Humidity

A psychrometer works by a number of different sensing methods, adsorption, wet and dry measuring temperatures, dew point measurement etc...

And it displays the result as either percentage relative humidity or dew point temperature.

Wikipedia probably defines humidity for you.

John.

http://www.encyclopedia.com/doc/1E1-psychrom.html

In my opinion, not very well. The primary advantage of psychrometers is that they are low cost. If you want to know the amount of water vapor in air, I suggest you measure the dew point with a chilled mirror hygrometer.

I am familiar with two types. Sling and motorized. Both involve measuring the drybulb and wetbulb temperature and referring to the phsychrometric chart or look-up tables.

The accuracy of the sling phsychrometer is highly dependent on the operator. One takes a thermometer with a cotton swab on the bulb end. Another reference drybulb thermometer without. The thermometers have to be accurate and matched such that at the beginning of the operation the temperature reading is the same for both. The cotton swab is soaked with distilled water and the thermometer is slung around in circles on the end of a string to cause evaporative cooling. After a few minutes of slinging, the temperature is quickly read and compared to the ambient thermometer.

The motorized phsychrometer uses a fan to pull air over the wetted cotton swab. One can watch and see the temperature fall and stabilize and then take a reading. Motorized units are usually much more accurate than sling units, which typically do not sufficiently depress the wetbulb temperature and therefore give overly high humidity readings.