The Psychrometer is made up of two thermometers; determines relative humidity by measuring ambient temperature and the temperature of an evaporating water source. The bulb of one of them is wrapped in a fabric that is always kept moist.

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Operating principle

Evaporation from the surface of the wet bulb into the air stream cools the wet bulb to a steady state temperature such that there is a balance between heat lost by evaporation and heat gained by convection and radiation. This temperature depends on the pressure, temperature and humidity of the atmosphere. Thus, when an approximate value of pressure is available, the humidity can be obtained from the observed temperatures of the wet and dry bulbs.

There are two types of psychrometers: Natural ventilation and forced ventilation.


Naturally ventilated psychrometers are made up of two thermometers, usually RTD's or mercury thermometers. The elements are not subject to forced air movement, they are less accurate, however they are used in some applications.

The dry-bulb thermometer is used to measure the ambient temperature and the wet-bulb thermometer (covered by a cotton wick that is in contact with a reservoir of water) is used to measure the difference in temperature due to the evaporation of surrounding water. of the wet bulb. Evaporation cools the bulb of the cold-bulb thermometer. Knowing the temperatures of both thermometers, the relative humidity can be easily determined by means of psychrometric charts, tables or calculated by means of the corresponding expressions.


In a forced-ventilated psychrometer, the two temperature sensors are exposed to an air stream. These are usually thermistors, RTD's, thermocouples or mercury thermometers.

Spin psychrometer

As the psychrometer is rotated, air flows over the wet and dry bulbs. This type of psychrometer is not as accurate as one vented by other methods, because the temperature of the wetted element begins to rise as soon as motion stops to read the thermometers.

Aspiration psychrometer (Assman type)

Most commonly used, a small fan driven by an electric motor or clockwork drives air axially over mercury thermometers. Water must be manually added to the wet element five to fifteen minutes prior to measurement under normal ambient conditions.

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