For inside-out leak detection, the test part can be filled with helium and sealed; it can be “bombed” with helium after it is sealed; or it can be pressurized with helium during the actual test. Then, the part is placed in a vacuum chamber; the chamber is evacuated; and the test instrument (a mass spectrometer) looks for helium escaping from the part into the chamber.
For outside-in leak detection, the test part is evacuated and connected to the mass spectrometer. Then, helium is sprayed around suspected areas outside of the part. If the part has a leak, helium will be drawn into the test part and be detected by the mass spectrometer.
The inside-out method with a vacuum chamber has some advantages, as a sniffer is not used. Testing is automatic and not dependent on the operator. The sensitivity achievable with this test is very high.
Even with these advantages, however, cost is a consideration with the vacuum chamber inside-out method. While the sniffing test requires only the leak detector and the ability of the operator, the hard-vacuum test requires more complex and expensive equipment.
Ultimately, the best method to use comes down to the application. The hard-vacuum method vastly reduces the incidence of human error, which makes it appealing in many industrial applications. The sniffing test is better for maintenance applications and for identifying the location of leaks in defective parts.
Production rates vary with the hard-vacuum test method. Rates range from 100 parts per hour to more than 700 parts per hour, depending on the configuration of the system (single chamber, dual independent chambers, dual alternating chambers, or even multiple test stations).
Systems differ in complexity, from manual loading and unloading by an operator to automatic loading and unloading with a robot. Depending upon the size and weight of the test part, various cranes might even be used to load and unload parts.
Some systems include modular vacuum chambers with wheels and quick connects for ease of changeover. In addition, some models even provide a charge-head tooling feature for changeover to accommodate various test part shapes, sizes, and configurations.