Wet (And Dry) Bulb Thermometer

Dictionary of Environmental Health

Used to measure RELATIVE HUMIDITY. It comprises two thermometers, one that measures the AMBIENT air temperature (the ‘dry bulb’) and another whose bulb is wrapped in gauze or absorbent material that is wet by water (the ‘wet bulb’). The reading of this latter thermometer is affected by the rate of evaporation of the water from the gauze, which itself is determined by the relative humidity. The drier the air (another way of saying the lower the relative humidity), the greater the rate of evaporation and hence the greater the loss of LATENT HEAT.

This is manifest as a lower temperature reading on the wet-bulb thermometer when compared to the dry-bulb thermometer reading. A chart is then used to translate the recorded difference into an equivalent relative humidity. At 100 per cent relative humidity there is no difference in the recorded temperatures between the two thermometers because no water can be lost by evaporation.

Simple wet-bulb thermometers rely on natural evaporation from the gauze, while more sophisticated and accurate instruments are equipped with a small fan that supplies a known air movement across the wet bulb. Static wet-bulb thermometers are not useful to measure rapid changes in relative humidity as they require time to stabilise; they should only be used therefore as an indication of the average relative humidity over time.

See also: WHIRLING HYGROMETER