What is a Hygrometer?
"Hygrometer" is a general term for an instrument used to measure humidity. There are different types of hygrometers that work on different principles. One kind is the so-called sling psychrometer.
The sling psychrometer consists of two identical thermometers mounted side by side. One of the thermometers is fitted with a wick type of material fastened around the thermometer's bulb. This bulb with the wick material is called the "wet bulb" because, to use the psychrometer, the wick material is soaked with water while the other bulb is left dry. After the wick is soaked on the wet bulb side, the unit on which the two thermometers are mounted is spun or slung around through the air by a handle with a swivel attaching it to the thermometer unit. The "sling" part of the name refers to the action involved in using the instrument.
What is Humidity?
Humidity is the amount of water vapor in the atmosphere caused by condensation and evaporation. It can be measured as absolute humidity (the amount of water vapor in a unit volume of air), or as relative humidity (the ratio of moisture in the atmosphere to the maximum moisture the atmosphere can hold). It is what gives you that horrible sticky feeling on a hot day and can cause heat stroke. We feel most comfortable with relative humidity between 30% and 60%.
How do Hygrometers Work?
Wet and dry bulb psychrometers are the most simple and common way of measuring humidity. This type of hygrometer uses two basic mercury thermometers, one with a wet bulb one with a dry bulb. Evaporation from the water on the wet bulb causes its temperature reading to drop, causing it to show a lower temperature than the dry bulb. Relative humidity is calculated by comparing the readings using a calculation table that compares the ambient temperature (the temperature given by the dry bulb) to the difference in temperatures between the two thermometers. A mechanical hygrometer uses a slightly more complex system, based on one of the first hygrometers designed in 1783 by Horace Bénédict de Saussure. This system uses an organic material (usually human hair) that expands and contracts as a result of the surrounding humidity (that also explains why you always seem to have a bad hair day when it’s hot and humid!). The organic material is held under slight tension by a spring, which is linked to a needle gauge that indicates the level of humidity based on how the hair has moved.