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Examples Of The Pyschrometer




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Moisture Meter Guide

The complete moisture meter website

The psychrometer is a form of hygrometer and is used specifically to measure relative humidity or moisture content in air. The psychrometer is an older form of humidity measuring, and though newer forms of electronic sensors that depend upon changes in electrical resistance and capacitance are replacing them, they are still widely used and found in many different industrial environments.

A psychrometer is made up of two thermometers of which one is dry and the other is wet. The wet thermometer is encased in a cloth which has been saturated with distilled water. This means that the wet thermometer will always be colder than the dry thermometer due to evaporation. To calculate relative humidity in air both themometer readings are taken and the difference between them both is then used to calculate the actual relative humidity.

A psychrometer can be used to measure humidity in both general areas and specific areas like inside a chamber.

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How A Psychrometer Works

As water evaporates, the surface it was on will cool. Now air can only hold a certain amount of water and when it is saturated (humid) the water does not evaporate so easily. The psychrometer makes use of these facts.

The psychrometer is the simplest form of hygrometer that measures relative humidity. There are 2 thermometers, one which is the dry bulb and the other the wet bulb. The dry bulb is left exposed to the air and measures the temperature of the air. The wet bulb is covered with a wick material and wetted with water until it is ready to use. To measure the temperature in a room the wet bulb will be removed from the water. As the water evaporates it will cool the bulb. By taking a measurement of the cooling of the wet bulb it is possible to see how much water has evaporated. This can then inform you how humid the air is. Moist air will only allow a little water to evaporate. Dry air will absorb a lot of moisture and hence cool the bulb more significantly.


This UEi psychrometer was designed and manufactured with the professional user in mind. It can measure temperature and humidity and show the results on its triple display screen. Measures temperature in both C and F, relative humidity from 0 to 100 %. Other features include an auto power off, data hold and it is supplied with a protective sensor cap.


UEi DTH31 Digital Psychrometer


On the whole though a psychrometer tends to be used in a laboratory situation to determine exact measurements during chemical reactions and experiments.

The stationary psychrometer is the most used version. The mobile psychrometer, which is known as the sling psychrometer, has two thermometers which are attached to a handle. The thermometers are spun around for a few minutes to determine the relative humidity in the air present.

As effective as they are, the psychrometer does have a couple of disadvantages. They need to be perfectly calibrated in order to obtain an accurate reading. This can prove to be very difficult because of ambient heat that is in the air. One combatant to this is the psychrometer where the thermometers are encased in metal tubes. The metal tubes insulate the thermometers from any ambient heat.

Freezing temperatures will also affect a psychrometer. The wet thermometer will develop a thin sheet of ice over it which will actually make it warmer than the dry thermometer. In order to counter this situation the psychrometer will be used with an electric heater to keep the wet thermometer above freezing.

Psychrometers though can prove to be very difficult to calibrate correctly. It is not a rare occurence for them to be off as much as 5 %.

A psychrometer chart can be printed on the back of the instrument or come with the product packaging for the user to be able to take readings and then convert these readings in to the actual relative humidity. Those who have made their own psychrometers can also use these charts to ascertain results. Just a simple home made unit consisting of 2 thermometers, with one thermometer wrapped in muslim wrap to wick water, can also use these charts or check online for results.



Psychrometer Readings Table


Bacharach Red Spirit Filled Sling Psychrometer, 25° F to 120° F


This sling psychrometer can accurately determine relative humidity without you needing to refer to complex tables. There is a slide rule calculator which can correlate the wet and dry bulb readings to give the user a direct relative humidity reading.

This instrument is well built, accurate and very easy to use. An excellent tool to use for HVAC testing.


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