WADS 202 – Dew Point Sensor

Dew Point is the Temperature to Which Air Can be Cooled Down without Condensation. DewPoint is Pressure Dependent and Would Change when Air is Compressed.

The Term “pressure dew point” is encountered when measuring the dew point temperature of gases at pressures higher than atmospheric pressure. It refers to the dew point temperature of a gas under pressure. This is important because changing the pressure of a gas increases the dew point temperature of the gas. Consider an example of air at atmospheric pressure of 1013.3 mbar with a dew point temperature of -10C. The partial pressure of water vapour (designated by the symbol “e”) in this case is 2.8 mbar. If this air is compressed and the total pressure is doubled to 2026.6 mbar, then according to Dalton’s law, the partial pressure of water vapour, e, is also doubled to the value of 5.6 mbar. The dew point temperature corresponding to 5.6 mbar is approximately -1C, so it is clear that increasing the pressure of the air has also increased the dew point temperature of the air. Conversely, expanding a compressed gas to atmospheric pressure decreases the partial pressures of all of the component gases, including water vapour, and therefore decreases the dew point temperature of the gas. The relationship of total pressure to the partial pressure of water vapour, can be expressed as follows,

P1/P2 – e1/e2

BY converting dew point temperature to the corresponding saturation vapour pressure, it is easy to calculate the effect of changing total pressure on the saturation vapour pressure. The New saturation vapour pressure value can then be converted back to the corresponding dew point temperature. These calculations can be done manually using tables, or performed by various kinds of software.