Wet-bulb Temperature | Building Energy Analytics

In layman terms, when a wet wick is placed over a standard thermometer and air is blown across the surface, the water evaporates and cools the thermometer below the dry-bulb temperature. This coolest temperature achievable (called the wet-bulb temperature) depends on how much moisture is in the air.

In engineering terms, wet-bulb temperature (WBT) is the lowest temperature to which air can be cooled by the evaporation of water into the air at a constant pressure.

Finding Wet-Bulb Temperature

In ambient air situation, the heat required for evaporation is supplied by the air. This is called the heat of vaporization. During this heat removal process, the air is cooled. For engineers the wet-bulb temperature is found by three means:

1. Sling psychrometer - It consists of two glass thermometers containing a liquid, usually mercury. One thermometer measures the air temperature while the other one measures the wet-bulb temperatures.You can buy one from [ Amazon.com ]
2. Portable electronic psychrometer - A hygrometer is an instrument used for measuring the moisture content in the atmosphere. A device combining thermometer and hygrometer functions, described as psychrometer, is therefore giving both dry-bulb and wet-bulb temperatures.You can buy one from [ Amazon.com ] or [ Amazon.CN ].

   

   Portable electronic thermometer

   psychrometer

3. From your local observatory – Outdoor wet-bulb temperature is usually available online, such as on this web address for Hong Kong application:   Daily Normals of Meteorological Elements for Hong Kong, January 1981 - 2010. This Hong Kong Observatory web page provides figures represent daily mean values of the weather elements calculated from data in the 30 years from 1981 to 2010 for the 5-day period centred on the day specified.