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Last updated date: 16th May 2024
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Definition of Psychrometer

A Psychrometer is a basic hygrometer composed of two identical thermometers, according to the definition. The bulb of one thermometer is kept moist by evaporative cooling, resulting in a lower temperature reading than the dry-bulb thermometer. A psychrometer is a device that determines the amount of humidity in the air. It does so by comparing the temperatures of a dry thermometer bulb with a wet thermometer bulb that has evaporated moisture.

A psychrometer is used to simultaneously measure the temperature of the dry and wet bulb. A wet wick placed above the thermometer bulb determines the temperature of the wet bulb.

Hygrometer and Psychrometer

A hygrometer is a device that detects the presence of water vapour in the air, soil, or restricted areas. Humidity measuring instruments frequently use temperature, pressure, mass, and a mechanical or electrical change in a substance as a result of moisture absorption. Using calibration and computation, these observable quantities can be utilised to determine humidity. The temperature of condensation (also known as the dew point) or changes in electrical capacitance or resistance are used by modern electronic equipment to assess humidity variations.

A psychrometer, commonly known as a wet and dry thermometer, consists of two calibrated thermometers, one of which is dry and the other of which is moistened with distilled water on a sock or wick. At temperatures over the freezing point of water, evaporation of water from the wick lowers the temperature, leading the wet-bulb thermometer to read lower than the dry-bulb thermometer. When the air temperature is below freezing, however, a tiny layer of ice must be applied to the wet-bulb to ensure accuracy. Due to the heat of sublimation, the wet-bulb temperature will eventually be lower than the dry bulb, albeit this may take many minutes of continuous psychrometer running.

Kappeller Psychrometer

The temperature differential, as demonstrated by the wet-bulb and dry-bulb thermometers, and the ambient temperature, as represented by the dry-bulb thermometer, are used to calculate relative humidity (RH). On a psychrometric chart, the intersection of the wet and dry-bulb temperatures can also be used to calculate relative humidity. The dry and wet thermometers will match when the air is completely saturated, and the larger the difference, the dryer the air.

Psychrometers are widely used in meteorology and the HVAC industry to verify that refrigerant levels in residential and commercial air conditioning systems are appropriately maintained.

Manually rotating a sling psychrometer with thermometers attached to a handle in open air flow, on the other hand, until both temperatures stabilize. Although electronic sensors are becoming more handy, this is still utilized for field measurements. A spinning psychrometer functions similarly to a ratchet or football rattle, with the exception that the two thermometers are coupled to a device that resembles a ratchet or football rattle.

How Does a Psychrometer Work and How to Calibrate It?

The principles of psychrometry can be applied to any physical system, including gas-vapor mixtures, although the most common system of interest is the mixing of water vapour and air, which is used in heating, ventilation, and air conditioning, as well as meteorology.

In human terms, our thermal comfort is significantly controlled not just by the temperature of the surrounding air, but also by the amount of water vapor present. The functioning mechanism of a psychrometer, as is well known, necessitates the use of two thermometers.

Each thermometer is suspended within a polished metal vertical tube, which is subsequently suspended within a second metal tube of a slightly larger diamete, which serves to separate the thermometers from radiant heating in this setup. A fan pulls air through the tubes at a constant speed, which is controlled by a clockwork mechanism (some modern versions use an electric fan with electronic speed control). "Air is sucked between the concentric tubes as well as through the inner one.

It's difficult to achieve the maximum theoretical depression of the wet-bulb temperature, especially at low relative humidity. The World Meteorological Organization's Guide states that "The heated psychrometer works on the assumption that heating does not modify the water vapor content of an air mass. This feature may be used to the benefit of the psychrometer by eliminating the requirement for an ice bulb to be kept frozen.

Uses of Psychrometer

  • Psychrometers have long been used in the design of architectural ventilation systems.

  • On some ships, the main wet bulb temperature measuring method is an electric psychrometer.

  • They use it as a backup for the air temperature and dew point sensors that are automatically mounted.

  • In musical instruments like pianos, guitars, and violins, psychrometers are used to measure humidity levels.

  • Paints are sensitive to humidity, hence it's used a lot in the coatings industry.


Evaporative cooling keeps the bulb of one thermometer moist, resulting in a lower temperature reading than the dry-bulb thermometer. Each thermometer is suspended within a polished metal vertical tube, which is then suspended within a second metal tube with a slightly bigger diameter, which in this setup serves to separate the thermometers from radiant heating. "Air is pulled between the concentric tubes as well as through the inner one," says the clockwork mechanism that pulls air through the tubes at a consistent speed.

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FAQs on Psychrometer

1. What is a psychrometer used for?

A psychrometer measures humidity by obtaining temperature readings from both the wet and dry bulbs. The remaining qualities of the air, including its moisture content, may be calculated or read from a psychrometric chart once those two numbers are known.

2. What is the difference between hygrometer and psychrometer?

There is no difference between a hygrometer and a psychrometer in reality. The earliest way of measuring humidity is used using a psychrometer. Hygrometer, on the other hand, employs a whole new digital psychrometer technique. The department's priorities differ when it comes to employing a hygrometer or psychrometer.

3. What is a psychrometer in simple words?

A psychrometer is a device that measures and compares two thermometers: dry-bulb and wet-bulb air temperatures to determine relative humidity in the atmosphere. One type of hygrometer is the psychrometer. Psychrometers are used by meteorology departments to forecast weather and precipitation.