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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.
To meet an urgent demand for a hygrometer capable of use at all heights in the atmosphere, the dew-point hygrometer, which is shown to have many advantages, has been developed primarily for use in aircraft to measure dew-points, or rather frost-points, down to -90° c.
It is necessary that the instrument should operate at the lowest possible frost-points as it has been discovered that the air in the stratosphere is very dry Laboratory studies of the deposition of water and ice from the vapour at low temperature are described.
Below -90° c. it is not possible to operate a frost-point hygrometer because the deposit is in the form of an invisible glassy layer, but the instrument gives correct results at temperatures close to this limit.
Details are given of the construction of different forms of hand-operated hygrometers, and work is now going on to develop a fully automatic frost-point hygrometer.
Incluye bibliografía e Indice
The relation between relative humidity and wet-and-dry-bulb readings, at dry-bulb temperatures between 40° and 100°C., has been studied experimentally. The relative humidity was measured both by absorbing and weighing the moisture in a known volume of air, and by the dew-point method. The results are finally expressed by means of a skeleton table for the wet-and-dry-bulb hygrometer in the temperature range referred to.