What Is a Psychrometer?
A psychrometer is an instrument that measures both the wet bulb and dry bulb temperatures. Two thermometers are needed to measure these parameters. Using these readings the relative humidity can be obtained. Relative humidity is the percentage of moisture in the air relative to the maximum that can be in the air at the current temperature. It is the most accurate type of hygrometer, which is any instrument that measures humidity.
History
According to Institute and Museum of the History of Science, psychrometer means "cold measurer" in Greek and the term was originated by German inventor Ernst Ferdinand August in 1818. Meteorology, Practical and Applied states Sir John Leslie (1776-1832) is to whom we really owe the psychrometer.
Identification
Recognizing a psychrometer is easy once you know what to look for. There will always be two thermometers one or both of which will have a metal extension at the bottom. This is the wet bulb. Instruments for placing in a traditional weather station box will simply have a hole above the thermometers for mounting onto a pivot within the station. A hand held device has a handle, and either a chain connected to some sort of swivel, or the handle will be connected directly to the swivel.
Operation
The "sock" is placed on the bottom end of one thermometer to make the wet bulb. Next the instrument is either swung within the weather box or at arms length for about 30 seconds to reset both thermometers. The sock is then dipped into distilled water until saturated. Swinging the instrument once again for about one minute should allow enough time for the moisture to evaporate. The user will then look at both thermometers and mentally record the readings. Finally it is swung again for about 30 seconds and looked at to see if the wet bulb is still dropping. If the wet bulb has stayed the same both numbers are recorded, if not it is swung for 30 seconds again until the temperatures don't change.
Figure Relative Humidity
Obtaining exact relative humidity and dew-point from a psychrometer requires calculations that can get time consuming without the use of software. The easiest route to glean this information is through the use of a psychometric chart. These can be purchased, often with your psychrometer, or one can be downloaded from the Internet. Generally these charts require you to find both of your readings on the chart and match them up to the corresponding relative humidity. Dew point temperatures can also be found on these charts.
Warning
Mercury has many well know negative health and environmental effects. Swinging a sling psychrometer can thus be dangerous. Extra caution must be taken when operating these weather instruments.
Tags: relative humidity, about seconds, again about, both thermometers, instrument that, instrument that measures
