What is a psychrometer and what is it for?

A thermometer attached to a window frame is usually sufficient for the average person to determine the weather outside.

Sometimes a barometer, a device that predicts changes in the weather based on fluctuations in atmospheric pressure, is added to it. But for a forecaster, this set of measuring equipment is not enough – at least a psychrometer is needed.

What is a psychrometer?

The name of this device is composed of two words in the ancient Greek language: “psychros” – cold and “metreo” – to measure.

It measures temperature and humidity, which is often necessary in meteorological measurements of the environment, as well as in many areas of industry – for example, when weighing hygroscopic materials to find their exact weight.

How does a psychrometer work?

As you know, any liquid, including water, gradually evaporates and the fastest molecules vaporize, i.e. the total temperature of the liquid decreases. This physical property of liquids is the basis of the design of any psychrometer.

Imagine two thermometers, either alcohol or mercury thermometers. The surface of one of them remains dry, while the other is wrapped in a wet cloth. To prevent the cloth from drying out, one end of it is dipped in a container of water. When the water evaporates from the cloth, the wrapped thermometer cools down and therefore shows a lower temperature than the second thermometer whose surface remains dry and records the air temperature. The less moisture in the ambient air, the more intense the evaporation process.

The difference between the readings of the two thermometers can judge the level of humidity in the air. There is a special psychrometric table for this purpose, from which you can find out the relative humidity and then, using a special formula, determine the absolute air humidity.

Psychrometer and hygrometer.

Is a psychrometer necessary, when the air humidity can be determined with another device – a hygrometer?

The working principle of this device is based on other properties of the environment: Hygrometers can be a resistive, capacitive, thermistor, and optical. The simplest of them use the property of some materials to twist or straighten when humidity fluctuates.

Sometimes, the readings of the hygrometers are not sufficiently accurate. The simplicity of the device psychrometer provides a high level of protection against extraneous influences, so the data got with it is more accurate and reliable. Therefore, to refine the measurement results, the hygrometer readings are often checked from time to time with a psychrometer.

Hygrometers most times are more convenient to use. Thus, capacitance hygrometers can measure humidity not only in the air but also in solid substances. This is necessary for controlling the parameters of tablets, concrete products, etc.

Varieties of psychrometers

Nowadays, psychrometers of three basic types are used for measurements, which differ from each other by constructional features.

A stationary-type psychrometer is a pair of mercury thermometers fixed on a solid tripod. The mercury flask of one of them is tied with a cambric cloth, the end of which is lowered into a beaker filled with water. The entire structure is placed in a weather booth or wherever measurements are usually taken. The major disadvantage of the stationary psychrometer is the dependence of the readings on the mobility of the airflow, which affects the evaporation rate.

The aspiration-type (Asman) psychrometer, although composed of the same two thermometers, has a more complex and protected design. The thermometers in this version are housed in a rugged housing that cuts off extraneous thermal radiation. The evaporation of the liquid is done by forced air blowing by a small fan, generating an airflow of about 2 m/s. This is the most accurate and protected device against external influences.

A remote-type psychrometer does not use mercury glass thermometers, but electrical temperature gauges based on material resistivity measurements, on the properties of thermistors and thermocouples, and on manometric systems. But in either case, one sensor is left dry when measuring, and the other is wetted to measure the temperature difference.

Thus, Asman and stationary psychrometers use familiar mercury thermometers made of glass, so their common disadvantage is the increased fragility of the device. Remote psychrometers measure air temperature based on other principles, so their readings can be monitored remotely, which is often a vital circumstance.

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