BIBLIOGRAPHY 



Humidifiers 



Air is not efficiently humidified by being blown over water; it can be most 

 effectively humidified by bubbling through an immersed sinter-glass funnel 

 or by being blown through meshes across which water is flowing. It is 

 essential to use distilled water. Steam injection involves great quantities of 

 heat release from condensation; the essential to accept is that it is useless 

 merely to give air the opportunity to take up water — it must be humidified. 



Driers 



Apart from refrigeration condensation drying, which has every kind of 

 disadvantage, about the only available drying process is chemical adsorption, 

 and silica gel is the only readily recoverable chemical drier. It is quite 

 sufficient for biological work and gives an excellent indication of its active 

 life when impregnated with cobalt salts. 



Control 



Humidistats using every variety of element and principle are in commerical 

 use. Typically, a mechanical element may be used such that a change in its 

 length operates a pair of contacts, thus energizing a moisture, or drying, 

 source through a relay. The nature of the time constant of these systems is 

 very variable, but is typically long; they have great hunting instincts. 

 Wherever possible mark-to-space systems, utilizing the humidity equivalent 

 of the temperature regulating circuits shown in Chapter 29, should be used. 



Capacity methods 



Commercial moisture-measuring devices exist, basing their measurements 

 on the relation between capacity and moisture content. They have excellent 

 applications to the routine analysis of uniform samples, such as of grain, 

 fabric, etc., but of course, a change in every constituent of air, and also both 

 its temperature and pressure, will also alter its dielectric constant. It is 

 therefore not a reliable method of measuring the humidity of an air sample 

 which may vary in respect of temperature, barometric pressure, carbon 

 dioxide, or other gas content. 



Conclusion 



This account of humidity measurement and control is necessarily brief 

 because of the paucity of research and development in it in biological work, 

 coupled with the technical difficulties and expense of such projects. Biolo- 

 gists are sincerely advised to regard all measurements of humidity with sus- 

 picion, to enter the field of humidity control as though it were at least the 

 eighth circle of the Inferno, and to realize that however near to a living surface 

 they may attempt to measure or control, the living surface will always have 

 the last word. 



BIBLIOGRAPHY 



For references to the authoritative v^ork of Professor H. S. Gregory in this field, 



see: 



Gregory H. S. The measurement of humidity Instrument Practice (1947) 



— and RouRKE, E. Trans. Soc. Inst. Tech. 4 (1952) 



417 



