The Whirling Psychrometer is an instrument for the measurement of the temperature and humidity of the air. It consists of two thermometers exposed side by side. One is an ordinary dry bulb thermometer and the other wet bulb thermometer whose bulb is surrounded by a piece of thin muslin kept moist with distilled water. The thermometer mount can be rotated with a handle to ensure draught if air across the thermometer bulb. The temperature is measured when the equilibrium condition is reached. From a knowledge of dry bulb reading and wet bulb reading, the dew point temperature, relative humidity and vapour pressure can be calculated using appropriate formulae.
