Sec. 1-7] MECHANICAL INPUT TRANSDUCERS 141 



error that does not exceed il.5 per cent are commercially available. 

 Variations of relative humidity as small as 0.15 per cent can be 

 detected. The calibration curve is nonlinear; empirical calibration 

 is required. 



The speed of response is high. If exposed to an abrupt, large 

 change of humidity at room temperature, the element will assume 63 

 per cent of its equilibrium resistance within several seconds. But the 

 speed of response depends upon the velocity of the air current. 1 It 

 also depends upon the magnitude of the humidity change and upon 

 the temperature. The resistance changes faster when the humidity 

 varies from a low to a higher value than it does in the opposite 

 direction. The response at low temperature is slower than at high 

 temperature. 



Transducer systems with very fast response have been described 

 by Wexler and Krinsky. 2 Such systems consist of a thin layer of 

 potassium dihydrogen phosphate, which is deposited on glass by 

 evaporation in vacuum and which is converted into potassium meta- 

 phosphate. If exposed to a humidity change from S3. 3 to 100 per 

 cent, these transducers will reach 63 per cent of the final resistance in 

 less than J sec at room temperature and in about 10 sec at — 20°(!. 

 The response time of this transducer is decreased by a factor of about 

 10 to 30 as compared to the lithium chloride transducer. 



An electrical hygrometer based upon the variations with hu nudity 

 of the electrical properties of anodized aluminium oxide layers has 

 been suggested by Ansbacher and has been investigated and de- 

 scribed by Cutting, Jason, and Wood. 3 



The transducer consists essentially of a capacitor having as a 

 dielectric a porous film of aluminum oxide. At least one capacitor 

 plate should be permeable for the water vapor to permit the pene- 

 tration of humidity to the aluminum oxide. Both the capacitance 

 and the resistance of such a capacitor change in response to the 

 ambient humidity. 



The porous oxide layer is formed on aluminum by anodization in 

 acid electrolyte (17.5 volume per cent H 2 S0 4 , current density 10 to 

 100 mA/cm 2 , 30 min). The apparent dielectric constant of such a 



1 Gliickauf, ibid., p. .'544. 



2 A. Wexler and A. Krinsky, \'atl. Bur. Standard* Tech. News Bull., June, 

 1954; also A. Wexler et al., ./. Research Natl. Bur. Standards, 55, 71 (lit.")")). 



3 C. L. Cutting, A. C. Jason, arid J. J,. Wood, J. Sci. Instr., 32, 425 ( 1 955); see 

 also C.R. Underwood and It. ( '. Honslip,./. »SW. Instr., 32, 432 ( 1 955). The varia- 

 tion with humidity of the dielect rical properl ies of aluminum oxide has been 

 observed formerly by A. V. Ast in, ./. Research Natl. Bur. Standards, 22, 690 

 (1939). 



