Sec. 1-7] MECHANICAL INPUT TRANSDUCERS 137 



condensation of water vapor on a cooled mirror surface causes a 

 change of light reflection, which is detected photoelectrical ly, or the 

 condensation changes the resistance between two metal electrodes. 



Other indirect systems have been constructed from hygrometers 

 made of dimensionally varying materials, such as the hair hygrom- 

 eter in combination with displacement transducers. Some systems 

 of this type are reviewed by Wexler. 1 Humidity can also be found 

 from optical measurements, e.g., infrared absorption in connection 

 with photoelectric devices. 



The moisture content in dielectric liquids and solids changes their 

 dielectric properties, such as the resistivity and the dielectric con- 

 stant. Methods for moisture determination in solids, based upon the 

 variation with humidity of these electric properties, are described in 

 l-73a. Another method for moisture determination by means of 

 nuclear magnetic resonance measurements is described in 1-736. 



The humidity in solids can sometimes be determined by forming 

 a closed gas space adjacent to the solid. The gas is allowed to come 

 into humidity equilibrium with the solid, and the humidity of the 

 gas is then measured. In some cases, humidity in solids can be 

 determined by means of the beta gauge (1-16). 



For general information see A. Wexler and W. B. Brombacher, Methods of 

 Measuring Humidity and Testing Hygrometers, Natl. Bur. Standards Circ. 512, 

 September, 1951; Wexler, loc. cit. 



1-71. Resistive Teansducers Responding to Humidity in 

 Gases 



a. Resistance-variation Systems. Most resistive humidity trans- 

 ducers consist essentially of a hygroscopic body which absorbs water 

 from the surrounding atmosphere and which contains a salt that 

 dissociates in the presence of water. The concentration of dissociated 

 ions is determined by the measurement of the electric resistance 

 between electrodes applied to the test specimen. 



An example of a resistive humidity transducer is shown in Fig. 

 (1-7)1. An insulating plate carries two strips of metal; the space 

 between the strips is coated with a thin layer of a hygroscopic mater- 

 ial. Figure (1-7)2 shows a similar transducer; the arrangement with 

 the interdigitating metal strips has the effect of reducing the resist- 

 ance between the metal electrodes. Moreover, small local injuries 

 of the resistive layer do not greatly affect the results. A picture of a 

 commercial humidity transducer of this type is shown in Fig. (1-7)3. 



1 A. Wexler, Electric Hygrometers, Natl. Bur. Standards Circ. 586, sec. 5, 

 p. 13, September, 1957. 



