temperature (45°C) f o r high l i g h t output ( B u l l e t i n Number 2B).  Fluorescent bulb l i g h t output f a l l s to nearly 10%  o f maximum when the chamber temperature approaches 5°C.  The mylar b a r r i e r  blocks the c i r c u l a t i o n o f chamber a i r around the bulbs.  Fans mounted  above the l i g h t system draw ambient a i r past the bulbs which i s then ducted outside the b u i l d i n g .  As long as the ambient temperature remains  constant, bulb cooling and surface temperature w i l l be f a i r l y constant i f the chamber temperature i s held constant. However, the disadvantages of the b a r r i e r exceed the b e n e f i t s .  The  mylar sheets used as a b a r r i e r by Percival are translucent and, when new, absorb a-minimum of 12% o f the v i s i b l e radiant energy from the l i g h t system.  With ageing, mylar tends to become more opaque and transmit less  l i g h t (Rohm and Haas, 1968).  Other problems associated with b a r r i e r s are  the c o l l e c t i o n o f d u s t , water and other materials on the b a r r i e r and the p o s s i b i l i t y o f exceeding the optimum surface temperature o f the fluorescent  49  bulbs.  High ambient temperatures are very possible in a small room  with several chambers operating.  The removal of the mylar b a r r i e r i s  a cumbersome task often r e q u i r i n g a s h e l f to be moved*. I t has been shown by several other growth chamber manufacturers t h a t l i g h t systems without b a r r i e r s are more e f f e c t i v e over the chamber temperature range, require fewer components, and cost less (ShererG i l l e t t , Controlled Environments, Engineered Environments).  (B) Temperature Control For cooling chamber a i r , the Percival Model PGC-78 u t i l i z e s a 1 horsepower r e f r i g e r a t i o n compressor with an a i r cooled condensing u n i t and dual evaporator c o i l s .  For h e a t i n g , a 300 watt s t r i p heater  is mounted below each of the evaporator c o i l s .  Two separate h y d r o s t a t i c  temperature controls (Penn Controls) operate the heating and cooling systems.  Percival s p e c i f i e s a temperature range and control at 8°C to  40°C ± 1.5°C with a f u l l l i g h t load. Measurements o f temperature u n i f o r m i t y were taken by mounting a thermocouple t o the remote sensing apparatus.  The measurements represent  the average o f two complete heating and cooling cycles by the conditioning equipment.  Associated with each cooling cycle is a minimum temperature,  and with each heating c y c l e , a maximum temperature, at each measurement location.  Measurements every e i g h t inches in a horizontal plane provided  s u f f i c i e n t d e t a i l o f the temperature u n i f o r m i t y (Figure 3 - 3 ) .  After  discovering the nearly symetrical nature o f the temperature p r o f i l e , only one traverse was made down the center of the chamber to obtain the v e r t i c a l p r o f i l e since these measurements were very time consuming.  50  -8'  •]  21.5  25.0  26.0  25.5  26.0  25.5  24.5  22.5  22.5  23.5  24.0  23.5  22.5  21.5  *  CO I  |  25.5  26.5  26.5  2SD  25.5  24.5  22.5  23.5  2U3  24.S  24.0  23-0  22.0  24.5 22.S  25.0  26.0  26.5  27.0  26.S  25.5  22.5  23.5  24.5  2 5.0  24.0  22.5  25.5 22.5  26.5 2 4.5  26.5  26.5 24.5  27.0 2 5.0  26.5  25.5  2 4.5  24.0  22.5  25.0  Fig. 3-3  Horizontal a i r temperature p r o f i l e 14" below the l i g h t canopy barrier.  25.0  25J  25.5  260  2 6.0  255  25.5  22  22S  23.0  23.0  23.0  23.5  234)  22.5  INCHES  24.S  24.5  2 5.0  25.0  24.5  24.5  24.5  30  22/S  22.5  22.5  22.5  22.5  21. S  21.5  INCHES  F1g. 3-4  24.0  24.5  24.0  24.0  24.0  21.0  21.S  21.0  21.0  21.0  Horizontal a i r temperature profiles through counter of chamber for 22", 30" and 38" from light canopy surface.  38 INCHES  51  Percival s p e c i f i e s ± 1.5°C control with no f u r t h e r q u a l i f i c a t i o n which implies t h a t any point reasonably d i s t a n t from the l i g h t source (approximately 10 inches) should be w i t h i n ± 1.5°C o f the control  point.  The data o f Figures 3-3 and 3-4 indicate t h a t at any single point in the growth area, the temperature f l u c t u a t i o n i s about ± 1.5°C around some temperature t h a t may be above or below the desired control point by as much as 2°C.  In f a c t , the actual s e t - p o i n t of the r e f r i g e r a t i o n  c o n t r o l l e r was 19°C, nearly 5 1/2°C cooler than the average temperature at 14" from the l i g h t system, and below any temperature a c t u a l l y measured by 2°C.  This s i t u a t i o n could e x i s t because the hydrostatic temperature  controls have no c a l i b r a t i o n adjustment, and they are also affected by ambient c o n d i t i o n s . I t should be noted that temperature u n i f o r m i t y w i t h i n a growth chamber is very much dependent upon a i r c i r c u l a t i o n , which i s discussed in the f o l l o w i n g s e c t i o n .  (C) A i r C i r c u l a t i o n The a i r c i r c u l a t i o n pattern w i t h i n a plant growth chamber is an important but often ignored environmental v a r i a b l e .  Uniform a i r  c i r c u l a t i o n is a p r e r e q u i s i t e to uniform temperature and humidity coaditions throughout the plant growth area.  The Percival Company states  t h a t in the Model PGC-78, chamber a i r i s r e c i r c u l a t e d over the cooling and heating elements at a v e l o c i t y o f 50 to 100 surface feet per minute. The s p e c i f i e d a i r flow w i t h i n the plant growth area i s a uniformly d i s t r i b u t e d 75 f e e t per minute.  52  Measurements of a i r v e l o c i t y were taken at 8 inch i n t e r v a l s on horizontal planes at 14, 22, 30, and 38 inches from the l i g h t system (Figure 3 - 5 ) .  A Flow Corporation hot-wire anemometer probe was attached  to the remote sensor apparatus, allowing the probe to be moved without opening the chamber door.  Each reading represents an average o f two  minutes observation with the f l u c t u a t i o n s about the average indicated as a plus and minus q u a n t i t y .  The f l u c t u a t i o n was such t h a t the damped  mode (Position 2) o f the anemometer had to be used to obtain the data of Figure 3-5.  The hot-wire o f the probe was held horizontal to the  f l o o r of the chamber so t h a t the l a r g e s t v e l o c i t y component of an observation should have been the v e r t i c a l component.  A l l shelves and  instruments except probes were removed from the chamber.  The d i s t r i b u t i n g  screens on the f l o o r o f the chamber were cleaned, straightened and t i g h t l y f i t t e d in t h e i r proper p o s i t i o n s . Figure 3-5 indicates t h a t there were s i g n i f i c a n t a i r v e l o c i t y v a r i a t i o n s and f l u c t u a t i o n s across the horizontal planes.  