By Phil Rains
“Wet Bulb temperature” is a term used by HVAC technicians when checking cooling operation with a “sling psychrometer”. The result indicates the temperature of air measured using a standard mercury-in-glass thermometer, with the thermometer bulb typically wrapped in gauze or a muslin-type material, with air passing over its surface. The covering is kept wet during testing. The evaporation of water from the thermometer has a cooling effect (Wet Bulb Depression), so the temperature shown is less than the temperature shown on a Dry Bulb thermometer used during the same test.
When compared to the Dry Bulb temperature, the difference indicates to the technician the approximate relative humidity of the occupied space against a known scale, as well as the operating conditions of the system during cooling.
The wet bulb temperature is very dependent on the moisture content of the air. What happens in a wet bulb temperature measurement is that water begins evaporating from the bulb of the thermometer. The evaporation of water causes a cooling effect on the thermometer bulb, and the temperature measured by the thermometer drops. If evaporation occurs easily, as with dry air, there is a temperature drop. However, if there is high humidity conditions, the temperature measured on a wetted thermometer bulb may drop only slightly or not at all, because there is too much moisture in the air. When the wet bulb temperature equals the dry bulb temperature, air is totally saturated with water and no further evaporation can occur. This is called 100% saturation. This is the condition that is met at the surface of the evaporator when air is cooled to its saturation temperature and begins to change state, giving up heat to refrigerant and condensing moisture out to be drained away during cooling. The leaving air is cooled and transferred to the conditioned space in the summer.
Phillip A. Rains
Master Trainer/Technical Developer