How to Measure & Record Temperature & Humidity
You can measure temperature and relative humidity simultaneously using a meteorological instrument called a "sling psychrometer." You would use its two thermometers to compare the actual temperature with the temperature if the water vapor in the air was at its condensation point. Tables derived from controlled lab conditions enable you to convert the two values into relative humidity.
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Instructions
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Dampen the little piece of cloth covering the mercury-filled bulb of the wet-bulb thermometer on the sling psychrometer with water.
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Grab the psychrometer's handle and swing the two thermometers around for about five minutes. This gives time for the moisture to evaporate off the cloth, leaving behind cooler water molecules and a lower temperature reading.
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Use the psychrometer's built-in slide scale to calculate the relative humidity from the wet- and dry-bulb readings. If your psychrometer doesn't have a built-in slide or calculator, then consult an online calculator or table.
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Record the temperature and relative humidity by hand at regular intervals, for example, in a ledger every 6 hours, or you can delegate the job to electronic equipment linked to your computer, which can take record and graph measurements continuously.
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Tips & Warnings
You can also purchase an electronic psychrometer, which eliminates the need for a slide or table.
A common misconception about humidity in the air is that, because it has a condensation point, it must therefore be dissolved in air somehow, like a salt that condenses out of water dissolves in solution. These two types of condensation, however, are not so analogous. Actually, water vapor is itself a gas. It can exist without air, on its own--for example, as the only gas in an otherwise evacuated gas chamber.
References
Resources
- Photo Credit hygrometer and barometer image by Richard J Thompson from Fotolia.com
