The wet-and-dry-plate dew point hygrometer is composed of a wet-and-dry-plate hygrometerand a cooling apparatus.
Being cooled properly, the air to be measured flows from the dry-part to the wet-part of micaplate set in the wet-and-dry-plate hygrometer. The temperature difference t-tw of both parts ismeasured by the thermopile, and the air temperature t (dry-part temperature) is measured bythermister thermometer.
When the air is cooled down with constant pressure, the dew point tD is always the same so faras air temperature is above the dew point. Therefore, the temperature difference t-tw is functionof the air temperature t and its dew point tD. As t-tw is small and the dew point tD is constant, the temperature difference t-tw is proportional to t-tD, in other words, it varies linearly with thetemperature t. This fact is proved by experiment as expected theoretically.
The dew point tD is determined by extrapolating the curve which shows the relation betweent and (t-tw).
