Vaisala DM500 - Manual (Page 130)

User's Guide ______________________________________________________________________
128 _________________________________________________________________ M210205EN-D
There are many ways of expressing the water vapor content of a gas.
Some water vapor quantities, such as dewpoint temperature, can be
measured directly and fundamentally. Other parameters, such as
relative humidity, require knowledge of additional information, such
as the temperature of the gas.
Saturation Vapor Pressure
Saturation vapor pressure is the maximum pressure of water vapor that
can exist at a given temperature. This quantity is expressed in pressure
units such as Pascals or millibars, and is often represented by the
symbol “e.” There is a unique saturation vapor pressure for any
temperature. Thus, if a gas is cooled until condensation forms (the
dewpoint temperature), it is possible to determine the saturation vapor
by measuring the temperature at which condensation forms or remains
in equilibrium.
Dewpoint Temperature (T
d
)
Dewpoint is the temperature at which condensation begins to form
when a gas is cooled. An object or surface is said to be at the dewpoint
temperature when condensation is maintained in an equilibrium
condition (amount of condensation is neither shrinking nor growing).
Dewpoint has a unique correlation to the saturation vapor pressure of
water. Accurate determination of dewpoint establishes knowledge of
the partial pressure of water vapor in a gas. Knowledge of additional
gas parameters, such as temperature and pressure, enable calculation
of commonly used humidity parameters (percent relative humidity,
parts per million by volume, mixing ratio, etc.).
Frost Point Temperature (T
f
)
Dewpoints below 0 °C are often referred to as frost points, although
the term “dewpoint” is often used interchangeably. Frost point is the
temperature at which a gas is saturated with respect to a plane surface
of ice. Saturation vapor pressure over ice is slightly lower than over
water. This difference can be important in the temperature ranges
below zero (0 ºC) where it is possible to have condensation either in
the solid phase (frost) or in the liquid state (dew, supercooled water).