GE Moisture Image 1 Series, Service Manual, Page: 73 / 107

Application of the Hygrometer (900-901D1) A-11
June 2003
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 on page A-15,
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
•
P
S
= saturation vapor pressure of water at the prevailing
temperature (mm of Hg)
•
P
W
= water vapor pressure at the measured dew point
(mm of Hg)
•
P
T
= total system pressure (mm of Hg)