7.2.5 Local leak detection
Local leak detection is used to identify leakage in a test specimen.
In the vacuum method, the test specimen (vessel) is connected to the leak detector, and helium is briefly sprayed onto a suspected area using a spray gun. If the pressure in the test specimen is in the molecular flow range, i. e. < 10-3 hPa, the test speed will be dependent on the volume of the test specimen and the effective pumping speed of the test setup for helium. The smaller the test specimen or the greater the pumping speed of the leak detector or auxiliary pump used, the quicker the result is obtained. At higher pressures, particularly in the laminar flow range greater than 1 hPa, the display speed will be much slower and will be governed by the pumping speed of the leak detector‘s backing pump.
In the sniffer method according to Figure 7.6 the test specimen (3) is filled with test gas overpressure. A sniffer probe (2) is connected to the test gas connection of the leak detector. The test gas that escapes through leaks in the test specimen can be detected by sniffing with the probe.
3
2
Testgas
Testgas
Figure 7.6: Local leak detection with sniffing and vacuum methods
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.
| Sniffer Leak Detection | Vacuum Leak Detection | |
|---|---|---|
| Methode | Sniffing the test gas-filled test object | Spraying with helium |
| Mechanical strength | Against overpressure | Against atmospheric pressure from the outside against vacuum (pressure difference 1000 hPa) |
| Detection limit | < 1 · 10-8 Pa m3 s-1 | < 5 · 10-13 Pa m3 s-1 |
Table 7.1: Local leak detection by sniffer and vacuum methods
