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Gibbons jargon buster #16: What are wet-bulb and dry-bulb temperatures?

Gibbons have been working closely with our friends over at E-co carrying out some testing work on the Steril-Aire™ UVC technology for HVAC systems. Used for the control of mould, germs and bacteria as well as an energy-saving device inside AHUs, it’s fast becoming an integral part of HVAC management for our customers. 

As part of the testing documentation, Gibbons have been asked to provide a layman’s guide to some of the terminology involved. Our HVAC Services Manager Andrew Knight stepped up to the plate and here he explains wet-bulb and dry-bulb temperatures in our latest jargon-busting blog.

Dry-bulb and wet-bulb temperatures (along with dew point temperatures) are used in HVAC systems to determine the state of humid air. Knowing only two of these values is enough to determine the state, - including the content of water vapour and the sensible and latent energy (referred to as enthalpy).

Dry-bulb temperature

The unit of measure for dry-bulb temperature is expressed as ‘Tdb’.

Dry-bulb temperature is usually referred to as ambient air temperature. It’s called ‘dry bulb’ because the air temperature is indicated by a thermometer that’s unaffected by the moisture of the air.

Dry-bulb temperature can be measured using a normal thermometer freely exposed to the air but shielded from radiation (normally direct sun) and moisture. The temperature is usually given in degrees Celsius (°C) in the UK. The dry-bulb temperature is an indicator of heat content and is shown along the bottom axis of a psychrometric chart.

Wet-bulb temperature

The unit of measure for wet-bulb temperature is expressed as ‘Twb’.

The wet-bulb reading is the temperature of adiabatic (water) saturation. This is the temperature indicated by a moistened thermometer bulb exposed to the air flow. Day to day, wet bulb is less commonly used than Tdb or dry bulb.

Wet-bulb temperature can be measured by using a thermometer with the bulb wrapped in wet muslin or by using a digital wet-bulb test meter. The adiabatic evaporation of water from the thermometer and the cooling effect is indicated by a wet-bulb temperature lower than the dry-bulb temperature in the air.

The rate of evaporation from the wet bandage on the bulb, and the temperature difference between the dry bulb and wet bulb, depends on the humidity of the air. The evaporation is reduced when the air contains more water vapour. The wet-bulb temperature is always lower than the dry-bulb temperature but will be identical with 100% relative humidity (because the air is at the saturation line).

Combining the dry bulb and wet bulb temperature in a psychrometric chart gives the state of the humid air.

If you have any HVAC technical queries, or would like us to quote you for HVAC products and services, including the Steril-Aire™ UVC technology for HVAC systems, you can call Andrew on 07850204915 or email

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