"Is the system tight?" is the wrong question – nothing is absolutely tight. The right one is: how tight does it need to be, and with which method do I prove it? Eight orders of magnitude lie between the simplest and the most sensitive leak detection method; choose wrongly and you either test past the requirement or pay needlessly.
The methods at a glance
| Method | Detection limit (mbar·l/s) | Effort | Typical use |
|---|---|---|---|
| Bubble test / soap solution | ~10⁻³–10⁻⁴ | Minimal | Gross leaks, assembly checks, domestic installations |
| Pressure decay test | ~10⁻³–10⁻⁴ | Low | Acceptance of pipe networks, vessel testing |
| Pressure rise test (vacuum) | ~10⁻⁴ | Low | Vacuum systems, first diagnosis |
| Sniffer leak detection (He or H₂/N₂) | ~10⁻⁶–10⁻⁷ | Medium | Leak location on pressurised systems |
| Helium vacuum method (integral) | down to ~10⁻¹¹ | High | High vacuum, semiconductors, vacuum chambers |
| Envelope / foil test | ~10⁻⁹ | High | Integral testing of individual components |
Coarse methods: fast and underrated
The soap solution (or leak detection spray) finds every leak you can hear or see within minutes – unbeatably fast for assembly checks and entirely sufficient for many compressed-air applications. The pressure decay test quantifies integrally: bring the network to test pressure, let the temperature stabilise (!), log the pressure curve. Its pitfall is temperature dependence – a 1 K temperature change fakes about 35 mbar of pressure change at 10 bar. Without temperature compensation, short-term measurements are worthless.
Sniffing: locating the leak
Integral methods say that it leaks, sniffing says where: put tracer gas on the system (helium or the cheaper 95/5 N₂/H₂ forming gas), scan the joints with the sniffer probe. Detection limit around 10⁻⁶ mbar·l/s – limited by the helium background of ambient air (5 ppm) and the probe speed. Move slowly: more than 2 cm/s misses small leaks.
The premier class: helium vacuum method
Evacuate the test object, spray helium on the outside, and the mass spectrometer in the leak detector counts every helium atom that enters. Detection limits down to 10⁻¹¹ mbar·l/s make the method the standard for semiconductor components, vacuum chambers and UHV technology. Two pitfalls: large leaks "flood" the detector and thereby disguise themselves (always pre-test coarsely!), and the result is a helium leak rate – comparison with air or process gas specifications requires conversion depending on the flow regime. That is exactly what the Leak Rate Converter (Leak Testing) is for.
Which method for which requirement?
The decision logic in three questions: 1. What leak rate does the specification demand? Keep one decade of safety margin between detection limit and acceptance limit. 2. Locate or quantify? Integral methods for the test report, sniffing and spraying for the repair. 3. What does an overlooked leak cost? For a high-purity gas line drawing in moisture for years, the helium leak detector pays for itself quickly – more on this in our article The 7 Most Common Mistakes in Handling High-Purity Gases. Rule of thumb: test as coarsely as possible, as finely as necessary – but derive the requirement honestly from the process, not from the instrument you happen to own.
