5.3 Application notes
Prior to beginning any leak detection process with helium, the user must clarify several fundamental questions:
- Is the workpiece vacuum-proof?
- Is the workpiece overpressure-proof?
- Is the total leakage rate of the workpiece to be determined?
- Is only the location of the leakage to be determined or should it be quantified?
An appropriate test method can be selected from among the methods indicated under 5.2.2 to 5.2.5.
Leak detection with helium
The leak detector must be calibrated prior to beginning. A helium test leak integrated in the Pfeiffer Vacuum SmartTest leak detector is used for this purpose. The calibration routine is started at the touch of a button and runs automatically. Following calibration, the leak detector is ready for use. For leak detection under the vacuum method, the best option is to use an audible indicator, where the frequency of the signal tone rises as the leakage rate increases, thus eliminating the need for a second person to read the display while the workpiece is being sprayed.
The following must always be observed when using helium as the test gas:
- Helium is lighter than air. So when helium is used in the atmosphere, the leak detection process should always begin at the highest point of the workpiece
- Excessive amounts of the test gas should not be sprayed, as this can increase the concentration of helium in the ambient air, which would constantly simulate leaks that do not exist
- Because helium accumulates in the backing pump, in the exhaust space and in the oil, and can return to the backing vacuum area from these points, the gas ballast in the backing pump must be energized if there are high leakage rates. This usually occurs automatically when the higher measuring ranges are selected in the leak detector. The gas ballast must be energized manually if auxiliary pumps are being used
Under the vacuum method, it is necessary to generate sufficiently good vacuum to allow the leak detector to be operated at maximum sensitivity.
Additional vacuum pumps (auxiliary pumps) with high pumping speeds must therefore often be used for large workpieces. In this case, the leak detector should be connected directly to the recipient pump ports for the large vacuum pump, or at least directly adjacent to them.
When the auxiliary pump is running, the measured leakage rate must be increased by the pumping speed ratio between the auxiliary pump and the leak detector in order to determine the leakage rate.
When working with the sniffer valve, the pressure in the vessel must be at least 100 mbar higher than the ambient pressure. Due to strong mixing with the air, the sensitivity of the sniffer method is lower than that of the vacuum method. Moreover, the delayed reaction of the leak detector to the inflowing helium must also be taken into consideration.
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