Sec. 1-7] 



MECHANICAL IX PUT TRANSDUCERS 



143 



10 7 



10 E 



h"10- 





10 ; 







100 



is established, on the average, in 10 to 100 sec. The response at 

 humidities higher than 80 per cent is slower, and also a slight hyster- 

 esis effect is noticeable at high degrees of relative humidity. An 

 aging effect occurs during the first few months after the formation of 

 the oxide layer, resulting in a 

 reduction of the capacitance and 

 an increase of resistance at any 

 given humidity. 



The transducer output is a 

 function of the ambient relative 

 humidity only. In the tempera- 

 ture range between — 15 and 

 + 80°C, the readings are practi- 

 cally independent of tempera- 

 ture. Changes of capacitance and 

 resistance with variation of am- 

 bient relative humidity can be 

 observed at temperatures above 

 1 00°C (even as high as 400°C), but 

 the exposure to such high tem- 

 peratures can cause an irrevers- 

 ible change of the characteristic. 



b. Thermal System (Dewcel 1 ). Although similar in appearance to 

 the preceding methods, this system operates on a different principle 

 and permits an absolute determination of humidity. The element is 

 illustrated in Fig. (1-7)9. A tubular wick made from glass fibers and 



impregnated with a hygroscopic salt 

 (lithium chloride) is mounted over a 

 thin-walled metal tube T electrically 

 insulated from the tube. Two parallel 

 silver wires S 1 and# 2 are helically wound 

 around the wick and connected to an 

 alternating-voltage source E s through 

 a current-limiting resistor R s . The lith- 

 ium chloride absorbs humidity from the 

 surrounding air and becomes conduc- 

 tive, so that a current passes from one 

 silver wire through the lithium chloride layer to the other silver 

 wire. The current generates heat in the lithium chloride layer and 

 tends to evaporate the humidity from it, a process which leads to an 

 increase of resistance and a reduction of the current. An equilibrium 

 1 Trade name, Foxboro Company, Foxboro, Mass. 



20 40 60 80 



Relative humidity, % 



Fig. (1-7)8. Capacitance and resistance 

 of an aluminum oxide humidity trans- 

 ducer plotted against the relative humid- 

 ity of the surrounding atmosphere [C. L. 

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

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





Fig. (1-7)9. Humidity trans- 

 ducer, thermal system. 



