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UBC Theses and Dissertations

Artificial environments for plant research Gibson, Jonathan Stephen 1972

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ARTIFICIAL ENVIRONMENTS FOR PLANT RESEARCH by JONATHAN STEPHEN GIBSON B . S c , Washington S ta te U n i v e r s i t y , Pu l lman, Washington, 1967 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in the Department o f P l an t Sc ience We accept t h i s t h e s i s as conforming to the requ i red standard THE UNIVERSITY OF BRITISH COLUMBIA January , 1972 In presenting t h i s thesis i n p a r t i a l fulfilment of the requirements for an advanced degree at the University of B r i t i s h Columbia, I agree that the Library s h a l l make i t freely available for reference and study. I further agree that permission for extensive copying of t h i s thesis for scholarly purposes may be granted by the Head of my Department or by his representatives. I t i s understood that copying or publication of t h i s thesis for f i n a n c i a l gain s h a l l not be allowed without my written permission. Department The University of B r i t i s h Columbia Vancouver 8, Canada i i ABSTRACT A review was made o f envi ronmental technology as a p p l i e d t o the eng ineer ing 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 phys io logy research . The r e s u l t s o f t h i s study were u t i l i z e d i n the development o f an a r t i f i c i a l environment which i n c o r p o r a t e d the n u t r i e n t m is t technique o f 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 p a r t l y dependent on the type o f c o n t r o l ins t ruments used. S o l i d s t a t e e l e c t r o n i c c o n t r o l devices o f f e r many advantages, p a r t i c u l a r l y w i t h respect to accuracy, respons iveness, r e l i a b i l i t y and remote c o n t r o l . A t r a v e l l i n g sensor was developed t o d e t e c t the environmental c o n d i t i o n s w i t h i n a r t i f i c i a l environments by remote c o n t r o l . This sensor g r e a t l y increased the r a p i d i t y and convenience o f measurement w i t h minimum d is tu rbance o f the env i ronment . 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 , tempera tu re , wind speed and humid i t y w i t h i n a commercial growth chamber, the Perc iva l Model PGC-78, were ana lysed. 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 nonuni form f o r a l l the v a r i a b l e s t e s t e d . The manu fac tu re r ' s s p e c i f i c a t i o n s f o r the chamber were considered to be l i m i t e d i n e x t e n t and to some degree m i s l e a d i n g . • The design o f the a r t i f i c i a l environment system cons t ruc ted f o r t h i s p r o j e c t i s desc r i bed . With t h i s system, temperature c o n t r o l o f ± h°C was achieved w i t h i n the p l a n t growth a r e a . In a d d i t i o n , the u n i f o r m i t y o f l i g h t i n t e n s i t y and a i r f l ow in the cons t ruc ted chambers was s u p e r i o r to the PGC-78. i i i TABLE OF CONTENTS PREFACE i x LITERATURE CITED Jo CHAPTER I . A r t i f i c i a l E n v i r o n m e n t D e s i g n INTRODUCTION 1 (A) L i g h t S y s t e m 2 ( B ) T e m p e r a t u r e C o n t r o l . 12 ( C ) H u m i d i t y C o n t r o l 20 (D) A i r C i r c u l a t i o n 30 ( E ) R e l i a b i l i t y a n d S e r v i c e 31 CHAPTER I I . I n s t r u m e n t s f o r C o n t r o l , I n d i c a t i o n , a n d M e a s u r e m e n t 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) C o n t r o l 33 ( B ) I n d i c a t i o n . 3 7 ( C ) M e a s u r e m e n t 39 • CHAPTER I I I . C o m m e r c i a l G r o w t h Chamber P e r f o r m a n c e INTRODUCTION 42 (A) L i g h t S y s t e m 42 (B) T e m p e r a t u r e C o n t r o l 49 ( C ) A i r C i r c u l a t i o n 51 . (D) H u m i d i t y 56 ( E ) M a i n t e n a n c e 57 CHAPTER I V . L a b o r a t o r y S y s t e m D i v i s i o n (A) W a l l T e m p e r a t u r e C o n t r o l S y s t e m 59 ( B ) T e m p e r a t u r e C o n t r o l S y s t e m 6 3 ( C ) A i r C o n d i t i o n i n g S y s t e m . 6 5 (D) P l a n t P l a t f o r m A i r D i f f u s i n g M a n i f o l d 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 Lighting System • 73 (1) Photoperiod Control 73 CHAPTER V. Laboratory A r t i f i c i a l Environment Performance (A) Light System 76 (B) Temperature 82 (C) A i r C i rcu lat ion 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 Spectral Energy D is t r ibu t ion of Direct Solar Radiation In tens i ty at Normal Incidence f o r the Upper Limit o f the Atmosphere and at the Earth's surface fo r Clear Days . . . . 3 1-2 Action Spectra of Phototropism, Phytochrome and Photosynthesis 4 1-3 Emission Spectrum of a Cool White Fluorescent Lamp 5 1-4 Mor ta l i t y of Fluorescent Lamps as a Percent of Rated Lamp L i fe 5 1-5 Typical Range of Fluorescent Lamp Depreciation with Time . . 7 1-6 Energy D is t r ibu t ion of a Typical Cool White Fluorescent Lamp 7 1-7 Energy D is t r ibu t ion of a Typical 40 Watt Incandescent Lamp . 8 1-8 Spectral Energy D is t r ibu t ion of a Typical Incandescent Lamp 8 1-9 Horizontal Light In tens i ty D is t r ibu t ion 24" Below an Uncurtained Light Canopy o f Fluorescent Lamps . . . . . . . 11 1-10 Horizontal Light In tens i ty D is t r ibu t ion 24" Below a Curtained Light Canopy of Fluorescent Lamps 11 1-11 Processes Occuring When Cooling A i r in a Condit ioning System 13 1-12 Temperature Variat ions About a Control Point Caused by D i f fe ren t Rates of Heating and Cooling 17 1-13 The re f r i ge ra t i on System of the Percival Model PGC-78 Growth Chamber U t i l i z i n g Two Evaporator Coils 18 1-14 A Typical Refr igerat ion System U t i l i z i n g the Hot-Gas Bypass Technique of Evaporator Coil Temperature Regulation . 18 1-15 - Processes Occuring When Air is Cooled and Condensation Results 20 1-16 Processes Occuring When A i r is Heated and Humidified . . . . 21 1-17 Processes Occuring When Two Ai 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 v i i Fi gures 1-19 Dual Damper-controlled Cooling Coils 28 1 - 20 The Ef fect of Varying Heat Load on Cooling Coil Performance . 29 2- 1 Apparatus fo r the Remote Posi t ioning of Environmental Sensors 40 3- 1 The Spacing and Arrangement of Fluorescent and Incandescent Lamps in the Percival Model PGC-78 44 3-2 The Horizontal Light In tens i ty D is t r ibu t ion in the Percival (a-h) Model PGC-78 Growth Chamber at Distances Between 12 and 40 inches Below the Light Canopy Barr ier 46 & 47 3-3 Horizontal A i r Temperature P r o f i l e 14" Below the Light Canopy Barr ier 50 3-4 Horizontal A i r Temperature Pro f i les Through Center of Chamber fo r 22" , 30", and 38" from Liqht Canopy Barr ier in the Percival Model PGC-78 50 3-5 Horizontal A i r Veloci ty Prof i les at 14" , 22" , 30" and 38" Below the Light Canopy Barr ier 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 . . 64 4-4 Diagram of the A i r Temperature Conditioning System . . . . . . 66 4-5 Top View and Perspective of the A i r D is t r ibu t ion Assembly . . 69 4-6 Nutr ient Mist Spray System • 71 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 . . 74 5- 1 Horizontal Light In tens i ty Pro f i les Above the A i r ( a - j ) * D is t r ibu t ion Manifold in the Laboratory System Upper Chamber 78 & 79 5-2 Lamp Canopy and Chamber Location Relative to Laboratory Walls 81 vi i i Figures 5-3 Or ig inal Non-uniform A i r Flow Pattern in Upper Chamber of Laboratory System 84 5-4 Asymmetrical Duct from Cooling Coil to Upper Chamber of Laboratory System 86 i x PREFACE This pro ject served as a stage in a continuing research programme aimed at developing an enclosed, accurately con t ro l l ed , a r t i f i c i a l environ-ment fo r plant growth. The ult imate goal is 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 cu la t i on v e l o c i t i e s , temperatures and humid i t ies , any combination of which is programmable by the operator. This work represents an attempt to reduce environmental condit ions as uncontrol led variables in experiments on the physiology of plant growth. To approach th is ob jec t i ve , 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 st ructure of plant growth chambers. In Chapter I I , some instruments f o r c o n t r o l l i n g , ind ica t ing and recording environmental variables in p lant growth chambers are described. A special technique f o r measuring the physical components of the in ternal environment o f growth chambers is also described. A detai led study of the performance of a commercially manufactured growth chamber (Percival Model PGC-78) was carr ied out and is reviewed in Chapter I I I . The Model PGC-78 chamber approached the performance levels speci f ied by the manufacturers. Nevertheless, several major def ic iencies in environmental qua l i t y were evident , and the chambers were considered to be unsat isfactory fo r c r i t i c a l research on many aspects of the physiology of plant growth. For th is reason, i t was decided to design and construct a new laboratory plant growth chamber. The design of the laboratory growth chamber is described in Chapter IV. This system was intended to provide improved uni formity and control o f environmental condi t ions. X In developing the laboratory system, i t was necessary at the outset to define the qua l i t y of environmental control which the system should possess. The requirements f o r environmental control and uni formity are largely dependent on the types o f future research contemplated. I t was expected that the system would mostly be applied to studies o f the ef fects of environmental condit ions on the growth and gas exchange of shoots and roots of small p lan ts . Growth chamber performance c r i t e r i a was therefore developed in the fo l lowing way from known information on the e f fec ts of environmental condit ions on the components of growth and gas exchange. Studies with many d i f f e r e n t p lant 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 tens i t y is 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 avai lable on the d i rec t e f fec ts of wind speed and humidity on growth. At low a i r ve loc i t ies ( i . e . between 10 and 80 fpm), a 10 fpm change in a i r ve loc i ty may cause as much as a 10% change in the rate of t ransp i ra t ion of sunflower (Martin and Clements, 1936). Also, psychrometric tables indicate t h a t , fo r example, at i n i t i a l l y 60% R.H., 26°C dry bulb, a 1% change in r e l a t i v e humidity could change the atmospheric water vapour pressure surrounding the lea f by 10%. While these examples omit the in teract ions between d i f f e r e n t environmental var iab les , 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 fects on growth. I t was therefore decided that sa t i s fac to ry environmental control would be provided by a chamber wi th the fo l lowing c a p a b i l i t i e s : temperature control to wi th in ± 1°C and a temperature range of 5°C to 35°C; l i g h t i n tens i t y range from 0 to 40,000 lux wi th less than 1000 lux nonuniformity; average a i r xi ve loc i ty var iable 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% re l a t i ve humidity, var iable from 20 to 95% re l a t i ve humidity. The system described above would permit the study of i n t a c t , whole plants where lea f chambers and excised t issue are presently u t i l i z e d . The addit ion of the nutr ient mist technique provides a system eas i l y adapted for nutr ient studies and for studies of the dif ferences in root and shoot resp i rat ion and development. Such a system would provide environ-mental control adequate for a wide range of studies on the physiology of plant growth. 1 Chapter I ARTIFICIAL ENVIRONMENT DESIGN INTRODUCTION The basic purpose of a plant growth chamber is to provide environ-mental condit ions s imi la r to natural condit ions which are reproducible, and which can be varied at w i l l . For t h i s , a l i g h t system of adequate in tens i t y and spectral q u a l i t y , as well as mechanisms fo r the control o f root and shoot temperature and humidity are required. The type of research contem-p la ted , the size and number of plants involved, and the l im i ta t i ons o f laboratory f a c i l i t i e s must be considered in the select ion of the physical components of a p lant growth chamber. The size o f cont ro l led 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 mul t is tory bui ldings to small desk top chambers and lea f chambers. In terest here w i l l be centered on what are usually referred 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 feet between the plant p lat form and the l i g h t canopy. The environmental condit ions which can be produced by a r t i f i c i a l systems are l im i ted and d i f f e r from natural condi t ions. This di f ference l ies in the complex in teract ions of natural environmental var iab les, 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 in teract ions of the components of an a r t i f i c i a l environ-ment must be known before desirable performance can be achieved. The fo l lowing discussion w i l l consider some factors involved in incorporat ing mechanical and e l e c t r i c a l devices in to 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 tens i t y and spectral qua l i t y on a horizontal plane, and as small an in tens i t y gradient v e r t i c a l l y as is possible. I t must have photoperiod c o n t r o l , and should not emit excessive heat or harmful rad ia t i on . The natural source of 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 fo r comparison are the spectra o f incandescent and f luorescent lamps (Figures 1-3, 1-8) and the action spectra of photo-synthesis, phototropism, and phytochrome (Figure 1-2). Usually, a combination of incandescent and f luorescent lamps are used as a source of l i g h t fo r plant growth. This combination most p rac t i ca l l y sa t i s f i es the l i g h t qua l i t y and in tens i t y requirements of plants as represented by the above action spectra. The fo l lowing 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 d i f ference from sunl ight and w i l l indicate some o f the problems of 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 f luorescent tube shows a peak emission at 580 nanometers wi th a secondary maximum at 475 nanometers. The output is very low at 700 nm (Figure 1-3) (General' E l e c t r i c , 1960). As wi th a l l f luorescent lamps, i n te rm i t ten t operation shortens lamp l i f e appreciably. Figure 1-4 shows percent o f burnouts of f luorescent lamps p lo t ted 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 is variable from lamp to lamp, and th is value may decrease rap id ly during the f i r s t one hundred hours o f operat ion. The lumen depreciation may amount to as much as ten percent in th is per iod. For commercial ra t ing purposes, the one hundred hour value is used as the i n i t i a l 3 WAVELENGTH - MICRONS Fig. 1-1 Spectral energy of d i rec t so lar radiat ion in tens i t y at normal incidence for the upper l i m i t of the atmosphere and at the earth ' s surface during c lear days. THRELKELD, 1970. \ 4 Fig. 1-2 Action spectra of phototropism (a), photochrome (b), and photosynthesis (c). ( (a), (b) Sal isbury & Ross, 1969; (c) Bui ley et a l , 1969 ). Coo l White WAVELENGTH - NANOMETERS F i g . 