sentences:  All chilled-mirror devices, both manual and automatic, are based on this same basic method.It is necessary to measure both the condensation and evaporation temperatures, because the dew point is actually the equilibrium temperature at which water molecules are both condensing and evaporating in the same rate.Since, when cooling the mirror, the temperature is passing through the dew point rather than stopping exactly at it, the condensation temperature measurement in itself is too low as the mirror will have reached a temperature slightly below the dew point before water condensation starts to form.