The Difference Between Wet and Dry Bulb

  October 12, 2021      Jeff Rosenblum

When starting up an air conditioner it is important to measure wet bulb and dry bulb temperatures. These temperatures are used to determine how well the AC unit is working and if it is charged properly. What exactly is the difference between wet bulb and dry bulb temperature and how do we measure it?

Wet bulb temperature is the temperature of the air read by a thermometer covered in a water-soaked cloth or by a digital hygrometer. If wet bulb temperature was read at 100% relative humidity the wet bulb temperature would be the exact same as the dry bulb temperature. As the humidity drops, the wet bulb temperature drops as well because of evaporative cooling. This means the humidity and wet bulb temperature are directly proportional. Wet bulb is also used in determining the required superheat for an air conditioner as this reading takes into consideration the amount of humidity in the air going across the evaporator coil. 

Dry bulb temperature is the reading you get without a water-soaked cloth on the end. Dry bulb is most commonly used for temperature rises and drops. Dry bulb is also known as sensible temperature (the temperature you feel).

Both temperatures are used in a psychometric chart (which shows the properties of moist air) to evaluate how effectively an AC is operating. 

Jeff Rosenblum 
Technical Support
21 Years Industry Experience
Cell (330) 962-2491
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