On the Psychrometric Constant

Abstract

The psychrometric formula is, in its simplest form, f=F-A(t-t')B, (1) in which t is the air temperature, t' the temperature of the wet-bulb-thermometer, f the tension of aqueous vapour in the air, F the vapour tension, saturated at t', B the barometric pressure and A the psychrometric constant. The psychrometric constant A is usually calculated by A=F-f/B(t-t'), (2) observing the actual vapour tension by a dew point hygrometer parallel with a psychrometer.
In the determination of A by Regnault Angot and others the difference of the saturated vapour tension of water and of ice was not taken into account. In the case when the wet-bulb was covered with ice the vapour tension of supercooled water was used for F in the above formula. This is by no means allowable. In the days of these authors the determination of ice vapour was not yet carried out, though Kirchhoff pointed out the existence of the difference between the saturated tension of water and of ice vapour. Recently Georgii proposed to use 600/680 A for A in the case under consideration, taking into account the difference of the latent heat of vaporization of water and of ice. But by this we can not get off the difficulty mentioned above.
In the present note it is proposed to make a new determination of A in the formulae (1) and (2) using the value of the vapour tension of ice precisely measured by Scheel and Heuse.