The DEWRay On-line is a precision chilled-mirror dewpoint hygrometer for the measurement of moisture in air or gases. The operating principle is based on continuous measurement of the dewpoint (or frostpoint) temperature of a gas.Inside the sensor head is a gold-plated copper mirror which is cooled by a two stage Peltier element and an optical dew detection system. A Pt100 resistance thermometer measures mirror temperature.
Formation of dew or frost on the mirror surface is detected by the optical system. The temperature at which this happens is known as the dewpoint temperature.It is a well known physical phenomenon that when water vapor in a gas condenses at temperatures below the freezing temperature, super-cooled water may condense instead of ice. The problem is unpredictable. On a new and clean mirror, super-cooled water may form even at temperatures as low as -48°C, in other cases ice may form immediately. This can give significant measurement errors of as much as 4°C in this type of instrument. Some manufacturers of cooled-mirror hygrometers add a microscope to the instrument so that an operator can confirm by human eye if ice or water is present. Others induce formation of ice by a deep freezing cycle.
