140 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 1 



which are wound in the form of two parallel helices. A hygroscopic 

 coating J I consisting of polyvinyl acetate or polyvinyl alcohol with a 

 dilute lithium chloride solution is applied to the surface of the insu- 

 lating carrier and makes contact with the wires. The resistance 

 between the wires is measured with an a-c method to avoid polari- 

 zation. The resistance decreases with increased humidity in a 

 function approximately proportional with the logarithm of the rela- 

 tive humidity. A calibration characteristic is shown in Fig. (1-7)5. 



10' 



10* 



o 



10 ! 



10' 



20 40 60 80 



Relative humidity, % 



100 



Fig. (1-7)5. Calibration curves of the humidity 

 transducer illustrated in Fig. (1-7)4. 



In a practical sense, the useful resistance variation of such a trans- 

 ducer element extends only over a limited range of relative humidity, 

 e.g., from 20 to 35 per cent; transducer elements for different ranges 

 can be made by using different concentrations of the electrolyte in 

 the hygroscopic layer. Several transducer elements are needed to 

 cover the total range from 2 to 99 per cent of relative humidity. 

 These transducers must either be exchanged for the measurements 

 in different ranges or used in a resistive network to cover the entire 

 range. 



The resistance of a single transducer varies from about 10 4 to 10 7 

 ohms. The resistance at constant humidity depends strongly upon 

 the temperature, so that either operation at constant temperature or 

 temperature effect compensation is required. A method of tempera- 

 ture compensation is shown by Gliickauf. 1 



Transducers of the described type are accurate and reproducible 

 to within ±2 to ±2.5 per cent; special sensing elements with an 



1 E. Gliickauf, Proc. Phys. Soc. (London), 59, 357 (1947). 



