136 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 1 



1-7. Humidity Transducers 



The field is divided into two groups: systems for humidity deter- 

 mination in gases (1-71 and 1-72) and systems for measurements in 

 liquids or solids (1-73). 



One group of direct-acting transducer systems that furnish an 

 electric output in response to the humidity of a surrounding gas 

 atmosphere is based upon the absorption of humidity in a probe, 

 causing dissociation of molecules into ions. Electrolytic conduction 

 takes place; the resistance of the probe is a function of the humidity 

 (1-7 la). An empirical calibration is usualfy required. However, two 

 methods have been described which permit the absolute determina- 

 tion of the absorbed water in the probe (1-716 and c). 



A second group of transducers is based upon the variation of the 

 dielectric constant of a gas-water vapor mixture (1-72). The dielec- 

 tric constant of dry gases is usually not much different from 1 , while 

 that of water is much higher. Thus, the concentration of water vapor 

 in a gas may be detected from the measurement of capacitance 

 (l-72a) or the resonance frequency of a cavity (l-12b). 



The humidity in gases can also be determined by measuring the 

 thermal conductivity of the gas-water vapor mixture, 1 the mobility 

 of ions, 2 or the absorption of nuclear radiation (see Radiation Thick- 

 ness Gauges, 1-16). A method for the measurement of very small 

 degrees of relative humidity (0 to 2 per cent) in air at reduced pressure 

 is described by Hinzpeter and Meier. 3 The method is based on the 

 variation with humidity of the cathode fall and the breakdown 

 voltage in electric gas discharges and is primarily applicable to humid- 

 ity determination during freeze drying. 



There exist also a number of indirect systems for the determination 

 of humidity in gases. One of these is the wet-and-dry bulb psychrom- 

 eter with electrical readout (the liquid thermometer being replaced 

 by resistance thermometers, thermistors, or thermoelements). In- 

 formation on such systems may be found in Guthmann; 4 another 

 indirect method, the dew-point hygrometer with electrical readout, 

 is described in a reviewing paper by Czepek. 5 The formation of the 



1 C. Z. Rosecrans, Ind. Eng. Chem. Anal. Ed., 2, 129 (1930). 



2 E. Griffiths and J. H. Awbery, Proc. Physic. Soc. (London), 41, 240 (1928). 



3 A. R. Hinzpeter and W. Meier, Z. angew. Physik, 3, 216 (1951). 



4 K. Guthmann, Arch. tech. Messen, V 1283-3, September, 1932, and V 1283- 

 11, October, 1953; also Arch. tech. Messen, V 1283-2, 1932, and V 1283-4, 

 July, 1933. 



5 R. Czepek, Arch. tech. Messen, V 1283-6, August, 1940; see also C. W. 

 Thornthwaite and J. C. Owen, Monthly Weather Rev., 68, 315 (1940); C. W. 

 Sisco, Instrumentation, 2, 14 (July-August, 1947), and Guthmann, loc. cit. 



