Vacuum Technology Know How - Triumf

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Test gas
V 1
q
S V
1) Recipient
2) Sniffer probe
V Recipient valve
1
V Mass spectrometer valve
2
Figure 5.2: Schematic diagram of a helium counterflow leak detector
1
V 2
A workpiece 1 is evacuated via the test gas connection with valve V open. Valves V and V 1 2 1
are connected in such a manner that the required backing vacuum pressure of the turbopump
always takes priority over evacuation of the workpiece. Once the workpiece is evacuated, it
can be connected to the backing vacuum, or to a tap on the turbomolecular pump via valve V . 3
Depending upon the pressure range in question, helium is now sprayed onto the workpiece
from the outside and together with the ambient air penetrates into the workpiece through
leaks. The helium present in the residual gas flows counter to the pumping direction through
valves V and V and through the turbopump to mass spectrometer MS, where it is detected.
1 2
The differing compression ratios of the turbopump for helium and air, which differ by multiple
powers of ten, are utilized in this regard. While the high compression ratio of the turbopump
keeps air away from the mass spectrometer, the helium arrives there at a relatively high
partial pressure. The turbopump thus acts as a selective amplifier for helium.
This is why a mass spectrometer enables helium to be detected in the workpiece even at
pressures of less than 1 mbar. Several powers of ten of the helium partial pressure, and thus
a leakage rate range of between 10 and 10 - 8 mbar l / s, can be covered by means of various
taps on the turbopump (V ), as well as by operating it at different speeds that exponentially
3
influence the compression ratio. A pressure of 10 - 3 mbar must be attained in the workpiece
for the highest sensitivity stage of the leak detector (inlet via V ). 3
V 3
2
MS
K
P V
T
He
3
P MS
He
Backing vacuum
V Tap valve
3
S Backing pump
V
K Turbopump
MS Mass spectrometer
Page 115
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