Sec. 1-7] 



MECHANICAL INPUT TRANSDUCERS 



139 



C-* 



magnitudes, e.g., from 10 8 to 10 4 ohms. This fact makes it some- 

 times difficult to find an electric system capable of measuring and 

 indicating the entire humidity range on one scale. The measuring cur- 

 rent should be small so as not to cause an undue heating of the probe. 



Several other difficulties are likely to arise in the construction and 

 the use of such transducers. The effect of water absorption as well as 

 that of dissociation depends strongly upon temperature. The humid- 

 ity-temperature relationship of different salts has been studied by 

 Wexler and Hasegawa. 1 



If the hygroscopic body is thick, it will take a long time for it to 

 come to equilibrium with the surrounding atmosphere; thus the 

 dynamic response is poor and the indication of the system will show a 

 lag, sometimes of several hours. If the hygroscopic body is thin, so 

 that the physical process is limited 

 to the surface or near-surface layer, 

 the system is easily affected by sur- 

 face contamination and mechanical 

 injury, and the measuring results 

 may become nonreproducible. The 

 sensing elements should be protected, 

 therefore, from exposure to 100 per 

 cent humidity, because the conden- 

 sation water is likely to damage the 



vulnerable surface. However, Verzar, Keith, and Parchet 2 have used 

 a 2 per cent solution of sodium polyacrylate. Transducers made 

 with this materia] are resistant to the formation of water droplets; 

 their highest sensitivity is in the range between 80 and 100 per cent 

 relative humidity. 



Probably the most successful humidity transducer as far as ac- 

 curacy, speed of response, and stability are concerned has been 

 described by F. W. Dunmore and coworkers. 



1 H. Diamond, W. S. Hinman, Jr., and F. W. Dunmore, J. Research Natl. 

 Bur. Standards, 20, 369 (1938); F. W. Dunmore, J. Research Natl. Bur. 

 Standards, 20, 723 (1938); F. W. Dunmore, Bull. Am. Meteorol. Soc, 19 (6), 

 225 (1938); F. W. Dunmore, ./. Research Natl. Bur. Standards, 23, 701 (1939); 

 H. Diamond, W. S. Hinman, F. W. Dunmore, and E. G. Lapham, J. Research 

 Natl. Bur. Standards, 25, 327 (1940). 



The instrument is illustrated in Fig. (1-7)4. It contains an insulat- 

 ing carrier C (polystyrene) supporting two wires W of noble metal 



Fig. (1-7)4. Resistive humidity 

 transducer; C, insulating carrier; W, 

 wires; H, hygroscopic coating. 



1 A. Wexler and S. Hasegawa, ./. Research Natl. Bur. Standards, 53, 19 (1954). 



2 F. Verzar, J. Keith, and V. Parchet, Arch. ges. Physiol., Pfliigers, 257, 

 400 (1953). 



