Two types of dew point sensors are commonly used today: a condensation dew point method and a saturated salt system.
The condensation dew-point hygrometer is a polished metal mirror that is cooled until moisture just begins to condense onto it. This occurs when the surrounding air reaches its dew point. The temperature of the metal is then the same as the dew point temperature. Knowing the atmospheric temperature and the dew-point temperature, the relative humidity can be determined by means of a table.
The saturated salt system operates at dew points between 10° to 100°F with an error of less than ± 2°F. The system costs less than the condensation system, is not significantly affected by contaminating ions, and has a response time of about 4 minutes. The condensation type is very accurate over a wide range of dew point temperatures (less than ± 1°F from -100° to 212°F). A condensation dew point hygrometer can be expensive.
