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The dissipation factor method of ascertaining the moisture content of newsprint Chu, Gan Dick

Abstract

Ever since newsprint was made on a mass production basis, there has been a real need for a simple and instantaneous measurement of the moisture content of the moving sheet. A knowledge of the moisture content is important both economically and technically to the mill operator. As newsprint is sold by weight, the moisture content of the paper must be maintained within a narrow specified range. The mill, naturally, strives to produce paper with as high a percentage of moisture as permissible. Without any scientific means to guide them, however, the machine operators tend to over dry the paper because an over dried sheet is not nearly so noticeable as one that is too moist. This means that less paper is being made than is practically possible for each cord of wood processed. In addition, the over dried paper is of inferior quality to that containing the proper amount of moisture. At present, the only reliable method of measuring moisture content in Canadian mills is the laborious oven method which, though undoubtedly very accurate, has the great disadvantage of time lag. The recent development of the Q-meter offers a method for the rapid measurement of the moisture content of the moving sheet by the dissipation factor method which has the advantage that no contact with the paper is required. The fact that the dielectric constant of water is very high compared to that of cellulose suggests that the dielectric losses in moist paper could be used to measure its moisture content. Tests were therefore conducted in the laboratory with a Boonton Q-Meter, type 160-A, to determine the dissipation factor of newsprint samples of various moisture contents. A pair of plates with the sample of newsprint between them, but not touching, constitutes the test condenser. The dissipation factor of this condenser depends largely upon the amount of moisture contained in the newsprint. To measure the dissipation factor, the test condenser is tuned to resonance with a high-Q inductor. The amplitude of resonance depends on its Q value which in turn is largely a function of the condenser losses. Hence the dissipation factor may be calibrated against the percentage moisture content. Results of laboratory tests at different humidities and various frequencies showed that the percentage moisture content can be measured with adequate accuracy. The speed tests showed that the speed of the paper up to a velocity of 1800 feet per minute between the condenser plates has no effect on the readings. All these preliminary tests in the laboratory indicate that it is feasible to apply the Q-meter for measuring the moisture content of the moving sheet by the dissipation factor method. Exhaustive field tests under actual mill production conditions should be made over a period of time to compile sufficient data for a fair appraisal of the practical value of this method.

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