15.3
Thermodynamic concepts
Term
Description
% rH
The relative humidity is the relationship of water vapour partial
pressure in the test gas to maximum possible water vapour partial
pressure (water vapour-saturation pressure) at the same
temperature and total pressure
Test temperature
The test temperature, T
t
, is the gas temperature at the measuring
location.
Test pressure
The test pressure, Pt, is the total pressure in the gas at the
measuring location. The specification defines it as the absolute
pressure against vacuum.
Dew point
The dew point is the temperature up to which the gas is to be
cooled down so that straight water vapour condenses out as liquid.
Normally dew point is valid only for temperatures above the freezing
point. In certain cases, water can also condense below the freezing
point of liquid. However, in such a case, the dew point temperature
is not identical with the frost point temperature.
Frost point
The frost point is the temperature up to which the gas is to be
cooled down so that straight water vapour gets condensed to ice.
The frost point is valid only for temperatures below the freezing
point.
Absolute Humidity
The absolute humidity is the mass of water vapour per unit volume
of humid gas.
Specific humidity
The specific humidity is the ratio of the mass of water vapour to the
total mass of the humid gas.
Wet bulb
temperature
The wet bulb temperature Tw, is the temperature of the moistened
thermometer during measurement of relative air humidity with the
Aspirations-Psychometer. A thermometer covered with a moistened
wool wick is exposed to a constant airflow. After some time,
temperature equilibrium is reached. Together with ambient
temperature and total pressure, the relative humidity can also be
calculated from the wet bulb temperature
Enthalpy
The enthalpy is a measure of the energy which is necessary to bring
the gas at a certain temperature, pressure and humidity condition
into another condition. The zero point of the enthalpy has been fixed
at 0°C and 0% rH. In practice, it is often not the absolute value but
the difference of Enthalpy between two climate-conditions of interest
ppm volume
ppm volume is the ratio of the number of water molecules to the
number of molecules of remaining gas components. This humidity
parameter is independent of the total pressure and temperature
ppm weight
ppm weight is the ratio of the mass of water molecules to the mass
of the molecules of all remaining gas components. This humidity
parameter is independent of the total pressure and temperature.