The greatest  v a r i a t i o n s were at 14 inches from the l i g h t system (Figure 3-5a) with a 150 ± 15 fpm maximum and a 10 ± 5 fpm minimum.  The horizontal planes at  22, 30, and 38 inches e x h i b i t e d less t o t a l v a r i a t i o n , but the r a t i o of highest to lowest v e l o c i t y observed in any single plane s t i l l exceeded 10. The s i g n i f i c a n c e o f such a i r v e l o c i t y v a r i a t i o n s w i t h i n a plant growth area also depends somewhat on the type o f experiment undertaken. In view of the f a c t that the above measurements were taken in an empty growth chamber, the problem o f spacing pots on shelves to obtain uniform a i r c i r c u l a t i o n becomes even more complex.  One user of the Model PGC-78  reported t h a t c e r t a i n pots w i t h i n the chamber dried out considerably  53  60 ±30  90130  150 ±15  155 ±10  30±5  75 ±15  100H5  55*10  15*10  15±10  40 + 5  45±10  20±5  35+15  55±15  100±20  22 1S±J0  20+-10  30 ±10  40+10  15 ±5  40110  30±10  25±10  10±5  40±10  10±S  10±5  2015  40±5  25H0  10±5  20t10  45±15  35±10  10±5  55±10  65±15  40±15  15110  95±5  65+15  35±10  10+5  110±10  90120  65H5  30+15  100±10  80 ±15  80 ±30  55+10  100+10  80±15  100 ±10  10±5  40 ±10  75±15  50 ±15  10±5  15±5  30+10  40±10  10*5  70±20  90±15  50110  30 ±15  35±1S  65115  90110  140 ±.10  30  II  _ _  //  38  20 ±5  65±20  35+10  20 ±10  40115  65110  30110  30110  4S±5  65±10  15110  10±S  2015  70 ±10  50±10  30110  75±10  85115  30+10  60±10  2015  90115  75±1S  40110  50 ±5  100±5  60±10  50+10  15±5  115±15  125115  60+15  10 ±5  - 80 ±15  60 ±15  30+10  10t5  40115  40110  25110  3-5 F i g . 3-5 Horizontal A i r V e l o c i t y p r o f i l e s 1 4 " , 2 2 " , 3 0 " , and 38" below the Percival Model PGC-78 Light Canopy (measurements spaced 8" apart)  54 t.  quicker than others containing plants at the same stage of development (Gates, 1970).  The non-uniform a i r flow often causes the leaves o f  some plants to f l u t t e r , while other plants in the same chamber are completely  still.  Although the Percival Company advertises t h a t the Model PGC-78 provides uniform a i r d i s t r i b u t i o n w i t h i n the plant growth area, no d e f i n i t e s p e c i f i c a t i o n s are quoted.  The engineering of the components  of the PGC-78 a i r c i r c u l a t i o n system appears to have several d e f i c i e n c i e s . Propeller bladed fans are used to c i r c u l a t e the a i r from the top o f the growth area, down past the cooling and heating elements, and back i n t o the growth area (Figure 3 - 6 ) .  The fans are mounted so that the a i r  first  h i t s the end wall of the chamber before going down through the c o n d i t i o n i n g system.  The fans do not blow a i r d i r e c t l y at the c o i l s .  This s i t u a t i o n ,  plus the flow resistance o f the c o i l s , the a i r d i s t r i b u t i n g screens, pots and shelves, creates a back pressure t h a t could e a s i l y exceed the pressure r a t i n g o f a propeller-bladed f a n .  As evidence of t h i s , blow-back i n t o  the chamber is observed around the fans. The a i r d i f f u s i n g screen supplied by Percival is a perforated aluminum sheet with .120 inch holes on .188 inch c e n t e r s , which y i e l d s a porosity of 38%.  NRC engineers (1962), have found t h a t at the pressures  and v e l o c i t i e s encountered in growth chambers, a f i v e percent porosity is the maximum f o r achieving a manifold pressure s u f f i c i e n t f o r achieving uniform a i r f l o w .  Figure 3-5 indicates t h a t the d i s t r i b u t i o n screen must  not have much influence on the a i r stream since the flow patterns are quite non-uniform with large (up to ± 50% of average) f l u c t u a t i o n s .  It  <7~  *i i lE 11  II  "i i  J  F i g . 3-6  v  V  General design of the P e r c i v a l Model PGC-78 growth chamber.  56  Another problem i s concerned with the fan motors, which are of the universal series wound type.  Decomposition o f the i n s u l a t i o n on the  wire in the motor is evidently caused by over-heating.. motor slows down and eventually stops.  Consequently, the  The above measurements were  complicated by t h i s d i f f i c u l t y in that some fans were revolving f a s t e r than o t h e r s .  Non-uniform fan speeds occur in a l l o f the Model PGC-78  growth chambers in the Plant Science Department.  (D) Humidity There is no provision f o r humidity control on the Percival Model PGC-78.  Special equipment f o r humidity control is an optional accessory.  The Percival Company specifies t h a t the r e l a t i v e humidity in the growth area w i l l be in the range of 50 to 70% R.H., depending upon ambient temperature outside the growth chamber. Measurements of r e l a t i v e humidity were taken with a PCRC Model 11 copolymer styrene sensor.  One series of measurements was taken in the  conditioning duct below the cooling and heating elements j u s t before the a i r enters the plant growth area.  The average readings were a low of 40%  R.H. at 25°C f o l l o w i n g a heating cycle and a high o f 80%. R.H. at 21°C f o l l o w i n g a cooling c y c l e .  In the chamber, centered between the walls and  30" from the l i g h t system, the humidity varied from 50% R.H. at 24.5°C f o l l o w i n g a heating cycle and 70% R.H. at 22°C f o l l o w i n g a cooling c y c l e . Humidity^fluctuations o f t h i s magnitude may have some e f f e c t on plant growth, but such e f f e c t s may be minimized by r e l a t i v e l y short periods o f heating and cooling ( t y p i c a l l y less than 2 minutes).  57  (E)  Maintenance The maintenance o f growth chambers can be d i f f e r e n t f o r each model  and m a n u f a c t u r e r .  The l o a d on a chamber w i l l  determine what component  i n t h a t chamber may be the l e a s t d e p e n d a b l e , a f a c t the o p e r a t o r w i l l seldom know. Peroral  The e x p e r i e n c e o f the P l a n t S c i e n c e Department w i t h  Model PGC-78 growth chambers i l l u s t r a t e s the demanding o p e r a t i o n  o f these chambers.  The growth chambers are u s u a l l y o p e r a t e d  24 hours a day f o r what c o u l d be y e a r s a t a time o r u n t i l fails.  eight  All  continuously  the equipment  o f the a c t i v e components o f the growth chamber have a f i n i t e  l i f e t i m e , t h e l e n g t h o f which i s dependent on the q u a l i t y o f component and the harshness o f the o p e r a t i n g  the  conditions.  Maintenance o f the e i g h t P e r c i v a l Model PGC-78 chambers has p r i m a r i l y i n v o l v e d the replacement o f l i g h t b u l b s , t h e r m o s t a t s , a i r c i r c u l a t i o n and r e f r i g e r a t i o n c o m p r e s s o r s .  