7.2.6 Integral leak detection
Integral leak detection is used to determine the total leak rate, i. e. the total leak rate of all leaks in the test specimen. Here, too, the vacuum method and the sniffer method can be used.
| Sniffer Leak Detection | Vacuum Leak Detection | ||
|---|---|---|---|
| Method | Accumulation test, collection of escaping test gas in an enclosing shell or chamber |
Test specimen under overpressure, measurement of escaping test gas in a vacuum chamber | Test specimen under vacuum, measurement of test gas escaping from an enclosing shell into the test specimen |
| Mechanical strength | Against overpressure of the test gas | Against overpressure of the test gas | Against atmospheric pressure from outside against vacuum (pressure differential 1000 hPa) |
| Speed | slow | fast | fast |
| Limit of detection | Use mainly > 1 · 10-5 Pa m3 s-1 | < 5 · 10-13 Pa m3 s-1 | < 5 · 10-13 Pa m3 s-1 |
Table 7.2: Integral leak detection by means of the sniffer and vacuum methods
In the integral vacuum method (method A1 in accordance with DIN EN 1779, Figure 7.7 right-hand side), the test specimen (e. g. a vacuum system) is evacuated and the surrounding space is filled with a defined quantity of test gas. The surrounding enclosure can be a plastic film or a rigid vessel. It is important that the test specimen is exposed to a defined quantity of the test gas to enable conclusions to made about the test gas concentration at the leak and a reliable quantitative conclusion to be reached.
When testing enclosed test objects (method B6 in accordance with DIN EN 1779, Figure 7.7 left-hand side) the test specimen is filled with helium and placed in an encasing vacuum vessel. The escaping test gas is identified and quantified by the leak detector.
Figure 7.7: Integral leak detection with the vacuum method
In the sniffer method, the test specimen is filled with the test gas (method B3 in accordance with DIN EN 1779, Figure 7.8) and placed in an enclosing vessel. Contrary to the method described previously, this vessel does not require to be evacuated and can remain at atmospheric pressure. This means that less stringent requirements are placed on the apparatus as in the previously described method. The escaping gas is collected in the enclosing shell and needs to be mixed well during the test (using a fan, for instance) to ensure that a uniform concentration of test gas is present in the analytical chamber. The sniffer probe of the leak detector is used to determine the increase in the concentration of the test gas escaping from the test specimen which collects in the enclosing shell. The detection limit for this method is determined by the concentration of the test gas in the dead volume of the enclosing shell and the additional increase in the test gas concentration. This means that this method is considerably slower than the integral method under vacuum and its use is normally restricted to small test specimens with limited part throughput.
Figure 7.8: Integral leak detection of enclosed objects with the sniffer method
A fuller description of the methods and procedures for leakage testing with test gases is contained in our leak detection compendium.
