Wet-Bulb Temperature: From the Psychrometer to the Modern Sensor

Wet-bulb temperature is the temperature to which a wet surface can be cooled by evaporation. The difference between the dry-bulb temperature (the ambient air temperature) and the wet-bulb temperature indicates the environment's humidity level: the smaller the difference, the higher the humidity; the larger the difference, the drier the air.

A psychrometer uses two thermometers: a dry-bulb (T1), which measures air temperature directly, and a wet-bulb (T2), covered with a damp cloth cooled by evaporating water.

In a typical demonstration, the wet-bulb depression exceeded 6°C, with readings stabilizing at T1 = 22.84°C (dry-bulb) and T2 = 16.56°C (wet-bulb). That temperature differential yielded a calculated relative humidity of 52.63% for that setup.

The method has three inherent error sources: it requires sufficient air circulation, needs constant monitoring and refilling of the wick's water, and is vulnerable to contamination from salts and solids. In practice, the test psychrometer showed a deviation of around 10% RH from calibrated reference instruments.

Vaisala's HUMICAP® sensor technology computes wet-bulb temperature from the measured relative humidity and temperature, eliminating psychrometer maintenance. Most Vaisala humidity and temperature instruments can output humidity directly as wet-bulb temperature, with an accuracy better than ±0.5°C across the full RH range, and with reliable performance even in condensing environments.

In cooling towers, wet-bulb temperature guides operating efficiency: when humidity rises and the wet-bulb approaches ambient temperature, there's no point running the towers because their cooling effect will be negligible.

In frost protection, agricultural sprinkler systems use this variable: as long as the wet-bulb temperature stays above 0°C, there's less risk of frost damage.