When gas flows over a chilled surface, or chilled mirror, the moisture will condense on it. The exact temperature at which this condensation begins is known as the dew point. The temperature of this mirror is reduced from high to low, and the temperature is read exactly when the dew is observed. By obtaining the dew point temperature, one can calculate moisture content in the gas. The mirror temperature is controlled by the flow of a refrigerant over the mirror or by using a thermoelectric cooler.
The detection of condensation on the mirror can be achieved using visual or optical means. For example, a light source can be reflected off the mirror into a detector and condensation detected by changes in light reflected. The observation can also be done visually; however the exact point at which condensation begins is not discernible to the eye. Since the temperature is passing through the dew point rather than stopping exactly at the dew point, the measurement tends to be high and with it a high standard deviation. Additionally, the condensation of moisture can be confused with condensation of other condensable such as heavy hydrocarbons, alcohol, and glycol. Automated on-line systems are not able to make these distinctions, and only highly skilled operators can use manual systems.