Humidity can be measured by using a hygrometer. Types of hygrometers include the basic psychrometer, an electrical impedance hygrometer useful for remote locations, the condensation principle hygrometer and mechanical hygrometers.
A simple type of hygrometer is called a psychrometer, which uses evaporative cooling and contains one thermometer for measuring the current air temperature and another thermometer that is kept moist with a wet cloth or wick. The second thermometer in a psychrometer becomes cooler when the water on the damp bulb evaporates, given the air is above freezing. The difference between the two temperature readings is found on a psychometric chart and determines the relative humidity.
An electrical impedance hygrometer uses an electronic component capable of absorbing water vapor depending on air humidity. Some electrical impedance hygrometers can measure tiny levels of water vapor in extremely dry gases. The condensation principle hygrometer uses cooling to trigger controlled condensation. It typically consists of a cooled mirror and manages the mirror temperature by using optical detection.
Mechanical hygrometers work based on the change in the length of an organic material, such as hair or paper, which indicates an increase in humidity. When the material’s length changes, the pointer on the equipment’s dial moves. Some mechanical hygrometers operate using clockwork or batteries and record the changes in length on a chart.