What is the difference between a hygrometer and a psychrometer?

Air humidity is measured by hygrometers and psychrometers. In principle, both of them measure relative humidity, but the fundamental difference lies in the design of these devices

Similarities and differences between the devices

A psychrometer consists of two mercury thermometers - dry and wet. Air humidity is measured using tables or formulas based on the difference between the readings of two thermometers. The principle of operation is based on the dependence of the intensity of evaporation from the water surface on the saturation deficit of water vapor in the air in contact with it.

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There are various types of hygrometers. Electrical hygrometers are used to measure the alteration in electrical resistance of a thin layer of lithium chloride, or of a semiconductor device, as the humidity changes. The main part of a mechanical hygrometer is human hair, fat-free with alcohol or ether. The operating principle is an alteration in hair length when the comparative humidity of the surrounding air alters. With a decrease in the relative humidity, the hair is shortened, and with its increase, it lengthens.

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It should be noted that a hygrometer is more often used at home, and a psychrometer is applied in factories, laboratories, and sanitary stations. Psychrometer readings are more accurate and the measurement error is less than that of the hygrometer.

How psychrometers work

These devices work due to the evaporation of liquid. During this process, dry and wet thermometers show different readings. The desired value of air humidity is determined by the difference in their indicators.

During intense evaporation, molecules leave the liquid in the form of vapor, resulting in losing some energy, due to which its temperature reduces. A wet thermometer registers the process, cooling during the period of intense evaporation.

In dry air, it will show lower values, therefore, the difference in the readings shown by two thermometers will be larger. Humidity is calculated using a psychrometric table or special formulas, and a dry thermometer will measure the current temperature.

Types of psychrometers

The August psychrometer. It is a simple device, with thermometers as the main elements. One thermometer is tied with a piece of cambric cloth. At one end, it sinks into a vessel filled with water so that you can moisten it completely. The wet bulb is cooled when the water evaporates. The advantage of such psychrometers is in their simple design and operation, and the disadvantage is that the reading of a wet thermometer depends on the exchange of the air in the booth.

The Assman aspiration psychrometer. It is a somewhat complicated device. The thermometers used are in a double-walled case that protects them from any damage or exposure to heat. They are blown by an aspirator (fan) at a speed of 2m/sec. The disadvantage of the device is the fragility of thermometers and the inability to control them at a distance.

Remote psychrometer. There are manometric and electrical devices. The first one uses a two-channel thermometer or 2 single-channel thermometers with a humidification system, which is arranged in one of the thermal cylinders. In the second device, the resolution of humidity is carried out using electrical elements - resistance thermometers, thermocouples, thermistors.

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Types of hygrometers

The well-being of people, especially weather-sensitive, hypertensive patients with diseases of the heart and blood vessels, is related to the ambient humidity. Excessively dry air also harms healthy people, causing itching and dry skin, drowsiness and often causes respiratory system diseases. Therefore, it is recommended to have a hygrometer in the house that determines the humidity.

Hygrometers are likewise essential for use in industrial premises because sometimes, it is necessary to maintain a certain humidity level so the products don’t spoil.

There are mechanical and electronic hygrometers:


This type of hygrometer works due to the ability of the human hair to change the length under the influence of the moisture concentration changing in the air. A hair of a certain length is pulled on a special frame. When the moisture level changes, the hair length also does, which pushes the arrow on the device scale, showing the current moisture value. It is used in residential, industrial premises where high data accuracy is not required. The disadvantage of the device is its sensitivity to shocks and shaking.


Humidity measured by an electronic hygrometer is made by the concentration of the electrolyte. There are devices whose measurements are tied to the dew point. It is usually measured using a lithium chloride solution that is sensitive to changes in humidity. Sometimes these devices are equipped with thermometers. The advantages of an electronic hygrometer include high measurement accuracy with a low level of error.

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The concept of operation of each of these types of hygrometers is quite simple and works based on the physical and chemical properties of the materials used. Both psychrometers and hygrometers are widely used, just choose a product for your specific application!

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