1-3 Emission spectrum of a cool white f luorescent lamp. (General E l e c t r i c , 1960) o 1 25-I/I t— O 2; 50 m z tu u or UJ o- 7 5 , 1004 — i -20 1 1 1— i 0 60 80 PER CENT RATED LAMP LIFE —i 120 100 F i g . 1-4 Mor ta l i t y of f luorescent lamps as a percent of rated lamp l i f e . (General E l e c t r i c , 1960) 6 value. Figure 1-5 shows typ ica l ranges of f luorescent lamp depreciat ion in l i g h t output with respect to time (General E l e c t r i c , 1960). A typ ica l Cool-White f luorescent 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 in f rared 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 iency of a Cool -White f luorescent lamp ranges from seventy-f ive to eighty lumens per wat t . 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 f luorescent lamps occurs when the coolest spot on the bulb surface is about f o r t y - f i v e degrees Centigrade. The bulb wall temperature is af fected 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 ven t i l a t i on condit ions (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 of approximately 460, and has a rated average l i f e of 1000 hours. For such a f o r t y watt incandescent bu lb , 7.4 percent of 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 iv ing a to ta l of 71.3 percent. A twenty percent loss of input energy is caused by convectional flow of the f i l l i n g gas in a stream past the f i lament (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 least e f f i c i e n t emi t ter of v i s i b le energy, but also emits the least amount of in f rared rad ia t i on . The spectral d i s t r i b u t i o n of a typ ica l incandescent bulb is given in Figure 1-8 (General E l e c t r i c , 1961). Reflector — I 1 — — — — I 1 I—————\ 2000 4000 6000 8000 10000 12000 BURNING TIME (hours) F ig . 1-5 Typical range of f luorescent lamp depreciation with . time. (General E l e c t r i c , 1960) INPUT ENERGY 100 % 60 V . EXCITING ULTRAVIOLET 60 V . AO V . I 38 V . I' 20 V . HEAT 78 V . 2 V . 36 V . 1 4 2 ' / . f I 1 LIGHT INFRARED DISSIPATED HEAT 22 V . 36 V . 42 V . Fig. 1-6 Energy d i s t r i bu t i on of a typ ica l cool-white f luorescent lamp. (General E l e c t r i c , 1960) 8 INPUT ENERGY 100 V . 7.4 V . FILAMENT RADIATION BEYOND BULB GAS LOSS 8.7 CONDUCTION 71.3 % 20 V . LOSS TOTAL HEAT LOSS 92.6 V . r UGH F ig . 1-7 Energy d i s t r i bu t i on of a typ ica 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 1 i 1— 300 400 500 600 WAVELENGTH - NANOMETERS 700 Fig. 1-8 Spectral energy d i s t r i bu t i on of a typ ica 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 sui table fo r plant growth chamber appl icat ions as non- re f lec tor bulbs. In natural condi t ions, the neg l ig ib le change of l i g h t i n tens i t y from the so i l surface to the top of a plant (assuming no absorption by the p lant) is a funct ion of the distance from the earth to the sun, assuming clear atmospheric condi t ions. 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 feet from p lan ts . 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 . Vert ical l i g h t i n tens i t y gradients can be par 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 in ishes (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 mater ia ls , a more uniform ver t i ca l l i g h t i n tens i t y pattern can be obtained in the plant growth area (Ka lb f le isch , 1963). I t is usually expensive to design a plant growth chamber with an extremely low ve r t i ca l l i g h t i n tens i t y gradient . Any objects in the chamber, inc luding p lan ts , shelves, pots , and instruments, d is turb the uni formity of l i g h t i n t e n s i t y . Special designs are avai lable i f unfform ver t i ca l l i g h t i n tens i t y is important (Control led Environments, 1970). The uni formity of l i g h t i n tens i t y on any horizontal plane between the plant p lat form and the l i g h t canopy is determined large ly by the pos' i t ion, dimensions and wattage of the lamps used. Ref lect ive walls may inf luence horizontal un i fo rmi ty , depending on how the chamber is constructed. Kalbf le ish (1963) has published extensive measurements on a var ie ty of lamp canopy arrangements using various lengths o f f luorescent lamps spaced from 1/8 inch to several inches apart . A typ ica l chart (Figure 1-9) indicates 10 that 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 tens i t y which decreased in l i g h t i n tens i t y towards the sides of the canopy. Figure 1-10 demonstrates the inf luence o f a 24 inch long highly r e f l e c t i v e cur ta in mounted around the perimeter of the same lamp canopy used in Figure 1-9. Measurement distance from the canopy was again 24 inches. I t is c lear ly evident that the r e f l e c t i v e cur ta in produces more uniform i n t e n s i t y . The l i g h t i n tens 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 te rna t i ve ly the lamp canopy may be mounted on a pul ley system, al lowing the l i g h t i n tens i t y to be adjusted without moving the p lan ts . Moving the lamp canopy or plants may change the a i r c i r cu la t i on and heat t rans fer in the chamber and thus may require a change in temperature and humidity control se t t ings . Moving the lamps or plants are the most commonly used techniques fo r varying l i g h t i n tens i t y in commercial plant growth chambers. Another method is to vary the in tens i t y of the lamps themselves. A special ba l l as t is required to vary the in tens i t y of f luorescent lamps. This ba l l as t is expensive and l im i ted to lower wattage lamps seldom used in plant growth chambers. The in tens i t y of incandescent lamps can be varied by a series potentiometer. This method is less sa t is fac tory since several addi t ional e l e c t r i c a l components are required and uniform spectral qua l i t y cannot be maintained. Photoperiod control is generally provided by on/o f f switches driven by 24-hour c locks. To simulate natural condi t ions, several clock-switches may be used to switch on and o f f both the incandescent and f luorescent 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 for plant growth chambers can consume several thousand watts of power. The influence of the heat input from the lamp canopy on other.environmental variables in a chamber is 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 great ly improve lamp canopy performance fo r plant growth chambers. Newly developed high in tens i t y lamps using mixtures of sodium, mercury and other gases produce sun- l ike emission spectra ( P h i l l i p s , 1969). Any lamp of sui table i n tens i t y and spectrum for plant growth that reduces in f rared output would considerably s imp 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 var iables. The temperature range of general purpose plant growth chambers usually extends well below and above the optimum growth temperatures fo r p lan ts , t y p i c a l l y -10° to 50°C. The precision and rep roduc ib 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 locat ion of the control device, the select ion of heating and cool ing elements, the rate of flow and volume of a i r c i r c u l a t i o n , and the var iat ions of the heat load wi th in the chamber. The uni formity of a i r temperature depends on the a i r c i r cu la t i on patterns w i th in the plant growth area and the ef fects of the l i g h t canopy. Three types of temperature control are commonly encountered. The simplest is the constant temperature type, which maintains temperature close to a s ingle control po in 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 W h ma W, 2 2 7T777777T77T7777T7777777777 Fig. 1-11 Processes occuring when cooling air in a conditioning system. 14 Ai r enters the condi t ioning section at temperature t-j and leaves at tr>. The enter ing and leaving humidity ra t ios ( l b . o f water/ l b . o f a i r ) and enthalpies (BTU/lb. o f a i r ) were W-j, W2 and h-j, h 2 respect ive ly . The heat added to or subtracted from the a i r stream is represented by q (BTU). Unless the control point is changing, enter ing a i r w i l l be warmer than a i r at the exhaust of the condi t ioning section due to the pos i t ive heat load of the l i g h t s and other equipment in the chamber. The humidity ra t i o W-j w i l l be equal to W2 i f no water condenses on the cool ing co i l or is 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 r a t i o . For the sensible heating or cool ing of moist a i r (not involv ing the addit ion or subtract ion o f water) the fo l lowing heat t ransfer equations summarize the possible conditions of Figure 1-11 (Threlkeld, 1970). ma-| = ma2 (1) ma-jh-j + q = ma 2h 2 (2) ma-jW^ = ma2W2 (3) q = ma(h 2 -h 1 ) (4) and i f W-j = W2 q = ma(0.24 + 0.45W) ( t ^ ) (5) A pound of dry a i r at 50% re la t i ve humidity and a barometric pressure of 14.696 Psia w i l l have a volume of 12 cubic feet at 16°F and a volume of 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 rcu la ted 1000 cfm at a l l temperatures and humid i t ies , a greater mass of a i r would be c i rcu la ted 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 th is e f fec 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 condit ioning process equals the product of the mass of a i r c i rcu lated per unit time and the change in enthalpy of the a i r as i t passes through the condit ioning 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 cu la ted per minute times a f ixed coe f f i c i en t re lated to humidity ra t io times the temperature di f ference across the co i l (Equation 5) . For the transfer of sensible heat, Equation 5 allows the ca lcu la t ion of the heat t ransfer required to balance the heat load of the chamber. The value calculated w i l l be the theoret ica l quantity of heat exchanged in a given condit ioning process. In p rac t i ce , however, the heat required w i l l d i f f e r from q since a cool ing co i l i s not a perfect heat t ransfer system. An ideal cool ing or heating element would have low thermal res is tance, extensive contact with the a i r moving through i t , uniform temperature across the ent i re surface area of the element, and large face area re l a t i ve to the volume of a i r c i r cu l a ted . The properties of a real cool ing co i l w i l l be discussed l a t e r (Page 20), since humidity changes almost always accompany the operation of a cool ing c o i l . Two heating and cool ing methods are commonly used on plant growth chambers. The f i r s t system involves a re f r igerated cool ing co i l and a separate heating element. When cool ing is demanded, the compressor of the re f r igera t ion system is act ivated and the heating element is turned o f f . The reverse occurs during the heating cyc le . I f the heating and cool ing elements are c lose ly matched in the i r rate of heat exchange, sa t i s fac tory operation can resu l t . However, there is a tendency for the thermal resistance 16 of the heating and cooling elements to cause overshooting of a control po in t . I f the overshoot and undershoot var iat ions about a control point are s u f f i c i e n t l y smal l , the problem can be ignored. Figure 1-12 shows how heating and cool ing 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 iency of heat t ransfer of a heating or cooling element varies with temperature. Consequently, over the f u l l temperature range in a p lant growth chamber, the re la t i on of the rates of heating and cool ing may vary. I t i s possible to calculate whether the rate o f e i ther heating or cooling may be too great and cause overshoot (Thre lke ld, 1970). Direct experimentation, however, is usually re l i ed upon in prototype stages to discover undesirable performance. In p rac t i ce , the a i r temperature is measured at a locat ion s u f f i c i e n t l y past the heating and cool ing elements so that remixing of the condi t ioned-ai r stream is nearly complete and before the influence o f the chamber heat loads. A typ ica l heating and cool ing system is shown in Figure 1-13. 