Vacuum Technology Know How - Triumf

Pfeiffer Vacuum
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Vacuum Technology
Because of the upstream turbopump, the mass spectrometer always operates at an
extremely low total pressure, and is thus well protected against contamination and failure.
5.2.2 Test methods
A distinction is made between two methods of leak detection:
Local leak detection, which is used to find leaks
Integral leak detection, where the leakage rate of workpieces is typically determined
for quality assurance purposes
Leak detectors are equipped for two operating methods:
The pump-down method, in which the workpiece is evacuated and helium exerts its
effect from the outside
The sniffer method, in which the workpiece is filled with a helium overpressure �p of
more than 100 mbar, and the escaping helium is sucked into the leak detector via a sniffer
valve (capillary, metering valve) and detected
5.2.3 Calibrating the leak detector
The leak detector must be calibrated in order to determine leakage rates. This is done by
means of a commercial test leak, a small helium-filled vessel with a shut-off valve that emits
the defined quantity of gas. This is usually integrated into the leak detector or connected to
the intake side of it. For calibration, an appropriate working cycle is often built in that automatically
performs the calibration.
To obtain precise measurements, the device should be calibrated before each use. To test
large workpieces for which additional vacuum pumps are in use, it is advantageous to use
an external test leak. The measurement accuracy can be a function of where the test leak is
attached. Consequently, it is necessary to take flow conditions within the vacuum area into
consideration.
5.2.4 Local leak detection
Local leak detection is used to identify leakage in a workpiece.
Under the pump-down method, the workpiece (vessel) is connected to the leak detector,
and helium is briefly sprayed onto a suspicious area by means of a spray gun. If the pressure
in the workpiece is in the molecular flow range, i.e. less than 10 - 3 mbar, there will be an
immediate display as a result of the high velocity of the helium atoms. At higher pressures,
particularly in the laminar flow range that starts at 1 mbar, the display speed will be much
slower and will be governed by the pumping speed of the leak detector‘s backing pump.
Under the sniffer method (Figure 5.3), the workpiece (3) is filled with a helium overpressure.
A sniffer probe (2) is connected to the test gas connection. The helium that escapes through
leaks in the workpiece can be detected by sniffing with the probe.
Individual leaks can be identified using local leak detection. However the sum of all leakage
cannot be determined. That is why this process offers only limited suitability for providing a
GO / NO GO indication for quality assurance purposes.
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