A schedule o f l i g h t bulb  replacement  ( d i s c u s s e d on page 2 ) would improve the c u r r e n t p r a c t i c e o f f l u o r e s c e n t and i n c a n d e s c e n t b u l b s as they burn o u t .  replacing  A predetermined  s c h e d u l e would p r o v i d e more u n i f o r m l i g h t c o n d i t i o n s as w e l l the c o s t o f l i g h t canopy m a i n t e n a n c e .  as  fixing  The t h e r m o s t a t s (Penn C o n t r o l s )  have been the second most f r e q u e n t l y r e p l a c e d components f o l l o w i n g l i g h t bulbs.  fans  the  F a i l u r e o f these h y d r o s t a t i c t h e r m o s t a t s has been n e a r l y  e x c l u s i v e l y due t o c o n t a c t s e i z u r e o f the m i c r o s w i t c h . .  The a d d i t i o n o f an  i n e x p e n s i v e component (a " c l i p p e r " d i o d e ) would m i n i m i z e s p a r k i n g caused by e n e r g i z i n g and d e - e n e r g i z i n g s o l e n o i d v a l v e s and s i g n i f i c a n t l y improve the l i f e t i m e o f the m i c r o s w i t c h c o n t a c t s . components v/ere a i r c i r c u l a t i o n f a n s .  The t h i r d most f r e q u e n t l y  replaced  Two d e s i g n problems appear t o have  58  affected the l i f e t i m e of the fan motors.  F i r s t , a i r flow past the  motor apparently i s not s u f f i c i e n t to prevent decomposition of the motor winding i n s u l a t i o n due to high temperature, leading to reduced motor speed.  Secondly, the l u b r i c a t i o n ports f o r the bronze bushings  of the fan motor are inaccessible unless the chamber is emptied and the end panels removed. i s the r e s u l t .  The motors are seldom l u b r i c a t e d and e a r l y  failure  59 Chapter IV  LABORATORY SYSTEM DESIGN  To provide uniform c o n t r o l l e d plant growth c o n d i t i o n s , an a r t i f i c i a l environment was developed which included the n u t r i e n t mist technique.  Two i d e n t i c a l chambers were constructed, each composed of  a lower and upper section with a i r c i r c u l a t i o n ductwork to the upper section (Figure 4 - 1 ) .  The range and q u a l i t y of environmental  control  eventually desired from t h i s system is o u t l i n e d in the Preface.  The  f o l l o w i n g discussion summarizes the system design in i t s present stage of development (December, 1970).  (A) Wall Temperature Control System The two chambers and associated equipment were i n s t a l l e d in a laboratory t h a t i s not a i r conditioned and i s t h e r m o s t a t i c a l l y temperature controlled.  Room temperature can be as low as 20°C in the w i n t e r with  a few periods as high as 30°C in the summer.  This difference made i t  necessary to i n s u l a t e the walls o f the chambers completely from ambient fluctuations.  The chambers are of r e l a t i v e l y small volume (base area is  four square f e e t ) , making i t p r a c t i c a l to use a water jacket f o r wall temperature control purposes.  The water jacket around the lower or root  chamber was very e f f e c t i v e in c o n t r o l l i n g the a i r temperature in the lower chamber very close to t h a t of the water j a c k e t .  The a i r temperature  control i s achieved by both conduction and r a d i a t i v e energy exchange with the water j a c k e t .  Thus, the a i r temperature in the lower chamber can be  Light  »  Concpy  «  *  •  Light  Conopy  •  Complete  A i r Flow  System  Scale: 3/4" = 1 '  F1g. 4-1  Design of the a i r flow system in the laboratory a r t i f i c i a l environment  61  changed by warming or cooling the water t h a t is being c i r c u l a t e d through the water j a c k e t .  The water j a c k e t on the upper or shoot chamber  supplements the temperature c o n t r o l l i n g action o f the a i r  circulation  system, and also assists i n removing heat that may be absorbed by the chamber walls from the l i g h t system. Most of the upper and lower sections o f the chambers were made o f P l e x i g l a s s , which was chosen because of i t s ease o f c o n s t r u c t i o n , modi f i c a t i o n , low upkeep, low cost and corrosion resistance.  The main  d i f f i c u l t y in working with Plexiglass is i t s f l e x i b i l i t y and low resistance t o stresses and s t r a i n s .  The water jackets had to be heavily reinforced  to prevent bulging and subsequent breaking.  Watertight seams were made  by using s i l i c o n rubber glue supplemented with 4-40 machine screws placed in tapped holes on two inch centers.  This procedure makes a  strong seam and s t i l l allows the pieces to be disassembled f o r f u t u r e modification.  A more permanent method is to use a commercial glue  (Cadco SC-94), which makes a Plexiglass to Plexiglass bond.  Bonding i s  quicker and less expensive, but is not as strong or r e l i a b l e under stress as the machine screw method.  The f i r s t method was used on the chamber  water jackets and the commercial glue was used on the ducting and manifold of the a i r c o n d i t i o n i n g system. The lower ( r o o t ) chamber was made with the outside dimensions o f 25" x 25" x 36" (Figure 4 - 2 ) .  A four square foot chamber base ( i n t e r i o r )  was thought to allow s u f f i c i e n t space f o r four plants to be grown s i m u l taneously without i n t e r f e r e n c e . length o f a root system.  T h i r t y - s i x inches was allowed f o r the  This size w i l l accommodate Phaseolus v u l g a r i s  (bush bean) approximately 8 to 10 weeks o l d .  One quarter inch t h i c k  4-2  Perspective view oif Lower Chamber ar>d Bas Scale: 1"= '1'  63  Plexiglass was used f o r the chamber w a l l s , with two sheets being spaced one inch apart to make the water j a c k e t . the rear wall were water j a c k e t e d .  Only the two side walls and  No jacket was made f o r the f r o n t  panel so there could be access i n t o the chamber.  The f r o n t panel and  top and bottom pieces o f the lower chamber were made from one-half inch thick Plexiglass. The upper (shoot) chamber was made with the outside dimensions of 25" x 25" x 2 5 " .  Again t h i s height allowance accepts Phaseolus v u l g a r i s  of approximately 8 to 10 weeks o f age.  For access i n t o the upper chamber,  the f r o n t panel was provided with 1/2" t h i c k Plexiglass door insulated w i t h 2" of styrofoam. The plumbing f o r the upper and lower chamber was made from one inch and one and one quarter inch p o l y v i n y l c h l o r i d e t u b i n g .  Large  diameter tubing was used on the exhaust to prevent pressure from b u i l d i n g up w i t h i n the water j a c k e t s .  Temperature c o n t r o l l e d water was pumped  i n t o the base o f the water jacket and an overflow drain was provided close to the t o p .  (B) Temperature Control System For t h i s p r o j e c t , a single source of temperature c o n t r o l l e d water was provided f o r c i r c u l a t i o n through the chamber water jackets (Figure 4 - 3 ) . A large capacity (approximately 80 l i t e r s ) s t a i n l e s s steel water bath formed the main r e s e r v o i r .  A Blue-M r e f r i g e r a t e d cooling c o i l immersed  in the r e s e r v o i r , provided the heating and cooling capacity (1000 BTU/hr). An e l e c t r i c s t i r r e r was added to mix and c i r c u l a t e the water around the cooling c o i l .  A magnetically coupled, p l a s t i c bodied pump drew water from  64 i.  F1g. 4-3  Wall  Temperature  Control  System  65  the r e s e r v o i r and c i r c u l a t e d i t through the water j a c k e t s .  A solid-state  temperature control device was used to control the r e f r i g e r a t i o n system. The water in the r e s e r v o i r was c o n t r o l l e d at the temperature desired f o r the water j a c k e t s .  (C) A i r Conditioning System A good cooling f a c i l i t y was required to achieve the intended control o f a i r temperature o f at l e a s t ± 1/2°C.  A face and by-pass damper-  c o n t r o l l e d cooling c o i l , a model o f units made f o r i n d u s t r i a l  applications  by Recold o f Canada, was u t i l i z e d f o r the control of a i r temperature. The operating p r i n c i p l e of t h i s cooling c o i l i s such t h a t when ambient a i r temperature is higher than the desired chamber temperature, a l l o f the a i r passing through the cooling u n i t goes across w a t e r - c h i l l e d f i n s . As the chamber temperature reaches the desired control p o i n t , a modulating motor opens a damper to allow uncooled a i r t o mix with t h a t which has been cooled, thus providing a proportional control of the a i r temperature (Figure 4 - 4 ) .  The temperature sensing element was located inside the  chamber manifold.  I t was connected t o a small t r a n s i s t o r a m p l i f i e r t h a t  operated the modulating motor o f the damper assembly. Since the desired a i r v e l o c i t y inside the chamber was of the order o f seventy f e e t per minute, the t o t a l volume o f a i r passing through the cooling system was correspondingly s m a l l .  The low a i r flow requirement  made f e a s i b l e the use of a four hundred cubic f e e t per minute c e n t r i f u g a l fan.  This type o f fan was required since a p o s i t i v e pressure was needed  to ensure smooth a i r flow and uniform mixing in the system, and to exceed the flow resistances of the cooling c o i l , the ductwork, and the plant  .  /  Ducts t o A i r F l o w Manifolds  A & B-  Air  Input  F i g . 4-4 Diagram of the a i r temperature c o n d i t i o n i n g system.  67 p l a t f o r m a i r d i f f u s i n g system.  The fan was driven by a shaded pole  motor, the speed o f which could be c o n t r o l l e d by a l t e r i n g the v o l t a g e . Good control o f the volume of a i r c i r c u l a t e d could be achieved in t h i s manner.  Ductwork was provided to d i s t r i b u t e the cooled a i r from the  cooling c o i l to the chamber manifold.  The system was designed so t h a t  the ducting f o r one chamber could be removed while the other remained in o p e r a t i o n .  The surfaces o f the ductwork were insulated with 1/2"  t h i c k styrofoam to reduce heat exchange.  (D) Plant Platform A i r D i f f u s i n g Manifold A i r flow is one of the more d i f f i c u l t variables to control w i t h i n a small enclosure.  As discussed e a r l i e r , standard commercially available  growth cabinets are l i m i t e d in providing good a i r flow properties by t h e i r basic design.  In p a r t i c u l a r , the a i r cooling f a c i l i t y is not able to  remove heat from the l i g h t canopy and s t i l l maintain stable temperature and humidity conditions in the chamber. The use of a v e r t i c a l a i r flow about the plants has several advantages with respect to the engineering of growth cabinets.  Most important is the  f a c t that a i r passes d i r e c t l y through the plant growth area and i s not c i r c u l a t e d in the single or double c e l l fashion used with wall mounted fan's ( P e r c i v a l , Sherer-Gi 1 l e t t , Controlled-Envi ronments).  Also, the  conditioned a i r passes over the plants and is e s s e n t i a l l y out of the plant growth area before the major heat source, the l i g h t canopy, i s encountered.  A t h i r d advantage, which applies to a l l open systems, is t h a t  the chance of carbon dioxide depletion in the growth area is reduced by the continuous addition o f f r e s h , f i l t e r e d , laboratory a i r .  The open  68  system i s best used when small volumes of a i r are required f o r c i r c u l a t i o n and the heat loads are l a r g e .  Recirculating techniques are best suited  f o r use in l a r g e r systems where a t o t a l loss c o n d i t i o n i n g system would be uneconomical. The size o f the chamber i s generally l i m i t e d by the length of the fluorescent tubes used.  The open system allows the use o f longer f l u o r -  escents by mounting the l i g h t canopy above the chamber wall s t r u c t u r e (Figure 4 - 1 ) . The method chosen to d i f f u s e the a i r from the plant platform came from a design by Kalbfleisch (1962).  He t r i e s several approaches to the  problem of producing uniform a i r flow through a f l a t p l a t e .  He concluded  that i f f i v e percent of the surface area o f the plate were e q u i d i s t a n t l y spaced holes, uniform flow would r e s u l t .  The c o n f i g u r a t i o n involves  holes d r i l l e d at one inch centers over the e n t i r e surface.  1/4"  For use in  plant growth chambers, corrugated sheet metal 1 1/4" on a side was used. This permitted the holes to be d r i l l e d in the v e r t i c a l part of the corrugation which made i t very d i f f i c u l t f o r foreign material to drop down i n t o the manifold.  This feature also made possible less interference to  a i r entering the chamber from obstructions near the surface of the plant platform.  For long periods of operation t h i s removed the necessity o f  cleaning the base of the growth chamber; t h i s , in t u r n , minimized d i s t u r b i n g the plants (Figure 4 - 5 ) . The requirement of a manifold below the a i r d i f f u s i n g assembly t h a t was no less than two inches t h i c k required that a plant be at least two weeks old or t a l l enough so t h a t the primary leaves were not disturbed by the manifold.  Top  View  F i g . 4-5 Top view and perspective of the a i r d i s t r i b u t i o n assembly.  Perspective  70 (E) N u t r i e n t Mist System The n u t r i e n t mist growth technique involves the suspension of the root system in a closed, dark, temperature-control led" chamber.  