142 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 1 



film in the vicinity of the saturation point often exceeds 1,000; 

 apparent dielectric constants as high as 8,000 have been observed. 

 The effect is explained by assuming a structure of the aluminum 

 oxide layer like that shown in Fig. (1-7)6. The deposition of water on 

 the inside walls of the tubular pores leads to a reduction of the resist- 

 ance E 1 and to an apparent increase of capacitance between the 

 applied electrodes. At high degrees of humidity, the capacitance is 



Porous cond. 

 layer ■ 



Ai oxide ■ 



Aluminum 



[a) 



lb) 



Fig. (1-7)6. Humidity transducer based upon the 

 variations with humidity of the dielectric properties 

 of aluminum oxide: («) schematic diagram; (b) equivalent 

 circuit [from C. L. Cutting, A. C. Jason, and J. L. Wood, 

 J. Sci. Instr., 32, 425 (1955); by permission}. 



-Aluminum rod 

 -Aluminum oxide 



'Porous conducting 

 layer 



Insulating 

 layer 



essentially that between the pore bases P and the aluminum on which 

 the oxide layer is formed. 



A practical form of the transducer is shown in Fig. (1-7)7. It con- 

 sists of an aluminum rod which is oxidized over a part of its surface. 



The rod serves as one electrode of 

 the probe ; a porous conducting layer 

 (graphite or a thin evaporated metal 

 layer) applied to the outside of the 

 alumina serves as the other elec- 

 trode. The probe behaves electri- 

 cally like a parallel combination of 

 a resistance and a capacitance. A 

 typical calibration curve is shown 

 in Fig. (1-7)8. The transducer can be 

 used for a range of relative humidity 

 from to 100 per cent. The sensi- 

 tivity (slope of the characteristic) decreases near the saturation 

 point. 



The error of the method is estimated to be about 3 per cent. The 

 dynamic response to a sudden variation of ambient humidity depends 

 upon the magnitude and direction of such a variation. Equilibrium 



Fig. (1-7)7. Construction of an alu- 

 minum oxide humidity transducer 

 [C L. Cutting, A. C. Jason, and J. L. 

 Wood, J. Sci. Instr., 32, 425 (1955); 

 by permission]. 



