ABSTRACT

The chilled mirror hygrometer, sometimes called an optical condensation hygrometer, is the most accurate, reliable, and fundamental hygrometer commercially available and is therefore widely used as a calibration standard. The ftrst commercial applications of the hygrometer started in the early 1960s after it became practical to use thermoelectric coolers and electro-optical detection methods. Prior to 1960, the use of chilled mirror hygrometers was primarily conftned to laboratories because of the difficulties in cooling the mirror with cryogenic liquids, controlling its temperature, and detecting the onset of dew, usually through visual means.