For the  purposes o f t h i s p r o j e c t , the crown of the p l a n t was placed in an expanded foam cork which was f i t t e d i n t o a hole in a plate at the top of the lower chamber.  The foam cork served as a b a r r i e r to water and heat t r a n s f e r .  Two pneumatic spray nozzles were placed at the bottom of the root chamber to provide the mist to saturate the chamber.  The n u t r i e n t m i s t ,  when s a t u r a t i n g the root chamber, c o l l e c t s r e a d i l y on the root systems (Figure 4 - 6 ) .  I t i s important t h a t the spray does not d i r e c t l y h i t the  root system at high v e l o c i t y , as several disturbances in root development can r e s u l t .  There i s much experimenting to be done y e t to f i n d the best  way to o r i e n t the spray nozzles in respect to the r o o t s . (1) N u t r i e n t Solution The n u t r i e n t s o l u t i o n selected f o r use in the n u t r i e n t mist was a modified Hoagland's No. 2 ( H e w i t t , 1965), c o n s i s t i n g o f : 50 ml.  1 M Ca(N0 )  50 m l .  2 M KN0  20 ml.  1 M MgS0  10 ml.  1 M KH P0  3  2  3  4  2  4  10 ml.  Fe EDTA*  10 ml.  Micronutrients**  * Each m l . o f the stock s o l u t i o n o f Fe EDTA contains 5 mg. o f Fe * * The micronutrient stock s o l u t i o n contains 2.86 gm. of H-^BO^ (Boric a c i d ) , 1.81 gm. of MnCl - 4H 0 (manganese c h l o r i d e ) , 0.11 qm. of ZnCl 2  2  c h l o r i d e ) , 0.05 gm. of CuCl  2  2  - 2H 0 (Sodium molybdate) per l i t e r . 2  (zinc  71  NUTRIENT SUPPLY  20 LITERS  SOLENOID VALVES  115 VAC  TEMPERATURE CONTROLLED WATER  Fig. 4-6  BATH  N u t r i e n t m i s t spray system.  72 The designated amount o f each stock s o l u t i o n was added to d i s t i l l e d water and made up t o 2000 ml. A complete n u t r i e n t supplement i s necessary when.using a mist technique f o r plant growth.  The modified Hoagland's No. 2 i s easy to  work with and provided s u f f i c i e n t n u t r i e n t s f o r both tomato and bean plants used in preliminary t e s t s .  The phosphate concentration was kept  purposely low to remove the p o s s i b i l i t y of a calcium phosphate p r e c i p i t a t e . This p r e c i p i t a t e can be avoided by adjusting pH, the value o f which is dependent upon the r e l a t i v e concentrations of calcium and phosphate in the n u t r i e n t s o l u t i o n . (2) Nutrient Mist Control System The n u t r i e n t mist applied to the roots o f a p l a n t must be c o n t r o l l e d at the same temperature as the root chamber water j a c k e t .  This was accom-  plished by passing the n u t r i e n t s o l u t i o n through a c o i l e d copper tubing immersed in the water reservoir (Figure 4 - 6 ) .  The copper tube containing  the n u t r i e n t s o l u t i o n was placed inside the over-flow drain from the root chamber water j a c k e t so t h a t the temperature o f the n u t r i e n t s o l u t i o n was not affected by the ambient temperature.  Subsequent temperature measurements  showed no change in root chamber temperature when n u t r i e n t spray was a p p l i e d . The amount o f n u t r i e n t s o l u t i o n necessary to maintain s a t i s f a c t o r y growth i s , at the present t i m e , hard to s p e c i f y .  The best method determined  so f a r i s to observe the root system and adjust the amount o f s o l u t i o n applied u n t i l the root system appears thoroughly covered with water droplets or a water f i l m .  More experience with the system should produce a b e t t e r  technique. The capacity of the pneumatic spray nozzle was four or f i v e per hour.  liters  Compressed a i r from the general laboratory supply provided the  73  pressure f o r the spray.  I n t e r m i t t e n t spraying was required to conserve  the n u t r i e n t s o l u t i o n , and excess n u t r i e n t s o l u t i o n was drained o f f the bottom of the chamber.  An inexpensive time delay mechanism was constructed  from a synchronous e l e c t r i c motor, a one-eighth inch t h i c k , 10 inches in diameter Plexiglass d i s k , and a microswitch.  The disk was rotated by  the synchronous motor, and tabs placed on the perimeter of the disk operated the microswitch at predetermined i n t e r v a l s .  The microswitch  operated two solenoid valves which c o n t r o l l e d the flow of n u t r i e n t s o l u t i o n and compressed a i r (Figure 4 - 6 ) .  (F) A r t i f i c i a l  L i g h t i n g System  The primary requirement of the l i g h t system f o r t h i s p r o j e c t was to provide an i n t e n s i t y of approximately 2000 to 2500 F.C. (20,000 to 25,000 lux) at a distance of 24 inches from the l i g h t source. by Kalbfleisch ( A r t i f i c i a l  From tests reported  Light f o r Plant Growth, 1963), i t was determined  t h a t twenty Cool-White 48T12R VH0 fluorescent l i g h t bulbs and f i f t e e n 40 watt incandescent bulbs would produce the desired i n t e n s i t y and q u a l i t y . Kalbfleisch also indicated t h a t side curtains would g r e a t l y a s s i s t in producing a h o r i z o n t a l l y uniform l i g h t i n t e n s i t y below the l i g h t canopy. Eighteen inch long side curtains o f sheet aluminum f i n i s h e d in a baked white epoxy were mounted on both o f the l i g h t canopies constructed. Figure 4-7 shows the spacing o f the fluorescent and incandescent l i g h t s in the canopies. (1) Photoperiod Control Three single p o l e , single throw time clocks were used to phase the l i g h t canopy on and o f f .  The incandescents were switched on f i r s t in the  morning, in an attempt to simulate the greater proportion of red wavelengths  74  F1g. 4-7  Laboratory l i g h t canopy consisting of 20 48T12 VHO cool-white fluorescent lamps spaced 1 5/8" on centers with 15 40 W incandescent lamps spaced 10" apart in three rows.  75  in a natural s u n r i s e .  Approximately one-half hour l a t e r , one c i r c u i t  of the fluorescents was switched on, and another one-half hour l a t e r , the second c i r c u i t o f fluorescents was switched on.  At the end o f the  plants "day", the fluorescents were phased o u t , and the incandescents switched o f f l a s t , in an attempt to simulate a natural sunset.  The  actual photoperiod duration is determined by the length o f time the incandescents are on.  High ampere rated switches must be used s i n c e ,  f o r example, 20 one hundred and f i f t y watt fluorescent lamps w i l l draw t h i r t y amperes when in operation.  76 Chapter V LABORATORY ARTIFICIAL ENVIRONMENT PERFORMANCE  The q u a l i t y o f environment t h a t was set as a goal f o r the a r t i f i c i a l environment developed in t h i s p r o j e c t was o u t l i n e d in the preface.  The  measurements presented in t h i s chapter are an attempt at determining whether the system constructed approaches the intended environmental quality.  (A) Light System A special twenty-two inch long motorized sensor apparatus s i m i l a r to t h a t described in Chapter I I I , was constructed f o r the measurement o f l i g h t i n t e n s i t y p r o f i l e s in the two foot square plant growth area.  After  several tests were made with s i l i c o n phototransistors and s i l i c o n photov o l t a i c c e l l s , a selenium photovoltaic c e l l was selected as a l i g h t sensor since i t offered greater l i n e a r i t y at l i g h t i n t e n s i t i e s above 2000 foot candles.  The s i l i c o n photovoltaic c e l l became very non-linear  above 2000 f o o t candles.  The s i l i c o n p h o t o t r a n s i s t o r could not cover the  required range without extra c i r c u i t r y , and i t showed greater s e n s i t i v i t y to temperature change than the selenium c e l l . The fluorescent and incandescent lamps in the canopy had been operated approximately 200 hours before the measurements were taken. The l i g h t system was turned on and operated f o r two hours before the f i r s t measurement was recorded.  This was done so t h a t the l i g h t output  had s t a b i l i z e d as much as possible.  Before the l i g h t output remains  s t a b l e , the lamps have to warm to operating temperature and evidently have to heat the surrounding equipment.  From preliminary measurements, i t was  77  i  also noticed t h a t the ambient temperature could noticeably a f f e c t the l i g h t output o f the lamp canopy.  Line voltage f l u c t u a t i o n s may also  account f o r some v a r i a t i o n in l i g h t output.  During a preliminary set  o f measurements, one h a l f of the chamber was measured in the morning ( a t 10:00 a.m.) and the second h a l f measured in the afternoon (3:00 p . m . ) . In t h i s case, the l i g h t i n t e n s i t y in the afternoon was found to be 20% less at the same point i n the chamber than in the morning measurements. To minimize t h i s problem, the data presented here were measured in as short a time as possible (3 hours) w i t h checks frequently made to positions measured e a r l i e r to make sure t h a t the l i g h t i n t e n s i t y gradients being measured were not an a r t i f a c t caused by changing c o n d i t i o n s . An equally d i f f i c u l t problem in measuring l i g h t i n t e n s i t y was the temperature s e n s i t i v i t y o f the l i g h t sensor.  This problem was circum-  vented by keeping a second l i g h t sensor, closely c a l i b r a t e d to the one in use, a v a i l a b l e f o r quick spot checks of the l i g h t i n t e n s i t y at a given location.  The quick spot checks did not allow time f o r the second sensor  to a t t a i n the temperature of the f i r s t and also served as an i n d i c a t i o n o f sensor " f a t i g u e " , the phenomenon of sensor output dropping with time in constant l i g h t i n t e n s i t y .  The selenium photocell did not require temper-  ature compensation nor did i t show any " f a t i g u e " . The r e s u l t s of the l i g h t i n t e n s i t y measurements are presented in Figures 5-la  j .  The ten graphs represent ten horizontal planes s t a r t i n g  at two inches above the plant platform manifold and continuing up in two inch i n t e r v a l s to w i t h i n two inches o f the top of the chamber w a l l s .  Each  horizontal plane consists o f 100 measurements (10 x 10) spaced two inches apart.  The measurements a t the edges were spaced two inches from the  Figure 5-1 ( a - f ) Horizontal l i g h t i n t e n s i t y planes from 32" to 22" below the l i g h t canopy. (54 lux i n t e r v a l s ) .  Rear  CHAMBER Left  ORIENTATION  Right  Front Panel ure 5-1 ( g - j ) Horizontal l i g h t i n t e n s i t y planes from 20" to 14" below the l i g h t canopy. (54 lux i n t e r v a l s ) .  80  chamber w a l l s .  The t o t a l o f 1000 measurements were taken in less than  a three hour period by means of the motorized sensor apparatus.  The  p o s i t i o n of the lamp canopy r e l a t i v e to the plant growth area was as indicated in Figure 5-2.  The lamps were a distance o f 38" from the  surface of the p l a n t p l a t f o r m .  The lowest horizontal plane (Figure 5 - l a )  was 32 inches and the highest plane (Figure 5 - l j ) was 14 inches from the lamps.  There was a 54 lux (50 f t - c ) change in l i g h t i n t e n s i t y between  each l i n e on the graphs in Figure 5-la  j .  Figure 5-la was the lowest in l i g h t i n t e n s i t y with each plane above i t increasing in i n t e n s i t y .  This type of v e r t i c a l i n t e n s i t y gradient  was expected and is a consequence of the inverse radius squared law of r a d i a t i o n .  Also, there was a gradual increase in the  d i f f e r e n c e in i n t e n s i t y between adjacent horizontal planes.  For example,  in the center of the chamber, the difference between Figures 5-la and 5-1b was 100 f o o t candles and between Figures 5 - l i and 5 - l j , 250 f o o t candles. The v a r i a t i o n of l i g h t i n t e n s i t y w i t h i n each of the horizontal planes i l l u s t r a t e s the importance o f symmetrical design.  Figure 5-2 shows t h a t  the chamber was overlapped by the l i g h t canopy on a l l s i d e s , but t h a t was not squarely centered under the l i g h t canopy.  it  As the upper horizontal  planes i n d i c a t e , the region of highest l i g h t i n t e n s i t y s h i f t s forward and to the r i g h t of the center of the chamber, so t h a t a p l a n t oriented in t h a t p o s i t i o n could receive a s i g n i f i c a n t l y higher average l i g h t than other plants in the chamber.  intensity  In t h i s case, the chamber was not  squarely centered under the l i g h t canopy due to a lack of surrounding laboratory space, a condition which w i l l be r e c t i f i e d in the f u t u r e .  81  LABORATORY  UPPER CHAMBER  Fig. 5-2  WALL  LIGHT CANOPY  Lamp canopy and chamber l o c a t i o n r e l a t i v e to laboratory walls (scale 3/32" = 1 ' ) .  82  Another f a c t o r not considered important at f i r s t was the presence of a dark walnut stained plywood box close to the l e f t side o f the chamber. lamps.  This box covered the b a l l a s t s required by the fluorescent The dark surface o f the box could possibly have influenced the  l i g h t i n t e n s i t y w i t h i n the chamber since the walls of the chamber were clear Plexiglass.  Part o f the s i g n i f i c a n t increase in non-uniformity of  l i g h t i n t e n s i t y in a single plane from lower to higher levels in the chamber may be due to t h i s i n f l u e n c e .  