A sling psychrometer holds a wet-bulb thermometer and a dry-bulb thermometer. A dry-bulb thermometer is an ordinary thermometer, whilst a wet-bulb thermometer is a thermometer that has its bulb wrapped in cloth and moistened with distilled water.
When a sling psychrometer is swung round in the air, moisture will evaporate from the wet-bulb thermometer, reducing its temperature depending on the humidity of the air it is exposed to. The higher the humidity, the lower the rate of evaporation and so the higher the temperature recorded. Wet-bulb temperatures are the same as dry-bulb temperatures at a relative humidity of 100%, but otherwise will be lower than dry-bulb temperatures due to the cooling effect of evaporation (described as wet-bulb depression).
The two thermometers should give steady-state readings after a few minutes.
A sling psychrometer can be used to determine the physical and thermal properties of moist air by using standard tables and charts. Typically it is used to determine relative humidity. Some sling psychrometers include mechanisms to allow relative humidity to be read directly without needing to refer to tables.
Psychometric charts are complex graphs that represent the dry-bulb temperature, wet-bulb temperature, relative humidity, specific volume, dew point temperature, humidity ratio and enthalpy of moist air at known atmospheric pressure. The state of moist air can be determined from any two of these properties (such as wet-bulb temperature and dry-bulb temperature which can be read on a sling psychrometer) from which all other properties can then be determined.
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