A hygrometer is the instrument that measures humidity. The two main types of hygrometers are analog and digital.
Analog hygrometers react mechanically to the ambient humidity and represent that value on a dial. Many high quality analog hygrometers use horse hair, which expands at a predictable rate when the humidity increases. Analog hygrometers may have to be calibrated approximately every six months to maintain accuracy. Many analog hygrometers have an adjustable dial in the back, or inside, that can be moved +/- with a small screwdriver.
Digital hygrometers use electric currents to measure the ambient humidity and require batteries. Conventional wisdom suggests that digital hygrometers tend to be more accurate than analog hygrometers because of factory calibration. Better still, some digital hygrometers are adjustable after they leave the factory.
Click on the link below for the Hygrometer Calibration FAQ.