How Does a Psychrometer Work?

A psychrometer is a device used to measure the humidity of air. It accomplishes this by comparing the difference in temperature between a dry thermometer bulb and a wet thermometer bulb that has lost some of its moisture through evaporation.

Using a Psychrometer

A psychrometer is an old-fashioned type of hygrometer, which is a device for measuring humidity. It consists of two thermometer bulbs: a wet bulb and a dry bulb. The dry bulb measures the ambient air temperature in the same manner as any household thermometer. The wet bulb is covered with a cloth, usually cotton, that needs to be moistened with water before use.

Once the wet bulb is ready, the scientist either swings the psychrometer or lets it stay still, depending on the design of the device, until the wet bulb has cooled to its final temperature. Then, by knowing the difference between the wet and dry temperatures and the atmospheric pressure in her location, she can determine the humidity of the air.

How Does a Psychrometer Work?

The amount of evaporative cooling on the wet bulb of the psychrometer is directly dependent on the amount of moisture in the air. Drier air absorbs more moisture from the bulb, in turn cooling it off more. Wetter air cannot absorb as much water from the bulb, so the temperature doesn't change as much. In other words, the smaller the change in the wet bulb temperature, the more humid the air.

The first psychrometers appeared in the 1600s. While the design of these devices hasn't changed much, today digital versions are also available. A digital device can display many details about atmospheric conditions at once, including humidity, air temperature, surface temperature and dew point. However, these devices need 20 to 30 minutes to calibrate to local conditions before use, and the wet bulb sensor can dry out after not being used for extended periods.

Therefore, the good old fashioned manual psychrometer hasn't lost its appeal, despite requiring a bit more legwork on the part of the user. After reading both bulb temperatures, they must refer to a psychrometric chart for the final humidity reading.

Psychrometric Charts

Psychrometric charts allow a scientist to look up the intersection of the dry bulb and wet bulb temperatures from the psychrometer to learn about the air's thermal properties. This information includes:

  • Relative humidity: How close the air is to a more desirable humidity.
  • Dew point: The temperatures at which the moisture present in the air will start to condense.
  • Enthalpy: How much heat the air contains in units of BTU per pound of dry air.
  • Humidity ratio: Pounds of moisture in the air per pounds of dry air.
  • Specific volume: How much space the air takes up.

The United States Department of Agriculture's Weather Bureau published its first tables of psychrometric data in 1900. Rather than showing plots of temperatures versus humidity, tables show numerical data for each pair of readings. A 1915 revised edition of the tables was 87 pages long.

Who Uses Psychrometers?

Although psychrometers are somewhat old-fashioned today, they are still occasionally used to calibrate humidistats, which control the internal humidity of building air conditioning and heating systems. In the past, psychrometers were used in designing building ventilation systems.

Psychrometers also have been used historically in various science disciplines, and students today may learn how to use these tools and interpret psychrometric charts, even if they are no longer the go-to method of data collection. Meteorologists or environmental scientists, for example, may learn to use psychrometers to measure atmospheric conditions, as might agricultural engineers who are designing spaces with specific humidity needs, such as greenhouses or barns for livestock.