sentences:  In this type of device, when gas flows across a reflective cooling surface, the eponymous chilled mirror, at the point that the surface is cold enough, the available moisture will start to condense onto it in tiny droplets.The exact temperature at which this condensation first occurs is registered and the mirror is then slowly heated until the water condensation begins to evaporate.This temperature is also registered and the average of the condensation and evaportion temperatures is reported as the dew point.