Wet-bulb temperature is a common term often heard around the HVAC industry, but sometimes misunderstood. Wet-bulb temperature is defined as the lowest temperature able to be achieved solely by the effect of evaporation.
Online — As water evaporates, it consumes energy from the surrounding air causing the air temperature to drop. The lowest temperature achievable due to evaporation will vary depending on the amount of water vapor present in the air.
The cooling effect is inversely proportional to the amount of water vapor present, thus we can determine relative humidity, dewpoint temperature and the other psychrometric parameters if we know the dry-bulb and the wet-bulb temperatures.
As a rule of thumb, the larger the wet-bulb depression (difference between the dry-bulb and the wet-bulb temperatures) the lower is the relative humidity of the air.
One common misunderstanding is that wet-bulb temperature and dewpoint temperature are equivalent. This is only true if the air is saturated or at 100% relative humidity, otherwise, the dewpoint temperature will not equal the wet-bulb temperature.
Wet-bulb temperature is measured with a psychrometer which is usually comprised of a thermometer (or any device to measure temperature), covered by a wet cloth or wick.
Air is passed over the thermometer bulb causing evaporation to occur. The air can be moved across the bulb with a fan, or the bulb can be moved through the air with a sling psychrometer.
Vaisala Conversion Tools for Calculating to Wet-bulb Temperature: RH_SlideRuleCalculator_72dpi_RGB
Free humidity slide rule calculator:
For a free customized psychrometric chart: Click here
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