In Figure 5-1 a , the horizontal  v a r i a t i o n is about 100 f o o t candles and in Figure 5 - 1 j , about 250 f o o t candles. The above measurements of l i g h t i n t e n s i t y w i t h i n the growth chamber do not come close to meeting the i n i t i a l v a r i a t i o n on any s i n g l e horizontal plane.  p r e r e q u i s i t e o f ± 50 f o o t candles As a r e s u l t , a new design w i l l  have to be developed to improve the l i g h t i n t e n s i t y u n i f o r m i t y in the chamber.  (B.) Temperature The u n i f o r m i t y o f temperature w i t h i n the plant growth area was w i t h i n the desired maximum f l u c t u a t i o n of less than ± 1/2°C.  Ten thermocouples  (copper-constantan, 28 gauge) were mounted at f i x e d positions every two inches from the plant p l a t f o r m to the top o f the chamber.  The f r o n t  panel was i n s t a l l e d and an average a i r v e l o c i t y of 80 fpm was passed through the chamber.  The t o t a l v e r t i c a l temperature gradient was 1°C,  with the 10 thermocouples placed in any l o c a t i o n in the chamber.  This  u n i f o r m i t y of temperature was due t o the u n i f o r m i t y of a i r c i r c u l a t i o n .  83  The t o t a l f l u c t u a t i o n of temperature at any given point was less than ± 1/4°C about a control p o i n t o f 20°C, which was approximately 5°C below ambient laboratory temperature.  A s i g n i f i c a n t d i f f e r e n c e between t h i s  system and commercial chambers i s the complete lack of heating and cooling cycles and the accompanying temperature changes. The temperature w i t h i n the root chamber was extremely stable and uniform with less than ± 1/8°C temperature change detected over a 24 hour period.  This i s a r e s u l t o f the chamber being e n t i r e l y enclosed (no  changing heat loads) and having three water jacketed w a l l s .  (C) A i r C i r c u l a t i o n Obtaining uniform a i r flow v e l o c i t y across a four square foot plant growth area was not as easy as o r i g i n a l l y thought.  The plant platform  manifold described on page 68 was g r e a t l y modified as a r e s u l t of the measurements described in t h i s chapter. were as described in Figure 5-3.  The i n i t i a l  a i r flow patterns  Two main problems were evident.  The  f i r s t was t h a t a i r entering the plant p l a t f o r m manifold continued through the d i f f u s i n g system i n t o the chamber c r e a t i n g a v e l o c i t y p r o f i l e as in Figure 5-3.  The r e s u l t was a tremendous v e l o c i t y gradient across the  chamber, from 10 fpm on the entrance side t o 200 fpm at the end o f the plenum.  A f t e r considerable experimentation, a series o f 150 b a f f l e  s t r i p s positioned by hand produced an a i r flow in the plant growth area of s a t i s f a c t o r y u n i f o r m i t y .  Measurements at f o u r , s i x , e i g h t , twelve,  s i x t e e n , and twenty inches from the p l a t f o r m surface indicated an average a i r v e l o c i t y o f 75 fpm ± 5 fpm on the undamped mode o f the Flow Corporation  84  F i g . 5-'3.  Original nonuniform a i r flow pattern in upper chamber of laboratory system (scale 1/8" = 1")  85  Model 55B hot-wire anemometer.  This was accomplished only under  experimental conditions and would not have permitted the growth o f plants in the chamber due to the obstructions in the manifold.  The  second problem with the basic design was the asymmetrical duct from the p o s t - c o n d i t i o n i n g plenum to the plant platform manifold (Figure 5 - 4 ) . Special b a f f l e s were required to balance the a i r flow across the duct as i t entered the platform manifold.  The asymmetrical duct was required  since the cooling c o i l was narrower than the chamber. As a r e s u l t o f the above d i f f i c u l t y in achieving uniform a i r f l o w , a new design w i l l have to be developed before plants are used in the system.  (D) Humidity Since the a i r used by the conditioning system i s drawn from the l a b o r a t o r y , and since these tests were made at below ambient temperatures, the humidity in the system ran c o n s i s t e n t l y higher than the humidity in the l a b o r a t o r y .  The laboratory humidity was usually between 40 and 45%  r e l a t i v e humidity with the windows in the lab closed.  A chamber humidity  of 55% was recorded with laboratory temperature at 25°C and the upper chamber's temperature at 20°C with 15°C water flowing through the cooling coil.  Like commercial growth chambers, the average level of humidity in  the upper chamber in i t s present stage o f development i s completely dependent upon ambient c o n d i t i o n s .  However, the absence o f heating and cooling cycles  in the laboratory chamber prevents the c y c l i n g of humidity t h a t occurs in commercial chambers.  86  TOP VIEW C00UN6  0  UPPER  F  CHAMBER  DUCT  F i g . 5-'4  Asymmetrical duct from cooling c o i l t o upper chamber of lab system.  87  (E) Summary of Laboratory System Performance The temperature control system o f the laboratory chambers provides more stable and uniform temperatures than the Percival PGC-78, and more than s a t i s f i e s the q u a l i t y o f temperature control o u t l i n e d in the Preface. The technique of modulating c o n t r o l , rather than On-Off control eliminates temperature and humidity f l u c t u a t i o n s due t o heating and cooling cycles. The l i g h t i n t e n s i t y measurements indicate t h a t a higher l i g h t i n t e n s i t y was a v a i l a b l e in the laboratory system than in the PGC-78, but t h a t both horizontal and v e r t i c a l gradients s t i l l e x i s t .  Further improve-  ments could r e s u l t from the use o f r e f l e c t i v e wall coatings and louvers at the top of the chamber to generate more uniform l i g h t d i s t r i b u t i o n . The achievement of a uniform 1000 lux i n t e n s i t y in a growth chamber s t i l l appears to be a very d i f f i c u l t engineering problem. A f t e r considerable experimentation, s a t i s f a c t o r y a i r flow conditions were achieved in the laboratory system.  The f i n a l design provided a  u n i f o r m i t y of a i r flow which was b e t t e r than the ±10 fpm v a r i a t i o n originally specified.  However, the use o f many b a f f l e s to achieve t h i s  u n i f o r m i t y is an adequate arrangement f o r one flow rate o n l y , and a more adjustable arrangement would be d e s i r a b l e . The control o f humidity was not attempted in the laboratory system mainly because o f expense and because o f inexperience with humidity control mechanisms.  E f f e c t i v e humidity contol requires s a t i s f a c t o r y temperature  control and t h i s was achieved in the present system.  A saturate and  reheat a i r c o n d i t i o n i n g system could be adapted to the e x i s t i n g design and would probably provide e x c e l l e n t humidity c o n t r o l .  