GE Moisture Image 1 Series, Service Manual, Page: 75 / 107

Application of the Hygrometer (900-901D1) A-13
June 2003
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:
(0-2)
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.
(0-3)
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 on page A-20, 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% .
Weight of Water per Unit
Volume of Carrier Gas
Three units of measure are commonly used in the gas industry to
express the weight of water per unit volume of carrier gas. They all
represent a vapor density and are derivable from the vapor pressure of
water and the Perfect Gas Laws. Referenced to a temperature of 60°F
and a pressure of 14.7 psia, the following equations may be used to
calculate these units:
(0-4)
(0-5)
(0-6)
Note: MMSCF is an abbreviation for a “million standard cubic
feet” of carrier gas.
PPM
W
PPM
V
M
W
M
T
---------- ×
=
RH
P
W
P
S
-------- 100 ×
=
mg of water
liter of gas
----------------------------- 289
P
W
T
K
--------
×
=
lb of water
ft
3
of gas
-------------------------- 0.0324
P
W
T
R
-------- ×
=
lb of water
MMSCF of gas
-------------------------------------
PPM
V
21.1
----------------
10
6
P
W
×
21.1 P
T
×
----------------------- = =