Direct relative humidity measurement usually uses an electric sensing element or a mechanical system. Electric hygrometers are based on substances whose electrical properties change as a function of their moisture content. As the humidity of the air around the sensor increases, its moisture increases, proportionally affecting the sensor's electrical properties. An accuracy of less than 2 percent of the actual humidity is often obtainable. Sensors lose their calibration if allowed to become contaminated, and some lose calibration if water condenses on them.
Most sensors have a limited life.
Mechanical hygrometers usually employ human hairs as a sensing element. Hair changes in length in proportion to the humidity of the air. The response to changes in relative humidity is slow and is not dependable at very high relative humidities. These devices are acceptable as an indicator of a general range of humidity but are not suitable for accurate measurements. The system costs less than the condensation system, is not significantly affected by contaminating ions, and has a response time of about 4 minutes.