Hygrometers (Figure 1-36) are used to measure the relative humidity in a controlled space. Hygrometers are also commonly called relative humidity meters. See Item 16 for a description of relative humidity.

Many types of hygrometers are available. The dial-type hygrometer uses a synthetic hair that expands and contracts as the humidity changes. Many humidifiers also work on the same principle. For field service work, the electronic hygrometer is the most common type. Electronic hygrometers have temperature-humidity sensing probes to measure the conditioned air.

The measured RH and/or temperature readings are usually displayed on an LCD. Some hygrometers are capable of reading dewpoint. Others, using two or more temperature probes, can calculate the differential temperature between probes. Some types of hygrometers can display temperature and RH readings simultaneously. Common electronic hygrometers can measure RH in the range of 0 to 98 percent with an accuracy of ± 2 percent. They typically measure temperatures in the range from -20° F to 160° F.

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