Application of the Hygrometer (900-901D1) A-15
4/9/98
Parts per Million by Volume (cont.)
2. Find the expected dew/frost point for a helium gas stream
having a measured moisture content of 1000 PPM
V
and a
system pressure of 0.52 atm.
Solution: In Figure A-1, connect 1000 PPM
V
on the Moisture
Content scale with 0.52 atm on the Pressure scale. Read the
expected frost point of –27°C on the Dew/Frost Point scale.
Parts per Million by Weight
The water concentration in the gas phase of a system, in parts per
million by weight, can be calculated directly from the PPM
V
and
the ratio of the molecular weight of water to that of the carrier gas
as follows:
(2)
Relative Humidity
Relative humidity is defined as the ratio of the actual water vapor
pressure to the saturation water vapor pressure at the prevailing
ambient temperature, expressed as a percentage.
(3)
1. Find the relative humidity in a system, if the measured dew
point is 0°C and the ambient temperature is +20°C.
Solution: From Table A-1, the water vapor pressure at a dew
point of 0°C is 4.579 mm of Hg and the saturation water vapor
pressure at an ambient temperature of +20°C is 17.535 mm of
Hg. Therefore, the relative humidity of the system is
100 x 4.579/17.535 = 26.1%.
PPM
W
PPM
V
M
W
M
T
----------
×=
RH
P
W
P
S
--------
100×=
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