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 condi t ioning technique more recent ly incorporated in plant growth chambers is the hot-gas bypass system (Figure 1-14). The dif ferences from the preceding system are that the compressor operates continuously and the hot or vaporized gas in the re f r i ge ra t i on system is shunted past the condensor d i r e c t l y in to the evaporator co i l thus making the evaporator co i l heat rather than cool . This technique is very e f f i c i e n t in reducing over- and undershoot problems. The e f fec t i ve thermal resistance of the 17 OVERSHOOT UNDERSHOOT • lu z a «"= o UJ O p a cc 2 ui U J D o y —i » - • +W5 c —COOLINS — - HEATING - — COOLING - - HEATING -Equal heating and cooling rates Cooling rate > heating rate - / COOLING - HEATING -Heating rate > cooling rate F1g. 1-12 Temperature variations about a control point caused by di f ferent rates of heating and cooling. EVAPORATOR PRESSURE REGULATING VALVE ( ) EVAPORATOR PRESSURE REGULATING VALVE C ) RECEIVER CONDENSOR DRYER F1g. 1-13 The refrigeration system of the Percival Model PGC-78 growth chamber u t i l i z i n g two evaporator co i l s . (Percival, 1963. •) EVAPORATOR ( ACCMULATOR HEAT EXCHANGER DRYER CONDENSOR HEATING ^ SOLENOID Q Lb COMPRESSOR PRESSURE RECUL.ATIN6 VALVE •o COOLING SOLENOID F1g. 1-14 A typical refrigeration system u t i l i z i n g the hot-gas by pass technique of evaporator co i l temperature regulation. (Controlled environments, 1967. | T9 evaporator co i l is reduced by the in t roduct ion of hot gas. I f a heating element is required, 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 th in ± 1/8°C can be real ized wi th the hot gas bypass system. Before proper condi t ioning capacity can be selected, the magnitude of the plant growth chamber's heat load must be determined fo r a l l pract ica l ambient and chamber condi t ions. Generally, the l i g h t system is the largest heat source. Each watt of f luorescent and incandescent l i g h t i n g y ie lds 4.09 and 3.41 BTU/hour, respect ive ly . These conversion f igures 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 lec t r i ca l and e lect ron ic devices in the a i r c i r cu la t i on system w i l l add heat r e l a t i v e to t h e i r duty cycles and wattage. Heat t ransfer through the chamber's w a l l , roo t , and f l oo r panels is the product of the surface area's involved (A) , the heat t ransfer coe f f i c i en t of the panels (K) , and the temperature di f ference across the panels ( t g - t ^ ) (K re i th , 1964): q = kA ( t 2 - t ^ ( 6 ) . The i n f i l t r a t i o n of a i r in to 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 condit ions are d i f f e r e n t . Ord inar i l y , chamber leakage becomes important only when extreme chamber condit ions are attempted. Fresh a i r make-up, which is the addit ion of a predetermined volume of a i r to a plant growth chamber to maintain normal C02 concentration l eve l s , also acts as a heat load. Once the general heat loads of a chamber are known, the e f fec 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 ef fects are presented in the fo l lowing sect ion. 20 (C) Humidity Control Knowledge of the humidity in the natural environment is important to the study of plant growth. For example, the gradient of pa r t i a l pressure of water vapor across the boundary layer of a leaf is the d r i v ing force of t r a n s p i r a t i o n , the loss of water by evaporation from p lants . A plant growth chamber should provide some degree o f humidity control to remove humidity f luc tuat ions caused by the condi t ioning equipment. Humidity is a very complex var iable of the environment, and therefore careful analysis of a chamber's proposed operating ranges is necessary to provide adequate humidity con t ro l . The inclusion of humidity control can double the cost of a plant growth chamber. I t is convenient to introduce addi t ional equations which summarize the physical processes involved and the humidi f icat ion and dehumidif icat ion of moist a i r . Table 1 gives the common terms used in psychrometric re la t ions along with an example of how each of the terms varies with temperature. An analysis of Figure 1-15 provides the fo l lowing re la t ions describing the cool ing of a i r which resul ts in condensation of water. ////////////\/\/////////////, W l m,. F ig . 1-15 Processes occuring when a i r is cooled and condensation r e s u l t s . 21 m-j = IU, = m q = m(h 2 - h-j) - m ch ( mc = m(W1 - W2) (7) (8) (9) In Equation 8, assuming ideal heat t ransfer , 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 humidi f icat ion and heating of moist a i r i s i l l u s t r a t ed 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 ra t io (Equation 11), ////////\////////////////// TTTTTTTTTT F ig . 1-16 Processes occuring when a i r i s heated and humidif ied. q = m(h2 - h^ - m wh w (10) (11) TABLE 1 P s y c h r o m e t r i c T e r m i n o l o g y r TERM SYMBOL UNITS T e m p e r a t u r e . t ° C 1 0 ° c 2 1 ° C 3 2 ° C H u m i d i t y R a t i o W l b , w' a . • » . 0 0 3 8 • . 0 0 8 0 . 0 1 5 2 R e l a t i v e H u m i d i t y R . H . 50% 50% 50% E n t h a l p y h BTU/lb d r y a i r 16.1 2 5 . 4 3 5 . 7 Volume V f t 3 / l b d r y a i r 1 2 . 9 . 1 3 . 5 5 • 1 3 . 9 S a t u r a t i o n p o t e n t i a l W s l b / l b w' a . 0 0 7 6 . 0 1 5 8 . 0 3 1 2 S a t u r a t i o n p e r c e n t -ages u - 50% 50% 49% Thermodynamic wet b u l b t e m p . ' t * ° C 6 ° C » • 1 4 ° C 2 4 ° C Dew p t . temp. V ° C 0 ° C 1 0 ° C 2 0 ° C ro ro S o u r c e : T h r e l k e l d P s y c h r o m e t r l c C h a r t , 1 9 7 0 . 23 The h u m i d i f i c a t i o n of moist a i r when no other energy i s added i s a s p e c i a l case o f the system i n Figure 1-16. The amount of heat added by the water to the a i r equals m times the t o t a l enthalpy change. The amount of water added equals m times the change i n humidity r a t i o . \ h w s m ( h 2 - V (12) = m(W2 - W.,) (13) The a d i a b a t i c mixing of two streams of moist a i r occurs when f r e s h a i r i s added to chamber a i r (Figure 1-17). For p l a n t growth chambers, the volume of fre s h a i r make-up i s u s u a l l y s m a l l , but 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 . The make-up a i r may represent a s i g n i f i c a n t water vapor load which should be considered when h u m i d i f i c a t i o n equipment i s designed. The e f f e c t s o f fre s h a i r i n f i l t r a t i o n can be determined i n the f o l l o w i n g manner, m^ , the mass of a i r r e t u r n i n g from the chamber, mixes with m^ , the mass of make-up a i r , producing the c o n d i t i o n s a t point 3 i n Figure 1-17 (Equation 14). F i g . 1-17 Processes occuring when two airstreams are mixed. 24 m l W 1 + m2W2 = m3^3 (14) (15) 0 6 ) 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^ Enterinq 50% R.H. A i r t e mP-18.5°C r Flow 12.5°C_ Entering L7ew~P~bTnt~ Cooling Coil (side view) Exhaust Air temp. 21 C 47% R.H. Coil surface temp. 10°C ExhaustTTew Point Fig. 1-18 Humidity and temperature changes occurring in a stream of air passing through a cooling coil. 25 Assume that the desired control point is 21°C and 47% re la t i ve humidi ty, and that such a i r leaves the co i l in Figure 1-18. A f te r c i r c u l a t i n g through the plant growth area, suppose that the a i r returns to the cool ing co i l a t 24°C and 50% re la t i ve humidity. The heat and moisture loads of the chamber w i l l determine the state of the a i r returning to the c o i l . Further assume t h a t , in the co i l of Figure 1-18, the temperature of the surface of the f i ns 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 funct ion of the face area of the c o i l , the to ta l outside surface area of the f ins 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 ve loc i t y , and the number of rows of tubes. A l i near gradient is shown in Figure 1-18 fo r convenience. The dewpoint o f the a i r enter ing the co i l in Figure 1-18 is 13°C and of the a i r leav ing, 10°C. This implies that condensation has occurred since a change in dewpoint is possible only when the humidity r a t i o changes (at constant atmospheric pressure). Furthermore, part of the surface of the co i l is above 13°C, and w i l l remain dry. Condensation w i l l occur on the port ion of the co i l which is below the dewpoint. A drainage system should be provided fo r such condensation. The surface area of the co i l which becomes wet is mostly determined by the surface area of the co i l below the dewpoint of the entering a i r . When the co i l surface temperature is lower than in Figure 1-18, even more condensation w i l l occur from each pound of a i r c i r cu la ted , since more co i l surface area w i l l be below the dew-point o f 13°C. The mass of water that condenses is given by Equation 9. 26 Some of the water that is condensed may also be re-evaporated. Since dehumidif icat ion is an inherent consequence of cool ing co i l operat ion, control o f humidity w i th in a chamber requires a source of water vapor to replenish the mass of water l o s t from the c i rcu la ted a i r due to condensation. The capacity of equipment selected fo r humidi f icat ion should be determined in re la t ion to the highest rate of humidi f icat ion expected to be required over the operating range of the chamber. In general, a plant growth chamber is designed to provide a pa r t i cu la r humidity range over a speci f ied temperature range. Close spec i f i ca t ion is 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 humidit ies at low temperatures are d i f f i c u l t to produce because the surface temperature of a cooling co 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 ransfer is great ly reduced, causing less e f f i c i e n t a i r temperature con t ro l . At high temperatures, high humidit ies 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 seal ing to reduce undesired water vapor i n f i l t r a t i o n or loss . I n s t a l l i n g dehumidif icat ion equipment is a much more d i f f i c u l t and expensive problem than humid i f i ca t ion . F i r s t , the water that is removed from the a i r must be drained away from the a i r c i r cu la t i on system so that re-evaporation is not s i g n i f i c a n t . Secondly, the temperature of 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 re la t ion to the lowest humidity desired at the highest temperature at which humidity control is spec i f ied . This condit ion w i l l require the highest rate of dehumidi f icat ion, in terms of pounds of water removed per pound of a i r , that w i l l occur in the 27 plant growth chamber. I t i s important to consider the e f fects of plant t ransp i rat ion as a source of water vapor when dehumidif ication i s performed. Several references contain data from which estimates of moisture load due to t ransp i rat ion can be obtained (Sal isbury and Ross, 1969; Geiger, 1965). To make an accurate estimate of t ransp i rat ion such factors as l i gh t i n tens i t y , rate of photosynthesis, temperature, r e l a t i ve humidity, wind ve l oc i t y , time of day, and atmospheric pressure should be considered. As mentioned e a r l i e r , i f f ros t accumulates on the surface of a cool ing c o i l , the heat t ransfer e f f i c i ency of the co i l is reduced. Defrost cyc les , usual ly only a few minutes long, are programmed on c o i l s when f ros t problems are encountered. A defrost cycle usual ly involves the shut-down of the re f r igera t ion compressor and fan, and possibly the act ivat ion of a heater to melt the accumulated ice or f r o s t . The length and required number of defrost cycles are usual ly found by experimentation. During a defrost cyc le , the a i r temperature of the plant growth area also r i s e s . This is an unavoidable problem in chambers with only one compressor and cool ing c o i l . One method that may be used to avoid the undesirable af fects 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 co i l can be processing the chamber a i r while the other i s being defrosted (Figure 1-19). Of course, the expense is greater but chamber performance is improved. 