146 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 1 



Invar 



Fig. (1-7)11. Capacitive humidity 

 transducer [from P. Webb and M. K. 

 Neugebauer, Rev. Sci. Instr., 25, 1212 

 (1954); by permission]. 



most practical cases, the temperature of the gas mixture must also be 

 kept constant; otherwise a variation of the temperature causes an 

 error of 0.3 per cent/deg. The instrument cannot be used for gases 

 that interact with phosphorous pentoxide. 



1-72. Dielectric Systems 



a. Capacitive System. The high dielectric constant of water {< — '80) 

 suggests the use of capacitive methods and transducers for the deter- 

 mination of humidity in gases. 

 However, the low concentration of 

 water in air, even at saturation, 

 causes only a small variation of 

 capacitance. The dielectric con- 

 stant of gases under normal con- 

 ditions changes with a variation of 

 water content only in the fourth 

 and fifth decimal position (e.g., di- 

 electric constant of dry air at 45°C 

 is 1.000247; at saturation it is 

 1.000593). Therefore, considerable 



experimental means are required for the detection and measurement 

 of such small capacitance variations. 



A capacitive system for the determination of humidity in air has 

 been described by Webb and Neugebauer. 1 The air flows through a 

 capacitor, illustrated in Fig. (1-7)11, consisting of two concentric 

 cylinders. The capacitor is thermally insulated on the outside and 

 slightly heated. The change of capacitance in response to the varia- 

 tion of water vapor in the air stream causes a frequency variation of 

 an oscillator operated at about 2 Mc, which is measured with a beat- 

 frequency method. The system detects changes of 1 mg water vapor 

 per liter of air. The dynamic response is fast and limited only bj^ the 

 flushing time; 50 per cent of the final response is obtained in 0. 1 sec. 



b. Operation at Microwave Frequencies {Microwave Ref Tactometer) . 

 The most accurate method for the determination of moisture in gases 

 is the microwave refractometer. An instrument of this type is de- 

 scribed by Sargent. 2 The method is based upon earlier work. 3 



The system is essentially an arrangement in which the resonance 



1 P. Webb and M. K. Neugebauer, Rev. Sci. Instr., 25, 1212 (1954). 



2 J. Sargent, Natl. Bur. Standards Rept. 4257, June, 1955. 



• 3 G. Birnbaum, S. J. Kryder, and H. Lyons, J. Appl. Physics, 22, 95 (1951); 

 G. Birnbaum and S. K. Chatterje, J. Appl. Physics, 23, 220 (1952); H. E. 

 Bussey and G. Birnbaum, J. Research Natl. Bur. Standards, 51, 171 (1953). 



