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A hygrometer measures humidity. hygrometer and barometer image by Richard J Thompson from Fotolia.com A scientific term most commonly used in weather forecasts and reporting, humidity represents oppressive heat to most people. Defined as the amount of water vapor actually in the air as a fraction of the amount of water vapor the air can hold, relative humidity shortens to humidity, as in the statement that "The humidity is 70 percent." Measuring humidity can be accomplished by several different types of sensors.
Psychrometer
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According to Kenyon College, a psychrometer measures relative humidity by means of the evaporative cooling of a thermometer bulb. This particular type of sensor dates back to 1827, when Professor John Frederic Daniell invented this technique for comparing the temperature of a thermometer bulb with that of a dry bulb. Once those temperatures are known, the relative humidity can be determined by reference to tables listing humidity variables. When the tables are built into such a device, it is known as a hygrodeik.
Dunmore Sensor
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Belonging to a class of sensors known as resistive, this sensor utilizes changes in chemical and electrical properties resulting from moisture absorption to measure humidity. Developed in the 1940s and used for air conditioning controls and telecommunications, Dunmore sensors' resistance changes inversely with the humidity, thus providing the means of measurement.
Dewpoint Hygrometer
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This sensor depends on the dew point, the temperature of air when saturation occurs, or when moisture in air reaches its maximum pressure. Consisting of a polished metal mirror cooled at constant temperature and constant vapor content until moisture starts to condense, this sensor measures that temperature, which represents the dew point. The dew point then becomes a direct index of the actual humidity in a region.
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