Theory of operation
CGS-240 Manual 6/15/2021
Page 5 of 148
THEORY OF OPERATION
2.1 TWO-PRESSURE PRINCIPLE
In an ideal two-pressure system, a stream of gas at an elevated pressure is saturated with respect to the liquid or
solid phase of water and then expanded isothermally to a lower pressure. Measurements of the pressure and
temperature of the gas stream, both at saturation and after expansion, are all that is required to determine the
resulting humidity content of the expanded gas stream. The two-pressure system is shown in Figure 1.
Figure 1—Simplified schematic diagram of the two-pressure principle where T
s
is saturator temperature, T
c
is
chamber temperature, and T
s
= T
c
.
A two-pressure generator is commonly used in the generation of a range of relative humidity values at fixed
temperatures. The saturator and chamber share a common fluid bath temperature and are, ideally, in thermal
equilibrium with each other. In this case, where saturator temperature, Ts, and chamber temperature, Tc, are of
nearly the same value, the generated relative humidity may be approximated by the ratio of the measured
saturated gas-stream pressure (saturator pressure) to the measured chamber pressure by using the simplified
expression
where P
c
is the absolute chamber pressure
P
s
is the absolute saturation pressure.
Please note that the above expression ignores the use of enhancement factors which account for non-ideal
properties of the air / water-vapor mixture, and is therefore only an approximation. More exacting equations
which include enhancement factors are used internally within the system.
Saturation
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