An investigation of the wet-and-dry-bulb hygrometer at low temperatures

Abstract

The humidity corresponding to various wet-and-dry-bulb temperatures between -2° C. and -19° C. has been measured. The wet-bulb temperatures were obtained by means of mercury thermometers, thermocouples and resistance thermometers, all three methods agreeing well. The actual humidity was generally obtained from a specially designed dew-point apparatus, thermojunctions being used to measure the temperature of the metal surface on which the deposit of dew was formed. In addition, tests by the gravimetric method were carried out. The results are utilized to prepare a table giving the relative humidity at various wet-and-dry-bulb temperatures. These agree, for the most part, with previous tables, but they differ in the region of low dry-bulb temperatures (below about -9° C.) and small wet-bulb depressions (less than 1° C.). The results are also examined from the point of view of the usual theory which asserts that (e'−e) = B₀P (θ − θ'), where e' is the saturation vapour pressure at the wet-bulb temperature θ', e the saturation vapour pressure at the dew point (i.e. the actual partial pressure of water in the air), θ the dry-bulb temperature, P the barometric pressure and B₀ the psychrometric constant. Although above 0° C. this formula is found to hold well, yet below 0° C. it is found that B₀ varies in a complicated manner with the temperatures. It tends to infinity when the wet-bulb depression tends to zero, and there is a further variation superimposed on this one; at a constant value of the depression, B₀ passes through a minimum and then through a maximum as the dry-bulb temperature decreases.