88  SUMMARY This research has led to the construction o f prototype growth chambers which incorporate the n u t r i e n t mist technique and which provide improved environmental control in comparison with a commercially available system. The development o f an a r t i f i c i a l environment f o r c r i t i c a l of the physiology of plant growth is a complex problem. artificial  studies  The design of an  l i g h t source f o r a plant growth chamber requires consideration  o f the c h a r a c t e r i s t i c s o f the lamps used, the r e f l e c t i v e n e s s of the chamber i n t e r i o r , and the influence o f the lamp canopy and chamber geometry on the horizontal and v e r t i c a l l i g h t i n t e n s i t y p r o f i l e s .  In a small chamber, the  w a l l s , whether r e f l e c t i v e or n o t , are the main cause of horizontal and v e r t i c a l l i g h t i n t e n s i t y gradients ( i n the absence o f p l a n t s ) . Temperature control systems f o r plant growth chambers must be engineered to provide precise control over a much wider range of conditions than are generally encountered i n conventional a i r c o n d i t i o n i n g .  Solid state  temperature controls provide the required c a l i b r a t i o n , s e n s i t i v i t y , set point adjustment and programming c a p a b i l i t i e s f o r a r t i f i c i a l plant growth envi ronments. Variations in r e l a t i v e humidity are inherent in a r t i f i c i a l  plant  growth environments using evaporative cooling r e f r i g e r a t i o n systems.  The  hot-gas bypass technique assists in reducing humidity . f l u c t u a t i o n s . Uniform a i r c i r y q l a t i o n in an a r t i f i c i a l environment i s dependent on the c h a r a c t e r i s t i c s o f the a i r , the design o f ducts and shelves, and the spacing and development of the plants being grown.  A special manifold can  be constructed to provide an a i r f l o w at 80 ± 10 fpm across the plant growth area.  89  Instruments used f o r the measurement and i n d i c a t i o n of environmental variables must be s t a b l e , rapid in response, and must not a f f e c t the environment being measured.  A t r a v e l i n g sensor was developed and was  found to be p a r t i c u l a r l y useful f o r the r a p i d , reproducible remote measurement o f the p r o f i l e s in plant growth chambers. The Percival Model PGC-78 growth chamber contained large gradients in environmental conditions and did not conform e n t i r e l y to advertised specifications.  The s p e c i f i c a t i o n s were considered to be u n s a t i s f a c t o r y  in extent and could be misleading to an operator. The laboratory growth chambers constructed during t h i s research produced more uniform environmental conditions than did the PGC-78, and s a t i s f i e d many o f the o r i g i n a l c r i t e r i a f o r a s a t i s f a c t o r y environment system.  artificial  The chambers were p a r t i c u l a r l y e f f e c t i v e with  respect to the u n i f o r m i t y of temperature and a i r f l o w .  In a small chamber,  water jacketted walls and a modulated a i r c o n d i t i o n i n g system that lacks heating and cooling cycles can be used to provide temperature control to w i t h i n ± 1/4°C o f a control p o i n t .  Nevertheless, a d d i t i o n a l  improvements  in the control and u n i f o r m i t y of l i g h t i n t e n s i t y , and in the regulation o f humidity, are required to improve the effectiveness o f the chambers in plant research.  90 LITERATURE CITED  1.  B u l l e y , N.R., Nelson, C D . and Tregunna, E.B. 1969. Photosynthesis: Action spectra f o r leaves in normal and low oxygen. Plant P h y s i o l . 45: 89-101.  2.  Cooper, J.P. and N.M. Tainton. 1968. Light and temperature requirements of t r o p i c a l and temperate grasses. Herb.Abstr. 38: 167-176.  3.  Controlled Environments L t d . 1969. Growth chamber design. B u l l e t i n No. 70. Winnipeg, Manitoba.  4.  Doebelin, E.O. 1966. Measurement systems: Application and desiqn. McGraw-Hill, New York. tfp. 502.  Technical  A  5.  Gates, J.Wayne.  1970.  Personal Communication.  6.  General E l e c t r i c .  1957.  Psychrometric Chart.  7.  General E l e c t r i c .  1960.  Fluorescent lamps.  8.  General E l e c t r i c .  1960.  Plant Growth L i g h t i n g .  1961.  Incandescent lamps.  Technical Manual No. T P - I l l . Technical Manual  No. TP-127. 9. 10. 11.  12.  13.  General E l e c t r i c .  Technical Manual No. TP-110.  General E l e c t r i c . 1963. Light measurement and c o n t r o l . Technical Manual No. TP-118. Hewitt, E.J. 1966. Sand and water c u l t u r e methods used in the study o f plant n u t r i t i o n . Technical Communication No. 22 (Revised 2nd e d i t i o n ) . Commonwealth Bureau of H o r t i c u l t u r e and Plantation Crops, East M a i l i n g , Maidstone, Kent. ^ p . 302. K a l b f l e i s h , W. 1963. Requirements of environmental f a c i l i t i e s f o r the growth of p l a n t s . Engineering Research Service, Research Branch, Can.Dept.Ag., Ottawa, Canada. K a l b f l e i s h , W. 1963. A r t i f i c i a l l i g h t f o r plant growth. Research Branch, Can.Dept.Ag., Ottawa, Canada.  14. ' M a r t i n , E.V. and F.E. Clements.  1936.  Engineering  Significance of t r a n s p i r a t i o n .  Plant P h y s i o l . 9: 165-172. 15.  Percival Manufacturing Co.  16.  Rohm and Haas Co.  17.  S a l i s b u r y , F.B. and C. Ross. 1969. Plant Physiology. Wadsworth Publishing Co. I n c . , Belmont, C a l i f o r n i a , pp. 513-518. T h r e l k e l d , J . L . 1970. Thermal environmental engineering. PrenticeHall I n c . , Englewood C l i f f s , N.J. 07632. pp. 205-256.  18.  1967.  1967.  Refrigeration System.  B u l l e t i n 2-B.  Plexiglass Desiqn Guide.  

Cite

Citation Scheme:

        

Citations by CSL (citeproc-js)

Usage Statistics

Share

Embed

Customize your widget with the following options, then copy and paste the code below into the HTML of your page to embed this item in your website.
                        
                            <div id="ubcOpenCollectionsWidgetDisplay">
                            <script id="ubcOpenCollectionsWidget"
                            src="{[{embed.src}]}"
                            data-item="{[{embed.item}]}"
                            data-collection="{[{embed.collection}]}"
                            data-metadata="{[{embed.showMetadata}]}"
                            data-width="{[{embed.width}]}"
                            async >
                            </script>
                            </div>
                        
                    
IIIF logo Our image viewer uses the IIIF 2.0 standard. To load this item in other compatible viewers, use this url:
http://iiif.library.ubc.ca/presentation/dsp.831.1-0101902/manifest

Comment

Related Items