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7.2.
Types of Gas Measured and Indication
As briefly explained in "7.1 Principles of Operation", the indication of the Pirani vacuum gauge
changes with the type of gas measured.
In the molecular flow region, A (free molecule thermal conductivity) is given by the following
equation.
Λ
=
C
(R /
MRT'
ν
π
+
2
2
)
.......................
(3)
where, M is molecular weight, R is a gas constant. T' is the average temperature of T and To, and
C
ν
is specific heat at constant volume. This equation shows that it is dependent on C
ν
and is in
inverse proportion to
MT'
. In a high pressure region, the number of molecules that carry heat
energy increases, while the free path of molecule becomes short, so that the amount of transport per
molecule decreases in effect and the change of the indication due to the type of gas measured
becomes complex.
Since the Pirani gauge measurement region is an intermediate region between molecular flow and
viscosity flow, the difference in indication due to the type of gas is not simple and is difficult to
express by an equation.
For your reference, Fig. 7-1 shows the relationship between the indication and pressure when
representative gases are measured.
