Application of the Hygrometer (900-901D1)
A-13
4/9/98
Calculations and Useful Formulas in Gas
Applications
A knowledge of the dew point of a system enables one to
calculate all other moisture measurement parameters. The most
important fact to recognize is that
for a particular dew point there
is one and only one equivalent vapor pressure
.
Note:
The calibration of GE Panametrics moisture probes is
based on the vapor pressure of liquid water above 0°C
and frost below 0°C. GE Panametrics moisture probes
are never calibrated with supercooled water.
Caution is advised when comparing dew points measured with a
GE Panametrics hygrometer to those measured with a mirror type
hygrometer, since such instruments may provide the dew points
of supercooled water.
As stated above, the dew/frost point of a system defines a unique
partial pressure of water vapor in the gas. Table A-1, which lists
water vapor pressure as a function of dew point, can be used to
find either the saturation water vapor pressure at a known
temperature or the water vapor pressure at a specified dew point.
In addition, all definitions involving
humidity
can then be
expressed in terms of the water vapor pressure.
Nomenclature
The following symbols and units are used in the equations that
are presented in the next few sections:
•
RH = relative humidity
•
T
K
= temperature (°K = °C + 273)
•
T
R
= temperature (°R = °F + 460)
•
PPM
v
= parts per million by volume
•
PPM
w
= parts per million by weight
•
M
w
= molecular weight of water (18)
•
M
T
= molecular weight of carrier gas
