937A Multi-Sensor System
B.1
Appendix B: Leak Testing
with the Series 937A
While the Series 937A Controller is not intended to replace mass
spectrometer leak detectors, it offers a simple and inexpensive method for
locating leaks in high vacuum systems. Under ideal conditions, a Pirani
sensor can detect leaks as small as 1x10
-4
Torr l/s and the cold cathode
sensor can be used to detect leaks as small as 1x10
-7
Torr l/s.
As with any leak testing, many factors can influence the sensitivity of the
test. Described in greater detail below, these include chamber volume; system
pressure; probe gas; type of vacuum pump; location of the Sensor, leak, and
pump; and others such as pumping speed and system tube size.
Reducing the search area by minimizing the chamber volume will
increase the efficiency of the test.
Sensitivity to gas leaks is also pressure dependent. In general, leak
test sensitivity is greater for lower system pressures.
The Pirani Sensor is sensitive to any leak probe gas lighter or
heavier than the gas in the system. For optimal sensitivity, select a
probe gas with the largest difference between its molecular weight
and that of the system gas.
A gas different than the system gas entering through a leak will change the
thermal energy transfer. Lighter gases increase thermal energy transfer while
heavier gases reduce it. Helium or argon gas are suitable to probe a system
pumping air or nitrogen.
Slowly and methodically probe with a small amount of gas. Flooding the leak
with gas or moving the gas quickly past the leak can confuse the search
since system time lags may be significant.
Since set points remain active in the Leak Test function, the
probe gas may change the indicated pressure enough to switch
the relay state. Disable any process control while probing the
vacuum system.
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, they may cause cyclical variations in
pressure with rotation of the vanes. This shows up as a large
background noise signal possibly masking the leak signal.