28 ///////////////////////// COIL A — 1 ' a* — COIL B — (a) COIL A Defrosting COIL B Processing 7/////////////////////V, COIL A COIL B '////////////////////, (b) COIL A Processing COIL B Defrosting F ig . I-19 Dual damper-controlled cool ing co i l The e f fec 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 gh t s are on, some heat i s added to the a i r which must be removed by the condit ioning equipment before the a i r can be re-introduced to the plant growth area at a given temperature. With the l i gh ts o f f , no such heat is added to the a i r , but the condit ioning equipment must recondit ion the a i r to the same temperature. Figure 1-19 indicates what occurs in the cool ing 29 coil in both lights-on (a) and lights-off conditions (b) Air Temp Air Flow Air Temp 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 air 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 rcu lat ion The qua l i ty of the environment of a plant growth chamber w i l l be a function of the uniformity of a i r ve loc i ty and temperature throughout the chamber. The provis ion of a proper quantity of conditioned a i r i s one problem, while that happens to the conditioned a i r within the plant growth area, under intense l i g h t s , with shelves and fo l iage blocking a i r f low, depends on the design of the a i r c i r cu l a t i on system. As mentioned e a r l i e r , a ver t i ca l a i r flow i s most e f fec t i ve in chambers of the reach-in type (about 12 sq. feet of growing area). I f the a i r flow i s uniformly d i s t r ibuted at the base of the chamber, much more uniform ver t i ca l a i r flow i s poss ib le . The a i r w i l l tend to go up through the plant f o l i age , and i f the plants are evenly d i s t r ibuted 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 within the chamber w i l l experience approximately the same resul tant a i r f low. The development of a ve r t i ca l a i r flow system is described in Chapter V. The ef fects of the a i r c i r cu la t i on system upon humidity mainly r e f l e c t the ef fects of a temperature gradient from the chamber base to the l i gh t system. For example, once conditioned a i r has entered the plant growing area, the absolute humidity i s constant (excluding t ransp i rat ion and so i l water evaporation for the moment). Therefore, i f a temperature gradient does ex i s t v e r t i c a l l y in the chamber (with ve r t i ca l a i r f l ow ) , a lowering of the r e l a t i ve 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 m a t e r i a l i n the chamber, i n c l u d i n g the chamber w a l l s . This v e r t i c a l 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 c i r c u l a t i o n . Because o f the above e f f e c t s , i t i s use fu 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. By us ing such a re fe rence p o i n t , the p o s s i b i l i t y o f reproduc ing a spec ia l environment i s g r e a t l y i n c r e a s e d . Temperature, h u m i d i t y , and a i r v e l o c i t y g rad ien ts occur w i d e l y i n n a t u r e , so the problems are not unique t o growth chambers. (E) R e l i a b i l i t y and Serv ice 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 c o n t r o l s should be i nco rpo ra ted wherever p o s s i b l e . The use o f e lec t romechan ica l devices should be reduced, as the number o f opera t ions in a con t i nuous l y runn ing growth chamber becomes enormous. For example, a r e l a y which operates once a minute (no t uncommon) w i l l exper ience 526,600 opera t ions in a y e a r . Such e lec t romechan ica l r e lays are used t o c o n t r o l the so leno id valves o f the r e f r i g e r a t i o n compressor. When a so leno id i s energ ized o r d e - e n e r g i z e d , a rad io - f requency 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 pulse h i t s the con tac ts o f a r e l a y , a spark can occur . As a r e s u l t , the con tac ts o f a r e l a y can erode u n t i l a s h o r t c i r c u i t occu rs . A r e l a y r a t e d a t 10 opera t ions (mechanical) may l a s t on ly 500,000 opera t ions 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 r e l a y contac ts can increase 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 pulse out o f the l i n e be fo re i t reaches the c o n t a c t s . A General E l e c t r i c t h y r e c t o r o r a " c l i p p e r " (d iode) accomplishes t h i s e a s i l y . 32 A schedule of l i gh t bulb replacement should be arranged so that a predetermined average l i g h t in tens i ty and qua l i ty is maintained continuously over any period of time. The average l i g h t in tens i ty that w i l l be pract i ca l to maintain w i l l depend on the number of l i gh t bulbs in the chamber, the cost, and the average l i f e t ime unt i l l i g h t output becomes too low. By fo l lowing a schedule of l i gh t bulb replacement, the researcher can have greater confidence that short- and long-term var iat ions in l i gh t in tens i ty and qua l i ty within a chamber and l i gh t in tens i ty dif ferences 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 ther constructed with sealed bearings or are s e l f -l ub r i ca t i ng . Thus, service of these devices is a matter of replacement at the end of the i r l i f e t imes . It is good pract ice to maintain an inven-tory of c r i t i c a l components that are not readi ly avai lab le from local r e t a i l ou t l e t s . A c r i t i c a l component i s defined as one such that i f i t f a i l e d , the operation of the chamber must stop unt i l i t is replaced. 33 Chapter I I INSTRUMENTS FOR THE CONTROL, INDICATION, AND MEASUREMENT OF ENVIRONMENTAL VARIABLES (A) Contro l At p r e s e n t , most commercial and custom b u i l t growth chambers prov ide on ly 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 ow 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 humid i t y c o n t r o l i s so expensive t h a t few chambers have such equipment. A i r f l o w , h u m i d i t y , and atmospher ic composi t ion (0 2 and C02 c o n c e n t r a t i o n i n p a r t i c u l a r ) are g e n e r a l l y assumed c o n s t a n t , which i s not u s u a l l y a safe assumpt ion. Cont ro ls 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 t ime c locks f o r v a r y i n g the p h o t o p e r i o d , and moveable shelves or p u l l e y systems f o r i n c r e a s i n g or decreasing the l i g h t i n t e n s i t y to which the p l a n t s are exposed. Temperature c o n t r o l in a p l a n t growth chamber i s more dependent on t o t a l 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 i s l a r g e l y determined by the p r o p e r t i e s o f lamps a lone . Three bas ic types o f temperature c o n t r o l s 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 , h y d r o s t a t i c t h e r m o s t a t s , and e l e c t r o n i c c o n t r o l s . 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 thermosta ts 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 be ing used on less expensive equipment. Both methods lack a s imple means o f v a r y i n g s e n s i t i v i t y ( t o the change i n temperature between an on and o f f c y c l e ) , which i s necessary to p rov ide proper balance between h e a t i n g and c o o l i n g l o a d s . The b i m e t a l l i c s t r i p does no t a l l ow remote s e t p o i n t l o c a t i o n , an impor tan t convenience 34 f o r the chamber operator. Neither method provides a ca l ib ra t ion adjustment fo r d r i f t s in ca l ib ra t ion with t ime, or a single pole, double throw output which is required fo r con t ro l l i ng both heating and cool ing loads. The small d ia l usually provided may or may not be in temperature units and seldom is i t possible fo r a spec i f ic temperature to be set d i r e c t l y on the d i a l . Several types of hydrostat ic controls are not compensated fo r changes in ambient condit ions around the chamber. The hydrostat ic cap i l l a r y tube extends from the sensor bulb ( in the chamber) to the body of the c o n t r o l , which is usually mounted on an ex te r io r chamber wall and is exposed to ambient temperature f l uc tua t ions . Users of plant growth chambers often require diurnal or program-mable temperature c o n t r o l . To provide such c o n t r o l , the Percival Model PGC-78, fo r example, uses a time clock that switches between two indep-endent hydrostat ic controls that are pre-set to day and night temperatures. Programming (mul t i -s tep) temperature control is not pract ica l wi th hydro-s t a t i c devices, since a separate control would be required fo r each step in the program. Cal ibrat ion o f hydrostat ic diurnal controls is more d i f f i c u l t than fo r a single control and since two controls are involved, there are more devices to f a i l in serv ice. So l id-s ta te e lect ron ic temperature controls are more complicated and- expensive than the above devices, but o f fe r r e l a t i v e l y simple solut ions to a l l of the problems associated with growth chamber temperature control in presently ex is t ing equipment. Nearly a l l so l id -s ta te 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 ( thermis tors ) , wire of a l loy composition (NiFe), 35 or pure metal (nickel or p lat inum). The sensor can be made small so that i t s time response to a temperature change is very f a s t . I f the resistance of the sensor is large (greater than 1000 ohms) lead r e s i s -tance is neg l ig ib le 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 plat inum, lead length compensation should be provided. Remote locat ion is r e l a t i v e l y simple when using a resistance sensor. Remote set -po in t adjustment is also eas i ly provided by so l id -s ta te con t ro ls . A potentiometer with a scale ca l ibrated according to the temperature coe f f i c i en t of the resistance sensor may be mounted on a panel convenient to the chamber operator. The length of the scale is usually about f i ve 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 losing the capab i l i t y of se t t i ng the temperature to wi th in 1/4°C. Cal ibrat ion and s e n s i t i v i t y adjustments are also accomplished with potentiometers and can be made avai lable on a convenient control panel. Cal ibrat ion is required fo r compensation of sensor aging and to balance out the non-uniformity between the o r ig ina l and replacement sensors. Resistance elements are t y p i c a l l y manufactured to tolerances of 1% or more. Sens i t i v i t y can be varied in several ways. A sensor may be wrapped with c loth 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 is to reduce the voltage across the Wheatstone br idge, which reduces the signal to the control amp l i f i e r . I t then takes more temperature change to make the ampl i f ie r switch from heating to cool ing. Changing the bridge voltage is not as sa t is fac to ry a method of varying s e n s i t i v i t y since the current f lowing through the sensor is changed, which a l te rs the se l f -heat ing er ror of the 36 sensor. Such a change can a f fec t the overal l ca l i b ra t i on of the c o n t r o l l e r . The best avai lable method of varying con t ro l l e r s e n s i t i v i t y is accomplished e l e c t r o n i c a l l y . By using a resistance-capacitance c i r c u i t in the feedback of the a m p l i f i e r , the di f ference signal of the wheatstone bridge is in tegrated. By varying e i ther the resistance or capacitance, the slope of the in tegrat ion can be changed, which d i r e c t l y changes the time response of the con t ro 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 so l id -s ta te temperature c o n t r o l . The most common is a s ingle-pole double-throw electromagnetic relay rated at 10 to 25 amps (non- induct ive) . This form of relay allows control of heating and cool ing elements. More recent so l i d -s ta te control designs incorporate so l id -s ta te a l te rna t ing current switches (Triacs) fo r the control of heating and cool ing elements. Triacs el iminate the electromagnetic re lay , a mechanical device with contacts that has proven to be the least r e l i a b l e component of so l id -s ta te cont ro ls . The use of t r iacs can extend the mean-time-between-failure ra t ing to years instead of months. Diurnal temperature control is eas i ly provided by so l id -s ta te control systems. For diurnal c o n t r o l , a time clock switches between two set -po in t potentiometers which are preset to the desired day and night temperatures. Instead of requi r ing two complete hydrostat ic c o n t r o l l e r s , a single poten-t iometer is the only extra pa r t . The same sensor and ampl i f i e r control both day and night temperatures, increasing accuracy while s impl i fy ing the to ta l control system. 37 The programming of temperature with a so l id -s ta te control i s an extension of diurnal cont ro l . One method is to switch a ser ies of set point potentiometers by means of a stepping relay sequenced by a time c lock. Proper switching must be provided (low contact res i s tance) , and a c i r c u i t for ca l i b ra t i on of the set-points devised. Only one cont ro l l e r i s required for th is type of programmer. The more f am i l i a r cam-type control is also adaptable to growth chamber temperature programming and gives stepless cont ro l . Maintenance of hydrostat ic temperature controls consists of replacement of the ent i re un i t . For so l i d state cont ro ls , 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 con t ro l , they should be inspected f i r s t in diagnosing f a i l u r e . So l id-s tate components have acquired a great reputation for r e l i a b i l i t y , and should exh ib i t a mean-time-between-fai1ure of more than 10,000 to 20,000 hours operat ion. In the future, i t w i l l be possible to have such controls on other environmental var iables such as l i g h t i n tens i t y , humidity, and a i r speed. Complexity and expense prevents common use of such controls at present. (B) Indicat ion The design of instruments for ind icat ion of environmental variables is more complex than for contro ls . Discussion here w i l l be l imi ted to those instruments and devices used in measuring l i gh t i n tens i t y , temperature, a i r c i r c u l a t i o n , and humidity for the data in Chapters III and V. In the co l l e c t i on of l i g h t in tens i ty information in the plant growth area of the Perc ival Model PGC-78, a photometric l i gh t sensor was u t i l i z e d , since the l i g h t qua l i ty (made up of f luorescents and incandescents) remained 38 essent ia l l y constant during the measurement per iod, and was fa r simpler to use than the only avai lable radiometric l i g h t sensor. An Internat ional Rec t i f i e r s i l i c o n photovoltaic photocell (Serial No. S1020E4PL) was cal ibrated with a Gossen Tri-Lux foot candle meter. The response