Vaisala DM500 - Manual (Page 129)

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
others water oxygen nitrogen total
P P P P P + + + = (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.