14S INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 1 



pressure of 0.07 per cent; tests at low vapor pressure (order of 0.2 mb) 

 an error of slightly more than 1 per cent. In terms of dew-point 

 measurements, the accuracy is better than 0.2°C in the range from 

 to 40°C, and better than 0.5°C in the range from —40 to 0°C. 



The response time is limited by the flow rate of the air through the 

 cavities. The time constant of the hygrometer (time to reach 63 per 

 cent of the final value) is approximately 10 sec. In 15 sec the re- 

 corded trace reaches 90 per cent of its final value. 



The wide range and the accuracy and sensitivity of this method are 

 reached by no other system. The hygrometer is suitable as a (second- 

 ary) standard for water-vapor pressure measurements, but the tech- 

 nical requirements are considerable. 



A microwave refractometer of somewhat different construction has also been 

 described by C. M. Crain, Phys. Rev., 74, 691 (1948), and Rev. Sci. Instr., 21, 

 456 (1950); C. M. Crain and A. P. Deane, Rev. Sci. Instr., 23, 149 (1952). 



1-73. Systems for Humidity Determination in Liquids or 

 Solids 



a. Dielectrical Systems. Both the resistivity and the dielectric 

 constant of dielectric liquids and solids vary with their water content. 

 Increased humidity in these substances usually has the effect of 

 decreasing their resistivity and increasing their average dielectric 

 constant. The dielectric constant of mixtures of pure dielectric 

 liquids and water can be computed from a simple mixing rule. The 

 variation with humidity of the electrical properties of more complex 

 materials, in particular those of organic origin, can only rarely be 

 found by computation. Electrical moisture determination in such 

 substances is usually based on empirical calibration procedures. 

 Even so, the results of measurements may vary strongly from one 

 specimen to another, and frequently they depend upon the concen- 

 tration variation of impurities. Small amounts of dissociable salts 

 can change the resistivity of a substance at a given moisture content 

 by large factors. 



Despite this difficulty, electrical moisture determination has found 

 many, in particular industrial, applications for materials such as 

 minerals, sand, coal, oil, salt, soap, food and dehydrated food prod- 

 ucts, coffee, tobacco, grain, flour, starch, wood, pulp and paper, 

 fibrous materials, and textiles. 



An example for the resistance variation with humidity in solids is 

 shown in Fig. (1-7)13, in this case for wood. The resistance decreases 

 nearly exponentially with increased moisture content. However, the 

 calibration is different for different types of wood, and even for the 