o f the photocell was found to be l i near between 300 and 2500 foot candles, a useable range fo r th is app l i ca t ion . The s i l i c o n photocell was attached d i r e c t l y across the input of a d i g i t a l voltmeter wi th an input impedence of greater than 1000 megohms. The various readings were d i g i t a l l y pr inted on command by a Solatron Data Logging system. The surface area of the photocell was 1/2 inch square (1 /2" by 1" ) , permit t ing measurements to be taken at close spacing ( four inches) without overlapping occurr ing. Temperature measurements were made with a copper-constantan thermo-couple (wire diameter .032 inch) . Radiation compensation was not necessary because of the small surface area of the thermocouple. Cold junct ion com-pensation was accomplished with a platinum regulated junct ion box (Sola t ron) , accurate to ± .15°C. The thermocouple output was also connected to the d i g i t a l voltmeter fo r measurement and recording. A i r c i r c u l a t i o n w i th in 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 that the hot-wire element was always hor i zon ta l . This was done in an attempt to obtain the ve r t i ca l a i r ve loc i ty component, as much as possible; and to improve the rep roduc ib i l i t y of readings at a given loca t ion . The re la t i ve humidity in the plant growth area was measured wi th a Phys-Chemical Research Corporation Model 11 precision copolymer styrene p las t i c sensor. The resistance of th is device is a funct ion of r e l a t i v e humidi ty. The sensor was connected in an a l te rna t ing current Wheatstone 39 bridge and capaci t ive ly coupled to a nu l l detect ing amp 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 of R.H. was less than 30 seconds in s t i l l a i r . Response time in an a i r flow of 50 to 100 feet per minute was less than 2 seconds. The p las t i c sensor was temperature compensated with a thermistor bead mounted very close to the surface of the wafer. The p las t i c sensor was sensi t ive to l i g h t i n tens i t y and required shield ing before measurements could be taken under the l i g h t system in the plant growth area. (C) Measurement The technique used fo r measuring the environmental variables was made as s im i la r f o r each as possible. To accomplish t h i s , a remote measuring system was developed to move sensors from one posi t ion to another without opening the door o f the chamber. A var iable length u-channel was made o f Plexiglass with rubber feet mounted on the ends (Figure 2 -1 ) . The u-channel could be suspended between the end walls of the chamber by applying a s l i g h t pressure on an adjustable s l i d e , and then t ightening a thumb screw. A miniature battery-operated motor with a gear reduction un i t was mounted at one end of the u-channel with a small pul ley facing the center of the channel. At the other end o f the u-channel, a second pul ley was mounted also facing the center of the channel. A s l ide that c losely f i t the u-channel was then pul led back and fo r th by the motor and pulley system, by reversing the bat tery p o l a r i t y . The various sensors could be attached to the s l ide and moved across the chamber. Wires were run from the motor and sensors to outside the chamber fo r manual operat ion. (C) SIDE VIEW > Fig. 2-1 Apparatus for the remote positioning of environmental sensors. 41 A four inch gr id was placed on both end walls of the chamber being measured (with .100 inch wide tape) so that 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 gr id as reference so that one end of the u-channel was in the same re la t i ve posi t ion as the other end. Along the length of the u-channel, reference marks were made so that the sensor could be stopped every four inches. The procedure fo r operating the device was to posi t ion the u-channel bar at one corner of the four- inch grids on the end wa l l s . The sensor s l ide was then run along the u-channel in four inch increments wi th measurements made every four inches. When the length of the u-channel was traversed (consist ing of th i r teen 4 inch increments), the ends of the u-channel were moved to the next gr id locat ion on the end walls and the sensor s l ide moved back again in four inch increments. The chamber door has to be opened to reposi t ion the ends of the u-channel bar, but only once every 13 measurements instead of once every measurement i f a motorized sensor s l ide wasn't used. A f te r opening and closing the chamber door, a s u f f i c i e n t amount o f time was allowed fo r the chamber condit ions to re -equ i l i b ra te . 42 Chapter I I I COMMERCIAL GROWTH CHAMBER PERFORMANCE INTRODUCTION A thorough examination of the performance of a Percival PGC-78 w i l l be presented in th is chapter. The information obtained is valuable fo 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 fo r performance analysis was one of 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 ig ina l spec i f icat ions as closely as possible. New f luorescent and incandescent lamps were i ns ta l l ed and the screens fo r d i f f us ing a i r flow were cleaned and stra ightened. The information gathered w i l l be compared to the advertised per for -mance of the Percival Model PGC-78, and to the standards of plant growth chamber design presented in Chapter I . Control led Environment Limited re f r i ge ra t i on and a i r c i r cu la t i on systems w i l l be discussed and compared to the Percival system. (A) Light System The Percival Company advertises a maximum of 5000 foot candles 4 (5.4 x 10 lux) l i g h t i n tens i t y w i th in the chamber u t i l i z i n g sixteen VHO 150 watt Cool-White type F72T12 f luorescent 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 spec i f ied . However, the maximum l i g h t 43 in tens 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 foot candles (2.7 x 10 l u x ) , wi th the measurement being taken s ix inches below the center of the l i g h t canopy. This measurement was repeated several times with a Gossen footcandle meter using f luorescent and incandescent lamps with approximately 100 hours operating time ( fo r burn-in purposes) and with chamber temper-ature at 20°C ± 1°C. The user of a plant growth chamber can easi ly be confused by l i g h t i n tens i t y speci f icat ions that do not include the conditions under which the manufacturer measured the performance of his system. In any case, a maximum in tens i t y ra t ing is of l i t t l e value, because of the progressive decrease in output of f luorescent lamps with age (General E l e c t r i c , 1960). An average l i g h t i n tens i t y ra t ing over a given period of time would be of more use to the researcher. Light i n tens i t y p ro f i l es wi th in the growth chamber were determined by measuring hor izontal planes with the remote sensing apparatus described in Chapter I I (Page 39). Eight horizontal planes, each including 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 Plexiglass. An empty chamber provides the most reproducible s i tua t ion fo r l i g h t measure-ment, and gives the operator a view of the to ta l chamber capab i l i t y . Measurements were taken with the chamber door and observation window closed. The spacing of the f luorescent and incandescent l i g h t bulbs and chamber door locat ion are shown in Figure 3 - 1 . Fig. 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 tens i t y gradients on hor izontal planes in four inch in terva ls below the l i g h t system. The l ines in Figures 3-2 ( a — * h ) represent points of equal i n tens i t y wi th the di f ference between one l i n e and the next being 50 foot candles (500 l u x ) . The di f ference in l i g h t i n tens i t y of hor izontal planes progressing down from the l i g h t system in the center of the chamber is approximately 150 to 200 foot candles (1500 to 2000 l u x ) . This indicates that the l i g h t system, in 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 fuse-hor izonta l l i g h t source. The e f fec t of distance from the l i g h t source on in tens i t y is extremely d i f f i c u l t to minimize with th is type of l i g h t system. The var iat ions of l i g h t i n tens i t y in any one horizontal plane resu l t from the di f ference in addi t ive ef fects of l i g h t i n tens i t y in the center of the l i g h t system re la t i ve to the sides or ends of the l i g h t system. Even per fec t ly r e f l e c t i v e walls would not el iminate the in tens i t y var iat ions near the edges of the l i g h t canopy. One s o l u t i o n , not always possib le, is to make the size of the lamp canopy much larger than the growth area p la t form, and th i s technique is of ten used in open chambers and walk- in rooms. The Percival Company speci f ies that up to 500 foot candles (50,000 lux) are obtainable in t h e i r Model PGC-78. I t is c lear that at 12 inches from the ba r r ie r between the l i g h t bulbs and the plant growth area, a distance that is much higher than would be pract ica l to use, an in tens i t y of 2000 foot candles (20,000 lux) is a l l that is avai lable under the d i rec t center of the l i g h t canopy (Figure 3-2a). This measurement was taken at 20°C using lamps that had operated 150 hours. At three thousand hours operating t ime, the f luorescents can be expected to give out 75% of the i n i t i a l 150 hour value and th is would resu l t in approximately 1500 foot 46 Fig. 3-2 (a-d) Horizontal l i g h t i n tens i t y p ro f i l es at 12" , 16", 20" , and 24" below the Percival Model PGC-78 Light Canopy (Door Closed) (500 lux in te rva ls ) "h) ^ ^ & n Z Z W l ° l ^ " ' T - 3 6 " ^ (500 lux in te rva ls ) 1 g h t CamW (Do°r Closed). 48 candles (15,000 lux) at the same locat ion (General E l e c t r i c , 1960). Because of these var iat ions of l i g h t in tens i t y in space and t ime, the spec i f ic inf luence o f l i g h t i n tens i t y var iat ions 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 task. The addi t ion of a few f luorescent lamps across the ends o f the chamber and one placed v e r t i c a l l y in each of the corners should consid-erably improve the horizontal uni formity of l i g h t i n tens i t y in the chamber. The Percival Company states that the purpose o f the mylar ba r r ie r between the l i g h t system and the plant growth chamber is to maintain optimum f luorescent bulb surface temperature (45°C) fo r high l i g h t output (Bu l le t in Number 2B). Fluorescent bulb l i g h t output f a l l s to nearly 10% of maximum when the chamber temperature approaches 5°C. The mylar ba r r ie r blocks the c i r cu la t i on of 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 is then ducted outside the b u i l d i n g . As long as the ambient temperature remains constant, bulb cool ing and surface temperature w i l l be f a i r l y constant i f the chamber temperature is held constant. However, the disadvantages of the bar r ie r exceed the benef i ts . The mylar sheets used as a bar r ie r by Percival are translucent and, when new, absorb a-minimum of 12% of 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 barr iers are the co l lec t ion of dust , water and other materials on the ba r r ie r and the p o s s i b i l i t y of exceeding the optimum surface temperature o f the f luorescent 49 bulbs. High ambient temperatures are very possible in a small room with several chambers operat ing. The removal of the mylar ba r r i e r is a cumbersome task often requ i r ing a she l f to be moved*. I t has been shown by several other growth chamber manufacturers that l i g h t systems without barr iers are more e f fec t i ve over the chamber temperature range, require fewer components, and cost less (Sherer-G i l l e t t , Controlled Environments, Engineered Environments). (B) Temperature Control For cool ing chamber a i r , the Percival Model PGC-78 u t i l i z e s a 1 horsepower re f r i ge ra t i on compressor wi th an a i r cooled condensing uni t and dual evaporator c o i l s . For heat ing, a 300 watt s t r i p heater is mounted below each of the evaporator c o i l s . Two separate hydrostat ic temperature controls (Penn Controls) operate the heating and cooling systems. Percival speci f ies 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 of temperature uni formi ty were taken by mounting a thermocouple to the remote sensing apparatus. The measurements represent the average o f two complete heating and cool ing cycles by the condi t ioning equipment. Associated with each cooling cycle is a minimum temperature, and with each heating cyc le , a maximum temperature, at each measurement loca t ion . Measurements every e ight inches in a horizontal plane provided s u f f i c i e n t de ta i l o f the temperature uni formity (Figure 3-3) . Af ter 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 ve r t i ca l p r o f i l e since these measurements were very time consuming. 50 21.5 - 8 ' •] 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 | 25.0 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 25.0 26.0 26.5 27.0 26.S 25.5 22.S 22.5 23.5 24.5 2 5.0 24.0 22.5 25.5 26.5 26.5 26.5 27.0 26.5 25.5 22.5 2 4.5 2 4.5 24.5 2 5.0 24.0 22.5 Fig. 3-3 Horizontal a i r temperature prof i le 14" below the l ight canopy barr ier. 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 24.0 24.5 24.0 24.0 24.0 38 21.0 21.S 21.0 21.0 21.0 INCHES F1g. 3-4 Horizontal a i r temperature profi les through counter of chamber for 22", 30" and 38" from l ight canopy surface. 