MKS HPS 945 Series, Operation And Maintenance Manual

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19
Digital Pirani vacuum Sensor 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. In
general, leak test sensitivity is greater for lower system pressures.
The Series 945 Pirani 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.
If the above detection method does not locate a leak, unexpected high
pressures may be caused by a virtual leak, i.e., outgassing of a system
component. You can locate outgassing parts, or "virtual leaks", as well as
true gas leaks, using the
rate-of-pressure-rise method.
1) With the Controller on, pump down the system to a base pressure.
2) Isolate the pump by closing a valve.
3) Measure the rise in pressure over a time interval. A very fast rise
indicates a leak.
4) Repeat this procedure as often as necessary.