Dry Bulb / Wet Bulb

Dry Bulb temperature is defined as the temperature of air, measured by a thermometer freely exposed to the air, but shielded from radiation and moisture.

Dry bulb temperature is the true thermodynamic temperature, measured by a regular thermometer exposed to the airstream. Unlike Wet Bulb temperature, dry bulb does not indicate the amount of moisture in the air.

Wet Bulb temperature is the lowest temperature that can be reached by the evaporation of water. It is the temperature you feel when your skin is wet and exposed to moving air.

Web Bulb is a temperature measurement that is used to determine and indicate the amount of moisture in the air. Dry Bulb and Wet Bulb Temperatures, used in conjunction with a Psychrometric Chart, can determine the amount of relative humidity in the air.

Interactive Psychrometric Chart

Wet Bulb is accurately measured using an analog device called a sling psychrometer.

Wet Bulb / Dry Bulb Measurement:

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We want to look over the means of measuring WB (Wet Bulb). Dry Bulb should not be an issue. If you have ever taken an air temperature in the conventional way, you are getting the dry bulb temperature. Wet bulb on the other hand, is a little different.

The Sling Psychrometer: Download this pdf for an extensive description and set of directions for a Sling Psychrometer.

Directions for using a Sling Psychrometer

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You can see that the wet bulb is exactly that, a "wet" bulb. The material is a cotton sock that slips over the bulb. You can see there is a water reservoir with a cap at the end of the sling psychrometer.

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Look at the sling scale to the left and see how the wet bulb and dry bulb are indexed to each other on the scales. Read the % RH at the pointer on the bottom scale.

This is a "real" sling psychrometer. These are used by technicians as well as the popular "electronic" slings that many manufacturers supply. The electronic ones are great; but can only be verified (accuracy checked) with an actual "water sling" from the old school.

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If you want to look at it from the "vapor pressure" angle, you will see that the soaked fabric around the "wet bulb" of the thermometer is at a higher vapor pressure. When twirled in the air, the high moisture content around the wet fabric is equalizing with the vapor pressure in the air. A process of "evaporation" is occurring until the wet fabric gets to the saturated temperature of the air, taking the other factors into consideration, like pressure. At that point (measured in temperature), the vapor pressure ceases to have a difference; so no more water leaves the soaking wet fabric around that thermometer bulb. This point would be 100% RH or an equalized vapor pressure difference.

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Most technicians have an electronic thermometer with two air probes. If you don't have a real sling, you can make a reasonable one by wrapping one of your air probes in a cotton swab (raid the first aid kit) and connecting them together with a wire tie or like this one a bread sack tie. Soak the "wick" in water and twirl it in the air (not the thermometer just the air probes). Read the resulting temperatures on your two temperature read outs. Use a wet bulb / dry bulb or psychrometric chart and find % RH.