51 Percival speci f ies ± 1.5°C control wi th no fu r ther q u a l i f i c a t i o n which implies that any point reasonably d is tant from the l i g h t source (approximately 10 inches) should be w i th in ± 1.5°C of the control po in t . The data of Figures 3-3 and 3-4 indicate that at any single point in the growth area, the temperature f l uc tua t ion is about ± 1.5°C around some temperature that may be above or below the desired control point by as much as 2°C. In f a c t , the actual set -po in t of the re f r i ge ra t i on con t ro 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 actua l ly measured by 2°C. This s i t ua t i on could ex is t because the hydrostat ic temperature controls have no ca l ib ra t ion adjustment, and they are also af fected by ambient condi t ions. I t should be noted that temperature uni formi ty wi th in a growth chamber is very much dependent upon a i r c i r c u l a t i o n , which is discussed in the fo l lowing sect ion. (C) A i r C i rcu la t ion The a i r c i r cu la t i on pattern w i th in a plant growth chamber is an important but often ignored environmental var iab le . Uniform a i r c i r cu la t i on is a prerequis i te to uniform temperature and humidity coadit ions throughout the plant growth area. The Percival Company states that in the Model PGC-78, chamber a i r is rec i rcu la ted over the cool ing and heating elements at a ve loc i ty of 50 to 100 surface feet per minute. The speci f ied a i r flow w i th in the plant growth area is a uniformly d i s -t r ibu ted 75 feet per minute. 52 Measurements of a i r ve loc i ty were taken at 8 inch in terva ls on hor izontal 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, al lowing the probe to be moved without opening the chamber door. Each reading represents an average of two minutes observation with the f luc tuat ions about the average indicated as a plus and minus quan t i t y . The f luc tua t ion was such that the damped mode (Posi t ion 2) of the anemometer had to be used to obtain the data of Figure 3-5. The hot-wire of the probe was held horizontal to the f l o o r of the chamber so that the largest ve loc i ty component of an observation should have been the ver t i ca 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 of 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 pos i t ions . Figure 3-5 indicates that there were s i g n i f i c a n t a i r ve loc i ty var iat ions and f luc tuat ions across the hor izontal planes. The greatest var iat ions 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 exhibi ted less to ta l v a r i a t i o n , but the r a t i o of highest to lowest ve loc i ty observed in any single plane s t i l l exceeded 10. The s igni f icance of such a i r ve loc i ty var iat ions w i th in a plant growth area also depends somewhat on the type of experiment undertaken. In view of the fac t that the above measurements were taken in an empty growth chamber, the problem of spacing pots on shelves to obtain uniform a i r c i r cu la t i on becomes even more complex. One user of the Model PGC-78 reported that cer ta in pots w i th in the chamber dr ied out considerably 53 60 ±30 90130 150 ±15 155 ±10 15*10 15±10 40 + 5 45±10 1S±J0 20+-10 30 ±10 40+10 10±5 40±10 10±S 10±5 20t10 45±15 35±10 10±5 95±5 65+15 35±10 10+5 140 ±.10 100±10 80 ±15 80 ±30 10±5 40 ±10 75±15 50 ±15 10*5 70±20 30 90±15 II 50110 20 ±5 65±20 35+10 20 ±10 4S±5 65±10 15110 10±S 75±10 85115 30+10 60±10 50 ±5 100±5 60±10 50+10 10 ±5 - 80 ±15 60 ±15 30+10 30±5 75 ±15 100H5 55*10 20±5 35+15 55±15 22 100±20 15 ±5 40110 30±10 25±10 2015 40±5 25H0 10±5 55±10 65±15 40±15 15110 110±10 90120 65H5 30+15 55+10 100+10 80±15 100 ±10 10±5 15±5 30+10 40±10 30 ±15 35±1S 65115 _ _ / / 38 90110 40115 65110 30110 30110 2015 70 ±10 50±10 30110 2015 90115 75±1S 40110 15±5 115±15 125115 60+15 10t5 40115 40110 25110 3-5 F i g . 3-5 Horizontal A i r Veloci ty p ro f i l es 14", 22" , 30", 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 f low often causes the leaves o f some plants to f l u t t e r , whi le other plants in the same chamber are completely s t i l l . Although the Percival Company advertises that the Model PGC-78 provides uniform a i r d i s t r i b u t i o n w i th in the plant growth area, no d e f i n i t e spec i f ica t ions are quoted. The engineering of the components of the PGC-78 a i r c i r cu la t i on system appears to have several de f ic ienc ies . Propel ler bladed fans are used to c i rcu la te the a i r from the top of the growth area, down past the cool ing and heating elements, and back in to the growth area (Figure 3-6) . The fans are mounted so that the a i r f i r s t h i t s the end wall of the chamber before going down through the condi t ioning 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 that could easi ly exceed the pressure ra t i ng o f a propel ler-bladed fan. As evidence of t h i s , blow-back in to the chamber is observed around the fans. The a i r d i f f us ing screen supplied by Percival is a perforated aluminum sheet with .120 inch holes on .188 inch centers, which y ie lds a porosity of 38%. NRC engineers (1962), have found that at the pressures and ve loc i t ies encountered in growth chambers, a f ive percent porosi ty is the maximum for achieving a manifold pressure s u f f i c i e n t for achieving uniform a i r f low. Figure 3-5 indicates that the d i s t r i b u t i o n screen must not have much inf luence on the a i r stream since the flow patterns are quite non-uniform with large (up to ± 50% of average) f l uc tua t i ons . It *i i 11 "i i <7~ lE I I J v V F i g . 3-6 General design of the Perc ival Model PGC-78 growth chamber. 56 Another problem is concerned with the fan motors, which are of the universal series wound type. Decomposition of the insu la t ion on the wire in the motor is evident ly caused by over-heating.. Consequently, the motor slows down and eventually stops. The above measurements were complicated by th is d i f f i c u l t y in that some fans were revolving fas ter than others. 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 fo r humidity control on the Percival Model PGC-78. Special equipment fo r humidity control is an optional accessory. The Percival Company specif ies that the re la t i ve humidity in the growth area w i l l be in the range of 50 to 70% R.H., depending upon ambient temp-erature outside the growth chamber. Measurements of re la t i ve humidity were taken with a PCRC Model 11 copolymer styrene sensor. One series of measurements was taken in the condit ioning 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 fo l lowing a heating cycle and a high of 80%. R.H. at 21°C fo l lowing a cooling cyc le. 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 fo l lowing a heating cycle and 70% R.H. at 22°C fo l lowing a cooling cycle. Humidi ty^f luctuat ions of th is magnitude may have some e f fec t on plant growth, but such ef fects may be minimized by r e l a t i v e l y short periods o f heating and cool ing ( 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 manufacturer . The load on a chamber w i l l determine what component i n tha t chamber may be the l e a s t dependable, a f a c t the ope ra to r w i l l seldom know. The exper ience o f the P l an t Sc ience Department w i th e i g h t P e r o r a l Model PGC-78 growth chambers i l l u s t r a t e s the demanding opera t i on of these chambers. The growth chambers are u s ua l l y operated con t i nuous l y 24 hours a day f o r what cou ld be years a t a time or u n t i l the equipment f a i l s . A l l 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 , the length o f which i s dependent on the q u a l i t y o f the component and the harshness o f the ope ra t i ng c o n d i t i o n s . Maintenance of the e i gh t P e r c i v a l Model PGC-78 chambers has p r i m a r i l y i nvo l ved the replacement o f l i g h t bu l b s , the rmos ta t s , a i r c i r c u l a t i o n fans and r e f r i g e r a t i o n compressors. A schedule o f l i g h t bulb replacement (d i scussed on page 2 ) would improve the cu r r en t p r a c t i c e o f r e p l a c i n g f l u o r e s c en t and incandescent bulbs as they burn out . A predetermined schedule would prov ide more un i form l i g h t c ond i t i on s as we l l as f i x i n g the cos t of l i g h t canopy maintenance. The thermostats (Penn Con t ro l s ) have been the second most f r equen t l y rep laced components f o l l ow i n g the l i g h t bu l b s . F a i l u r e o f these h y d r o s t a t i c thermostats has been nea r l y e x c l u s i v e l y due to con tac t s e i z u r e o f the microswitch. . The a d d i t i o n o f an inexpens ive component (a " c l i p p e r " d iode) would min imize spa rk i ng caused by ene r g i z i n g and de -ene rg i z i ng s o l eno i d va lves 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 mic rosw i t ch c on t a c t s . The t h i r d most f r equen t l y rep laced components v/ere a i r c i r c u l a t i o n f an s . Two des ign problems appear to have 58 affected the l i f e t i m e of the fan motors. F i r s t , a i r f low past the motor apparently is not s u f f i c i e n t to prevent decomposition of the motor winding insu la t ion due to high temperature, leading to reduced motor speed. Secondly, the lubr i ca t ion ports fo r the bronze bushings of the fan motor are inaccessible unless the chamber is emptied and the end panels removed. The motors are seldom lubr icated and ear ly f a i l u r e is the r e s u l t . 59 Chapter IV LABORATORY SYSTEM DESIGN To provide uniform contro l led plant growth condi t ions, an a r t i f i c i a l environment was developed which included the nu t r ien t mist technique. Two ident ica l chambers were constructed, each composed of a lower and upper section with a i r c i r cu la t i on ductwork to the upper section (Figure 4 -1 ) . The range and qua l i t y of environmental control eventual ly desired from th is system is out l ined in the Preface. The fo l lowing 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 ins ta l l ed in a labor-atory that is not a i r conditioned and is thermostat ica l ly temperature con t ro l led . Room temperature can be as low as 20°C in the winter with a few periods as high as 30°C in the summer. This di f ference made i t necessary to insulate the walls of the chambers completely from ambient f l uc tua t ions . 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 ract ica 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 fec t i ve in con t ro l l i ng the a i r temperature in the lower chamber very close to that of the water jacket . The a i r temperature control is achieved by both conduction and rad ia t ive energy exchange with the water jacke t . Thus, the a i r temperature in the lower chamber can be Light C o n c p y L ight C o n o p y » « * • • Comple te A i r Flow S y s t e m 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 cool ing the water that is being c i rcu la ted through the water jacke t . The water jacket on the upper or shoot chamber supplements the temperature con t ro l l i ng act ion o f the a i r c i r cu la t i on system, and also assists in removing heat that may be absorbed by the chamber wal ls from the l i g h t system. Most of the upper and lower sections of the chambers were made of P lex ig lass, which was chosen because of i t s ease of const ruc t ion, mod-i 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 to stresses and s t r a i n s . The water jackets had to be heavi ly reinforced to prevent bulging and subsequent breaking. Watert ight 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 fo r fu ture modi f i ca t ion . A more permanent method is to use a commercial glue (Cadco SC-94), which makes a Plexiglass to Plexiglass bond. Bonding is quicker and less expensive, but is not as strong or re l i ab le 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 condi t ioning system. The lower ( root ) chamber was made with the outside dimensions of 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 fo r four plants to be grown s imul-taneously without in ter ference. T h i r t y - s i x inches was allowed fo r the length of a root system. This size w i l l accommodate Phaseolus vulgar is (bush bean) approximately 8 to 10 weeks o l d . One quarter inch th ick 4-2 Perspective view o Scale: 1"= ' if Lower Chamber ar>d Bas 1' 63 Plexiglass was used fo r the chamber w a l l s , wi th two sheets being spaced one inch apart to make the water jacket . Only the two side wal ls and the rear wall were water jacketed. No jacket was made fo r the f ron t panel so there could be access in to the chamber. The f ron t panel and top and bottom pieces of the lower chamber were made from one-half inch th ick Plexig lass. The upper (shoot) chamber was made with the outside dimensions of 25" x 25" x 25" . Again th i s height allowance accepts Phaseolus vulgar is of approximately 8 to 10 weeks of age. For access in to the upper chamber, the f ron t panel was provided with 1/2" th ick Plexiglass door insulated wi th 2" of styrofoam. The plumbing fo r the upper and lower chamber was made from one inch and one and one quarter inch polyv iny lch lor ide tub ing. Large diameter tubing was used on the exhaust to prevent pressure from bui ld ing up w i th in the water jackets . Temperature cont ro l led water was pumped in to the base of the water jacket and an overflow drain was provided close to the top. (B) Temperature Control System For th is p ro jec t , a single source of temperature cont ro l led water was provided fo r c i r cu la t i on through the chamber water jackets (Figure 4 - 3 ) . A large capacity (approximately 80 l i t e r s ) stain less steel water bath formed the main reservo i r . A Blue-M re f r igerated cool ing co i l immersed in the reservo 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 rcu la te the water around the cool ing c o i l . A magnetically coupled, p las t i c bodied pump drew water from 64 i. F1g. 4-3 Wa l l T e m p e r a t u r e Control S y s t e m 65 the reservoi r and c i rcu la ted i t through the water jackets . A so l id -s ta te temperature control device was used to control the re f r i ge ra t i on system. The water in the reservoir was cont ro l led at the temperature desired f o r the water jacke ts . (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 least ± 1/2°C. A face and by-pass damper-contro l led cooling c o i l , a model o f units made fo r i ndus t r ia l appl icat ions by Recold of Canada, was u t i l i z e d fo r the control of a i r temperature. The operating p r inc ip le of th i s cool ing co i l is such that 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 cool ing un i t goes across water -ch i l led f i n s . As the chamber temperature reaches the desired control po in t , a modulating motor opens a damper to allow uncooled a i r to mix wi th that which has been cooled, thus providing a proport ional control of the a i r temperature (Figure 4 - 4 ) . The temperature sensing element was located inside the chamber manifold. I t was connected to a small t rans is to r ampl i f ie r that operated the modulating motor of the damper assembly. Since the desired a i r ve loc i ty inside the chamber was of the order of seventy feet per minute, the to ta l volume of a i r passing through the cooling system was correspondingly smal l . The low a i r flow requirement made feasible the use of a four hundred cubic feet per minute cent r i fugal fan . This type of fan was required since a pos i t ive pressure was needed to ensure smooth a i r flow and uniform mixing in the system, and to exceed the flow resistances of the cool ing c o i l , the ductwork, and the plant . / Ducts toAir Flow Manifolds A & B-Air Input F ig . 