MKS HPS 947 Series, Operation And Maintenance Manual, Page: 26 / 40

20
HPS
®
Products Series 947 CEP System
The pressure will either rise or fall, depending upon the thermal conductivity
of the probe gas relative to the system gas. The largest change in value
indicates the probe gas is nearest the leak location.
4) Repeat the test to confirm.
As with any leak testing, many factors can influence the sensitivity of the
test. Explained in further detail below, these include chamber volume;
system pressure; probe gas; type of vacuum pump; location of the Sensor,
leak, and the pump; and others such as pumping speed and size of tubing
in the system.
Minimize the chamber volume to reduce the search area and
increase the efficiency of the test.
Sensitivity to gas leaks is also pressure dependent due to the
complex nature of heat transfer at pressures above 10
-1
Torr
and the presence of significant radiation and end losses below
10
-3
Torr. Leak test sensitivity is generally greater for lower
system pressures.
The Series 947 CEP System is sensitive to any leak probe gas
lighter or heavier than the system gas, however, for optimal
sensitivity, select a probe gas with the largest difference between
its molecular weight and that of the system gas. Lighter gases
increase thermal energy transfer while heavier gases reduce it.
Helium or argon gas is suitable to probe a system pumping air
or nitrogen.
The type of vacuum pump used can also affect the accuracy of the
leak test. For moderate size leaks, pump down the system with a
high vacuum pump such as a diffusion or turbo pump if
possible (ion and cryo pumps are not recommended). Leak
testing can be done with a mechanical pump, however, cyclical
variations in pressure may result due to rotation of the vanes.
This shows up as a large background noise signal, possibly
masking the leak signal.
Place the pump away from the suspected leak source and place
the Sensor between the leak and the pump to reduce the Sensor
response time. Vacuum tubing between the suspected leak and
the Pirani Sensor should be as short and wide as possible to
shorten the time required for the probe gas to reach the tubing.