Chilled-mirror hygrometers

Chilled-mirror hygrometer (CMH) has been used for the past several decades in humidity transfer applications. The hygrometer is based on the measurements of the dew point temperature of a gas in a controlled environment. It condenses the reflective condensation surface by cooling and sensing the small condensed water droplets optically. Thermoelectric coolers are used to condense the gas.

A metallic reflective surface, chilled-mirror, is maintained in dynamic equilibrium with the water vapor pressure of the surrounding gas sample by maintaining a critical temperature. The critical temperature is at which the rate of dew condensation is equal to the rate of the evaporation of dew layer from mirror. This critical temperature is the dew point temperature of the sample where the mass of the dew layer is maintained constant. The intensity of a reflected beam of LED light from the chilled-mirror is measured by photodetector. The schematic diagram of CMH is given in Fig. 17. The dew point temperature is related to ambient RH and then to total suction using Kelvin's formula.

Fig. 4.17 Working mechanism of Chilled-mirror hygrometer