4-4 Diagram of the a i r temperature condit ioning system. 67 plat form a i r d i f f us ing system. The fan was driven by a shaded pole motor, the speed of which could be cont ro l led by a l t e r i ng the vol tage. Good control o f the volume of a i r c i rcu la ted could be achieved in th is manner. Ductwork was provided to d i s t r i b u t e the cooled a i r from the cooling co i l to the chamber manifold. The system was designed so that the ducting fo r one chamber could be removed while the other remained in operat ion. The surfaces of the ductwork were insulated with 1/2" th ick styrofoam to reduce heat exchange. (D) Plant Platform Ai r Di f fus ing Manifold Ai r flow is one of the more d i f f i c u l t variables to control w i th in a small enclosure. As discussed e a r l i e r , standard commercially avai lable growth cabinets are l im i ted in providing good a i r f low propert ies by t he 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 condit ions in the chamber. The use of a ve r t i ca l a i r f low about the plants has several advantages with respect to the engineering of growth cabinets. Most important is the fac t that a i r passes d i r e c t l y through the plant growth area and is not c i rcu la ted in the single or double ce l l fashion used with wall mounted fan's (Perc iva l , Sherer-Gi 1 l e t t , Controlled-Envi ronments). Also, the conditioned a i r passes over the plants and is essent ia l l y out of the plant growth area before the major heat source, the l i g h t canopy, is en-countered. A t h i r d advantage, which applies to a l l open systems, is that the chance of carbon dioxide depletion in the growth area is reduced by the continuous addit ion of f resh , f i l t e r e d , laboratory a i r . The open 68 system is best used when small volumes of a i r are required fo r c i r cu la t i on and the heat loads are large. Recirculat ing techniques are best suited fo r use in larger systems where a to ta l loss condi t ioning system would be uneconomical. The size of the chamber is generally l im i ted by the length of the f luorescent tubes used. The open system allows the use of longer f l u o r -escents by mounting the l i g h t canopy above the chamber wall s t ructure (Figure 4 - 1 ) . The method chosen to d i f fuse the a i r from the plant plat form came from a design by Kalbf leisch (1962). He t r i e s several approaches to the problem of producing uniform a i r f low through a f l a t p la te . He concluded that i f f i ve percent of the surface area o f the plate were equid is tant ly spaced holes, uniform flow would r e s u l t . The conf igurat ion involves 1/4" holes d r i l l e d at one inch centers over the en t i re surface. 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 ve r t i ca l part of the cor-rugation which made i t very d i f f i c u l t fo r foreign material to drop down into the manifold. This feature also made possible less interference to a i r enter ing the chamber from obstructions near the surface of the plant p la t form. For long periods of operation th is removed the necessity of cleaning the base of the growth chamber; t h i s , in t u r n , minimized d is turb ing 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 that was no less than two inches th ick required that a plant be at least two weeks old or t a l l enough so that the primary leaves were not disturbed by the manifold. Top View Fig. 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) Nutr ient Mist System The nu t r ien t mist growth technique involves the suspension of the root system in a closed, dark, temperature-control led" chamber. For the purposes of th is p ro jec t , the crown of the plant was placed in an expanded foam cork which was f i t t e d in to a hole in a plate at the top of the lower chamber. The foam cork served as a ba r r ie r to water and heat t rans fe r . Two pneumatic spray nozzles were placed at the bottom of the root chamber to provide the mist to saturate the chamber. The nu t r ien t mis t , when saturat ing the root chamber, co l lects readi ly on the root systems (Figure 4 - 6 ) . I t i s important that 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 is much experimenting to be done yet to f i nd the best way to o r ien t the spray nozzles in respect to the roo ts . (1) Nutr ient Solution The nu t r ien t solut ion selected fo r use in the nu t r ien t mist was a modified Hoagland's No. 2 (Hewi t t , 1965), consist ing o f : 50 ml. 1 M Ca(N0 3 ) 2 50 ml. 2 M KN03 20 ml. 1 M MgS04 10 ml. 1 M KH2P04 10 ml. Fe EDTA* 10 ml. Micronutr ients** * Each ml. o f the stock so lut ion of Fe EDTA contains 5 mg. of Fe * * The micronutr ient stock so lut ion contains 2.86 gm. of H-^ BO^  (Boric ac id ) , 1.81 gm. of MnCl2 - 4H20 (manganese c h l o r i d e ) , 0.11 qm. of ZnCl 2 (zinc c h l o r i d e ) , 0.05 gm. of CuCl 2 - 2H20 (Sodium molybdate) per l i t e r . 71 NUTRIENT SUPPLY 20 LITERS SOLENOID VALVES 115 VAC TEMPERATURE CONTROLLED WATER BATH Fig. 4-6 N u t r i e n t m is t spray system. 72 The designated amount of each stock solut ion was added to d i s t i l l e d water and made up to 2000 ml. A complete nu t r ien t supplement is necessary when.using a mist technique fo r plant growth. The modified Hoagland's No. 2 is easy to work with and provided s u f f i c i e n t nut r ients fo r both tomato and bean plants used in prel iminary tes 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 rec ip i t a te . This p rec ip i ta te can be avoided by adjust ing pH, the value of which is dependent upon the re la t i ve concentrations of calcium and phosphate in the nu t r ien t so lu t i on . (2) Nutr ient Mist Control System The nu t r ien t mist applied to the roots of a p lant must be contro l led at the same temperature as the root chamber water jacke t . This was accom-plished by passing the nu t r ien t solut ion through a coi led copper tubing immersed in the water reservoir (Figure 4 - 6 ) . The copper tube containing the nu t r ien t so lu t ion was placed inside the over-f low drain from the root chamber water jacket so that the temperature of the nu t r i en t solut ion was not af fected by the ambient temperature. Subsequent temperature measurements showed no change in root chamber temperature when nu t r ien t spray was appl ied. The amount of nu t r ien t solut ion necessary to maintain sa t is fac tory growth i s , at the present t ime, hard to spec i fy . The best method determined so fa r is to observe the root system and adjust the amount o f solut ion 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 bet ter technique. The capacity of the pneumatic spray nozzle was four or f i ve l i t e r s per hour. Compressed a i r from the general laboratory supply provided the 73 pressure f o r the spray. In te rmi t ten t spraying was required to conserve the nu t r ien t s o l u t i o n , and excess nu t r ien t solut ion 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 isk , 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 cont ro l led the flow of nu t r ien t so lut ion and compressed a i r (Figure 4 -6 ) . (F) A r t i f i c i a l L ight ing System The primary requirement of the l i g h t system fo r th i s project was to provide an in tens 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. From tests reported by Kalbf leisch ( A r t i f i c i a l Light fo r Plant Growth, 1963), i t was determined that twenty Cool-White 48T12R VH0 f luorescent l i g h t bulbs and f i f t e e n 40 watt incandescent bulbs would produce the desired in tens i t y and q u a l i t y . Kalbf leisch also indicated that side curtains would great ly ass is t in producing a hor izon ta l l y uniform l i g h t i n tens i t y below the l i g h t canopy. Eighteen inch long side curtains o f sheet aluminum f in ished in a baked white epoxy were mounted on both of the l i g h t canopies constructed. Figure 4-7 shows the spacing of the f luorescent and incandescent l i gh ts in the canopies. (1) Photoperiod Control Three single pole, s ingle 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 proport ion of red wavelengths 74 F1g. 4-7 Laboratory l ight 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 sunr ise. Approximately one-half hour l a t e r , one c i r c u i t of the f luorescents was switched on, and another one-half hour l a t e r , the second c i r c u i t o f f luorescents was switched on. At the end of the plants "day", the f luorescents were phased ou 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 of time the incandescents are on. High ampere rated switches must be used s ince, fo r example, 20 one hundred and f i f t y watt f luorescent lamps w i l l draw t h i r t y amperes when in operat ion. 76 Chapter V LABORATORY ARTIFICIAL ENVIRONMENT PERFORMANCE The qua l i t y o f environment that was set as a goal fo r the a r t i f i c i a l environment developed in th is project was out l ined in the preface. The measurements presented in th is chapter are an attempt at determining whether the system constructed approaches the intended environmental q u a l i t y . (A) Light System A special twenty-two inch long motorized sensor apparatus s im i la r to that described in Chapter I I I , was constructed fo r the measurement of l i g h t i n tens i t y p ro f i l es in the two foot square plant growth area. Af ter several tests were made with s i l i c o n phototransistors and s i l i c o n photo-vo l ta ic c e l l s , a selenium photovoltaic ce l l was selected as a l i g h t sensor since i t of fered greater l i n e a r i t y at l i g h t i n tens i t i es above 2000 foot candles. The s i l i c o n photovoltaic ce l l became very non-l inear above 2000 foot candles. The s i l i c o n phototransistor 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 f luorescent 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 fo r two hours before the f i r s t measurement was recorded. This was done so that the l i g h t output had s tab i l i zed as much as possible. Before the l i g h t output remains s tab le , the lamps have to warm to operating temperature and evident ly have to heat the surrounding equipment. From prel iminary measurements, i t was i 77 also noticed that the ambient temperature could noticeably a f fec t the l i g h t output o f the lamp canopy. Line voltage f luc tuat ions may also account fo r some var ia t ion in l i g h t output. During a prel iminary set o f measurements, one ha l f of the chamber was measured in the morning (at 10:00 a.m.) and the second ha l f measured in the afternoon (3:00 p.m.) . In th is case, the l i g h t i n tens i t y in the afternoon was found to be 20% less at the same point in the chamber than in the morning measurements. To minimize th i s problem, the data presented here were measured in as short a time as possible (3 hours) wi th checks frequent ly made to posit ions measured e a r l i e r to make sure that the l i g h t i n tens i t y gradients being measured were not an a r t i f a c t caused by changing condi t ions. An equally d i f f i c u l t problem in measuring l i g h t i n tens i t y was the temperature s e n s i t i v i t y of the l i g h t sensor. This problem was circum-vented by keeping a second l i g h t sensor, c losely ca l ibrated to the one in use, avai lable fo r quick spot checks of the l i g h t i n tens i t y at a given loca t ion . The quick spot checks did not allow time f o r the second sensor to a t ta in the temperature of the f i r s t and also served as an ind ica t ion of 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 resul ts of the l i g h t i n tens 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 