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How to Determine Relative Humidity From a Wet & Dry Bulb Thermometer

How to Determine Relative Humidity From a Wet & Dry Bulb Thermometer
Issue Time:2018-01-25

How to Determine Relative Humidity From a Wet & Dry Bulb Thermometer

Relative humidity shows how much moisturethe air could hold compared to how much it does hold. This percentage differsat various temperatures as warmer air has a greater capacity for holdingmoisture than cooler air. Determining the relative humidity using twothermometers lets you cheaply find out if your home or area has too much or toolittle moisture in it. Excess moisture can breed mold while not enough driesout skin. Prevent these problems by using dehumidifiers or vaporizers tomanually adjust the environmental humidity.


Tape two bulb thermometers side by sideonto the piece of cardboard with their tips hanging off the side of thecardboard.

Dip the facial tissue (or cloth) into waterto soak. Squeeze out the excess liquid.

Wrap the damp tissue around the bulb of oneof the two thermometers. Do not get the other thermometer wet.

Take the temperature from both thermometersin degrees Fahrenheit or degrees Celsius after 10 minutes. The dry thermometermeasures air temperature and the tissue-wrapped thermometer determinesevaporation temperature.

Convert both Fahrenheit readings to degreesCelsius by subtracting 32 from the Fahrenheit temperature and multiplying theresult by (5/9). For instance, for a temperature of 50 degrees F: 50 – 32 = 50;18 x (5/9) = 10 degrees Celsius.

Subtract the evaporation temperature indegrees Celsius from the air temperature in degrees Celsius.

Look on the left side of a relativehumidity chart to find the row for the air temperature (dry thermometerreading) in degrees Celsius.

Scan the column headings at the top of thechart to locate the difference between the air temperature and evaporationtemperature.

Find where the temperature difference columnintersects the row with the air temperature and use this number as the relativehumidity.