Why and How to Use Psychrometers May. 5, 2015


When water vaporizes, it cools whatever surface it was on. For example, sweat cools your body as it vaporizes. The air, however, can only contain a certain quantity of water. When it is humid, the air is saturated filled with as much water as it can contain, and water does not vaporize easily.

Psychrometers make use of these facts.

A traditional psychrometer is a type of hygrometer and is used exclusively to determine the relative humidity or moisture content in the air. Traditional psychrometers represent an older humidity measurement form and electronic sensors that depend on changes in electrical resistance and capacitance rather than condensation temperature have widely replaced them. However, psychrometers are still found in many industrial environments and are available in both stationary and mobile versions. It is a useful instrument to keep in the house. It measures the quantity of water vapor in the air or relative humidity. Relative humidity refers to the moisture content in the air compared to how much moisture the air can grasp at a given temperature. Information about the relative humidity is obliging in understanding the weather. A psychrometer is an instrument that counts both the wet-bulb and dry-bulb temperatures. Two thermometers are required to count these constraints. From the values obtained, the relative humidity can be known.

Knowing the relative humidity is helpful in understanding the weather and what to expect. It is also helpful if you keep several indoor plants.

Construction and Operation

The instrument consists of a dry-bulb and wet-bulb thermometer. The dry bulb measures temperature. It is left as it is, without any covering, for maximum exposure to air. The wet-bulb must be kept wet. It is covered with some wet cloth or sock or dipped in water until it is required for use. At the desired time, you remove the wet bulb from the water but retain the wet cloth (sock) on it.

Readings of both bulbs are taken immediately and also at intervals of a few minutes. Note is made of the cooling temperatures on the wet-bulb. The temperature of the wet-bulb will always be lower than the dry-bulb due to the evaporating water which creates a cooling effect. A comparison of the two temperatures gives you the relative humidity of the air.

Obtaining exact relative humidity and dew-point from a psychrometer requires calculations that can get time consuming without the use of software. The easiest route to glean this information is through the use of a psychometric chart. These can be purchased, often with your psychrometer, or one can be downloaded from the Internet. Generally these charts require you to find both of your readings on the chart and match them up to the corresponding relative humidity. Dew point temperatures can also be found on these charts.


Psychrometers measure the humidity in a general environment's or specific area's (such as a chamber or structure) air. Psychrometers are generally used in laboratory settings in order to determine precise measurements for experiments and chemical reactions.

A stationary psychrometer is the most common and is used in some forms of industrial equipment, while a mobile psychrometer, known as a sling psychrometer, consists of two thermometers attached to a handle and is spun in the air for several minutes in order to calculate the relative humidity in the field.


Compared to a sling psychrometer, a digital psychrometer is remarkably easy to use. There's no need to wave the psychrometer around for several minutes before you can make your calculations. It's all automatic at the push of a button. For those who need to use a psychrometer, this takes all the effort out of the task and makes the process a great deal faster.

You can obtain more data and perform more calculations with a digital psychrometer. At the push of a button you'll be able to find the percentage of relative humidity in the air, as well as the dew-point and the t°, including the wet-bulb t°. The differential between the two t°s (dry-bulb and wet-bulb) can be calculated very easily. A good digital psychrometer will let you store data including maximum and minimum readings, which allows you to make ongoing comparisons. For different areas you'll also be able to switch simply between measurements in °C and °F.

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