in terva ls to w i th in two inches of the top of the chamber wa l l s . Each hor izontal plane consists of 100 measurements (10 x 10) spaced two inches apar t . The measurements a t the edges were spaced two inches from the Figure 5-1 ( a - f ) Horizontal l i g h t in tens 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 Lef t CHAMBER ORIENTATION Front Panel Right ure 5-1 ( g - j ) Horizontal l i g h t i n tens 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 wa l l s . The to ta l o f 1000 measurements were taken in less than a three hour period by means of the motorized sensor apparatus. The posi t ion 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 of 38" from the surface of the plant p la t form. The lowest hor izontal plane (Figure 5- la) 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 tens i t y between each l i ne on the graphs in Figure 5- la j . Figure 5-la was the lowest in l i g h t i n tens i t y with each plane above i t increasing in i n t e n s i t y . This type of ver t i ca l i n tens i t y gradient was expected and is a consequence of the inverse radius squared law of rad ia t i on . Also, there was a gradual increase in the di f ference in in tens i t y between adjacent hor izontal planes. For example, in the center of the chamber, the di f ference between Figures 5- la and 5-1b was 100 foot candles and between Figures 5 - l i and 5 - l j , 250 foot candles. The var ia t ion of l i g h t i n tens i t y w i th in each of the horizontal planes i l l u s t r a t e s the importance of symmetrical design. Figure 5-2 shows that the chamber was overlapped by the l i g h t canopy on a l l s ides, but that i t was not squarely centered under the l i g h t canopy. As the upper horizontal planes ind ica te , the region of highest l i g h t i n tens 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 that a p lant oriented in that posi t ion could receive a s i g n i f i c a n t l y higher average l i g h t i n tens i t y than other plants in the chamber. In th is case, the chamber was not squarely centered under the l i g h t canopy due to a lack of surrounding laboratory space, a condit ion which w i l l be r e c t i f i e d in the fu tu re . 81 LABORATORY WALL UPPER CHAMBER LIGHT CANOPY F i g . 5 - 2 Lamp canopy and chamber locat ion r e l a t i v e to laboratory wal ls (scale 3/32" = 1 ' ) . 82 Another fac tor 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. This box covered the ba l las ts required by the f luorescent lamps. The dark surface o f the box could possibly have influenced the l i g h t i n tens i t y w i th in the chamber since the walls of the chamber were c lear Plexig lass. 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 tens i t y in a single plane from lower to higher levels in the chamber may be due to th is in f luence. In Figure 5-1 a, the horizontal var ia t ion is about 100 foot candles and in Figure 5 - 1 j , about 250 foot candles. The above measurements of l i g h t i n tens i t y w i th in the growth chamber do not come close to meeting the i n i t i a l prerequis i te o f ± 50 foot candles var ia t ion on any s ingle hor izontal plane. 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 tens i t y uni formity in the chamber. (B.) Temperature The uni formi ty of temperature w i th in the plant growth area was wi th in the desired maximum f luc tua t ion of less than ± 1/2°C. Ten thermocouples (copper-constantan, 28 gauge) were mounted at f ixed posit ions every two inches from the plant p lat form to the top of the chamber. The f ron t panel was i ns ta l l ed and an average a i r ve loc i ty of 80 fpm was passed through the chamber. The to ta l ve r t i ca l temperature gradient was 1°C, wi th the 10 thermocouples placed in any locat ion in the chamber. This uni formity of temperature was due to the uni formi ty of a i r c i r c u l a t i o n . 83 The to ta l f l uc tua t ion of temperature at any given point was less than ± 1/4°C about a control point 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 ference between th i s system and commercial chambers is the complete lack of heating and cooling cycles and the accompanying temperature changes. The temperature w i th in the root chamber was extremely stable and uniform with less than ± 1/8°C temperature change detected over a 24 hour per iod. This is a resu 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 wa l l s . (C) A i r Ci rcu la t ion Obtaining uniform a i r flow ve loc i ty 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 plat form manifold described on page 68 was great ly modified as a resu l t of the measurements described in th i s chapter. The i n i t i a l a i r flow patterns were as described in Figure 5-3. Two main problems were evident. The f i r s t was that a i r enter ing the plant p lat form manifold continued through the d i f f u s i n g system in to the chamber creat ing a ve loc i ty p r o f i l e as in Figure 5-3. The resu l t was a tremendous ve loc i ty gradient across the chamber, from 10 fpm on the entrance side to 200 fpm at the end of the plenum. Af te r considerable experimentation, a series of 150 ba f f l e s t r i ps posit ioned by hand produced an a i r flow in the plant growth area of sa t i s fac to ry un i fo rmi ty . Measurements at four , s i x , e igh t , twelve, s ix teen, and twenty inches from the plat form surface indicated an average a i r ve loc i ty of 75 fpm ± 5 fpm on the undamped mode o f the Flow Corporation 84 Fig. 5-'3. Original nonuniform a i r f low pattern in upper chamber of laboratory system (scale 1/8" = 1") 85 Model 55B hot-wire anemometer. This was accomplished only under experimental condit ions and would not have permitted the growth o f plants in the chamber due to the obstruct ions in the manifold. The second problem with the basic design was the asymmetrical duct from the post-condi t ioning plenum to the plant plat form manifold (Figure 5 -4 ) . Special baf f les were required to balance the a i r flow across the duct as i t entered the plat form manifold. The asymmetrical duct was required since the cool ing co i l was narrower than the chamber. As a resu l t o f the above d i f f i c u l t y in achieving uniform a i r f low, 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 condi t ioning system is drawn from the laboratory, and since these tests were made at below ambient temperatures, the humidity in the system ran consis tent ly higher than the humidity in the laboratory. The laboratory humidity was usually between 40 and 45% re la t i ve humidity wi th 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 f lowing through the cool ing c o i l . Like commercial growth chambers, the average level of humidity in the upper chamber in i t s present stage of development is completely dependent upon ambient condi t ions. However, the absence o f heating and cooling cycles in the laboratory chamber prevents the cyc l ing of humidity that occurs in commercial chambers. 86 TOP VIEW C00UN6 0 F UPPER CHAMBER DUCT F i g . 5-'4 Asymmetrical duct from cool ing co i l to 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 sa t i s f i es the qua l i t y of temperature control out l ined in the Preface. The technique of modulating c o n t r o l , rather than On-Off control el iminates temperature and humidity f luc tuat ions due to heating and cool ing cycles. The l i g h t i n tens i t y measurements indicate that a higher l i g h t i n tens i t y was avai lable in the laboratory system than in the PGC-78, but that both hor izontal and ver t i ca l gradients s t i l l e x i s t . Further improve-ments could resu l t from the use of 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 tens 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. Af te r considerable experimentation, sa t i s fac to ry a i r flow condit ions were achieved in the laboratory system. The f i n a l design provided a uni formity of a i r f low which was bet ter than the ±10 fpm var ia t ion o r i g i n a l l y spec i f ied . However, the use o f many baf f les to achieve th is uni formity is an adequate arrangement fo r one flow rate on ly , and a more adjustable arrangement would be desi rable. The control o f humidity was not attempted in the laboratory system mainly because of expense and because o f inexperience with humidity control mechanisms. Ef fect ive humidity contol requires sa t is fac to ry temperature control and th is was achieved in the present system. A saturate and reheat a i r condi t ioning system could be adapted to the ex is t ing design and would probably provide excel lent humidity c o n t r o l . 88 SUMMARY This research has led to the construct ion o f prototype growth chambers which incorporate the nu t r ien t mist technique and which provide improved environmental control in comparison with a commercially avai lable system. The development of an a r t i f i c i a l environment fo r c r i t i c a l studies of the physiology of plant growth is a complex problem. The design of an a r t i f i c i a l l i g h t source fo r a plant growth chamber requires consideration of the character is t ics of the lamps used, the ref lect iveness of the chamber i n t e r i o r , and the inf luence of the lamp canopy and chamber geometry on the horizontal and ver t i ca l l i g h t i n tens i t y p r o f i l e s . In a small chamber, the wa l l s , whether r e f l e c t i v e or no t , are the main cause of hor izontal and ver t i ca l l i g h t i n tens i t y gradients ( in the absence of p l a n t s ) . Temperature control systems fo r plant growth chambers must be engineered to provide precise control over a much wider range of condit ions than are generally encountered in conventional a i r condi t ion ing. Sol id 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 capab i l i t i es fo r a r t i f i c i a l p lant growth envi ronments. Variat ions in r e l a t i v e humidity are inherent in a r t i f i c i a l p lant growth environments using evaporative cool ing re f r i ge ra t i on systems. The hot-gas bypass technique assists in reducing humidity . f l uc tua t ions . 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 is dependent on the character is t ics of 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 ow at 80 ± 10 fpm across the plant growth area. 89 Instruments used fo r the measurement and ind icat ion of environmental variables must be s tab le , rapid in response, and must not a f fec t the environment being measured. A t rave l ing 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 rap id , reproducible remote measurement of the p ro f i l es in plant growth chambers. The Percival Model PGC-78 growth chamber contained large gradients in environmental condit ions and did not conform e n t i r e l y to advertised spec i f i ca t ions . The speci f icat ions were considered to be unsat isfactory in extent and could be misleading to an operator. The laboratory growth chambers constructed during th is research produced more uniform environmental condit ions than did the PGC-78, and s a t i s f i e d many o f the or ig ina l c r i t e r i a fo r a sa t i s fac to ry a r t i f i c i a l environment system. The chambers were p a r t i c u l a r l y e f fec t i ve with respect to the uni formi ty of temperature and a i r f low. In a small chamber, water jacketted walls and a modulated a i r condi t ioning system that lacks heating and cooling cycles can be used to provide temperature control to w i th in ± 1/4°C of a control po in t . Nevertheless, addi t ional improvements in the control and uni formi ty of l i g h t i n t e n s i t y , and in the regulat ion of humidi ty, are required to improve the effect iveness of the chambers in plant research. 90 LITERATURE CITED 1. Bul ley, N.R., Nelson, C D . and Tregunna, E.B. 1969. Photosynthesis: Action spectra fo r leaves in normal and low oxygen. Plant Physio l . 45: 89-101. 2. Cooper, J.P. and N.M. Tainton. 1968. Light and temperature requirements of t rop ica l and temperate grasses. Herb.Abstr. 38: 167-176. 3. Control led Environments L td . 1969. Growth chamber design. Technical Bu l le t in No. 70. Winnipeg, Manitoba. 4 . Doebelin, E.O. 1966. Measurement systems: Appl icat ion and desiqn. McGraw-Hill, New York. tfp. 502. 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. Technical Manual No. T P - I l l . 8. General E l e c t r i c . 1960. Plant Growth L igh t ing . Technical Manual No. TP-127. 9. General E l e c t r i c . 1961. Incandescent lamps. Technical Manual No. TP-110. 10. 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. 11 . Hewit t , E.J. 1966. Sand and water cul ture methods used in the study of 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 Hor t icu l ture and Plantat ion Crops, East Mai l ing , Maidstone, Kent. ^ p . 302. 12. Ka lb f le i sh , W. 1963. Requirements of environmental f a c i l i t i e s fo r the growth of p lan ts . Engineering Research Service, Research Branch, Can.Dept.Ag., Ottawa, Canada. 13. Ka lb f le i sh , W. 1963. A r t i f i c i a l l i g h t fo r plant growth. Engineering Research Branch, Can.Dept.Ag., Ottawa, Canada. 14. ' M a r t i n , E.V. and F.E. Clements. 1936. Signif icance of t r ansp i ra t i on . Plant Physiol . 9: 165-172. 15. Percival Manufacturing Co. 1967. Refr igerat ion System. Bu l le t i n 2-B. 16. Rohm and Haas Co. 1967. Plexiglass Desiqn Guide. 17. Sal isbury, 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. 18. Thre lke ld, J .L . 1970. Thermal environmental engineering. Prent ice-Hall I n c . , Englewood C l i f f s , N.J. 07632. pp. 205-256. 

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