4/9/98
A-14 Application of the Hygrometer (900-901D1)
Nomenclature (cont.)
• 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)
Parts per Million by Volume
The water concentration in a system, in parts per million by
volume, is proportional to the ratio of the water vapor partial
pressure to the total system pressure:
(1)
In a closed system, increasing the total pressure of the gas will
proportionally increase the partial pressures of the various
components. The relationship between dew point, total pressure
and PPM
V
is provided in nomographic form in Figure A-1.
Note: The nomograph shown in Figure A-1 is applicable only to
gases. Do not apply it to liquids.
To compute the moisture content for any ideal gas at a given
pressure, refer to Figure A-1. Using a straightedge, connect the
dew point (as measured with the GE Panametrics’ Hygrometer)
with the known system pressure. Read the moisture content in
PPM
V
where the straightedge crosses the moisture content scale.
Typical Problems
1. Find the water content in a nitrogen gas stream, if a dew point
of -20°C is measured and the pressure is 60 psig.
Solution: In Figure A-1, connect 60 psig on the Pressure scale
with -20°C on the Dew/Frost Point scale. Read 200 PPM
V
, on
the Moisture Content scale.
PPM
V
P
W
P
T
--------
10
6
×=
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