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MEASUREMENT AND CONTROL OF HUMIDITY 



J. W. L. BEAMENT 



INTRODUCTION 



The biologist is frequently interested in obtaining a relatively crude measure- 

 ment of air-moisture content; he may find occasional need to employ crude 

 control of the moisture in a plant-growing chamber or experimental enclosure, 

 which usually amounts to obtaining a 'high' humidity or literal saturation of 

 the air. However, the accurate assessment of air moisture content is an 

 exceptionally difficult — or laborious — business while its control to anything 

 like fine limits is therefore correspondingly difficult and usually has awkward 

 inter-actions with fine temperature control, which will naturally be a pre- 

 requisite of such an experimental device. 



UNITS 



Relative humidity (R.H.) is the percentage water in air of that which the air 

 would contain if saturated at that temperature. Saturation deficiency (s.d.) is 

 the difference between the saturation vapour pressure of water at a tempera- 

 ture and that which actually exists; it is usually expressed in mm Hg. This 

 latter is the most useful biological expression of humidity, for it indicates 

 both the 'evaporative power' and the 'carrying capacity' of air, and it is 

 further independent of temperature, whereas a percentage R.H. is only useful 

 information when the temperature is also quoted. 



MEASUREMENT 



Fundamental consideration 



A hygrometer may operate by indicating the rate of evaporation into the 

 air; in this case it is continuously increasing the humidity of an enclosed 

 space itself. Since evaporation automatically involves latent heat loss and 

 cooling, the hygrometer must properly be provided with a compensating 

 heater which exactly brings it back to ambient temperature — this would 

 constitute an excellent problem in feedback engineering. In addition, the 

 rate of evaporation will be entirely dependent on the rate of air-flow over the 

 element. 



A second method uses the dew-point principle : to cool a surface down to 

 the temperature at which the moisture content saturates the air and therefore 

 causes the surface to mist; the temperature difference and the true ambient 

 temperature then become transferable into humidity units by the use of 

 suitable tables. For conditions in which accurate temperature control is 



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