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Appendix A __________________________________________________________ Humidity Theory

VAISALA ______________________________________________________________________ 127

APPENDIX A

HUMIDITY THEORY

This Appendix provides you with a basic explanation of the terms

related to humidity theory that are used in many parts of this manual.

Measured and Calculated Results of DM500

DM500 measures the dewpoint temperature of a gas by condensing

water on a surface and using SAW technology to detect this

condensation. The temperature at which condensation occurs is

accurately measured by a four wire platinum resistance temperature

sensor and is reported as dewpoint/frostpoint temperature. Other

humidity parameters are calculated by using the measured

dewpoint/frostpoint temperature data and either measured or set

values for pressure and temperature.

Water Vapor in Air

Water vapor is water in the gaseous phase. It is an abundant

component of the earth’s atmosphere, and it is also common in many

industrial processes. At atmospheric pressure, water behaves like an

ideal gas.

Dalton’s law states that in a gas mixture, such as air, the total pressure

of the gas is the sum of the partial pressures of each of the component

gases

otherswateroxygennitrogentotal

PPPPP +++= (1)

Thus, atmospheric pressure is actually the sum of the partial pressures

of nitrogen, oxygen, water vapor, carbon dioxide, and small amounts

of other gases.

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Vaisala DEWCAP DM500 Guia do Utilizador Página 129