Ulvac GP-1G - Types of Gas Measured and Indication

APPENDIX
- 17 -
5.2. Types of Gas Measured and Indication
As briefly explained in "5.1 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
Cν is specific heat at constant volume. This equation shows that it is dependent on Cν 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. 5-1 (Indication of various types of gas) shows the relationship between the
indication and pressure when representative gases are measured.