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Helium is the most popular choice of tracer gas to find leaks for a number of reasons. It is non-toxic, inert, non-condensable, non-flammable and normally present in the atmosphere at trace levels of 5 parts per million. Due to its small atomic size (second smallest molecule), helium passes easily through leaks. Helium is also relatively inexpensive and is available in various size cylinders.
The Helium Leak Detector
A Helium Leak detector (HLD), also known as a Mass Spectrometer Leak Detector, is used to locate and measure the size of leaks into or out of a system or containing device. Helium, as a tracer gas, is introduced to a test part that is connected to the leak detector. The helium leaking through the test part enters through the system and this partial pressure is measured and the results are displayed on a front panel display.
HLDs consist of the following components:
- Mass Spectrometer to detect the level of helium
- High Vacuum system to maintain the pressure in the spectrometer
- Mechanical roughing pump to evacuate the test part or chamber containing the part under test
- Valves which enable the various stages of the test sequence such as evacuation, gross test, fine test, sniffing and venting
- Collector amplifier and readout instrumentation to monitor the output signal
- Power supplies and controls
- Chamber/Fixturing that attaches the part to be tested to the detector’s inlet test port
Methods of Leak Testing with Helium
There are three main methods to leak test parts using helium: Vacuum Testing (outside-in), Pressure Testing (Inside-out) and finally Pressure/Vacuum testing. In general whenever possible the detection method should be selected based on the working conditions of the part to be tested. It is important to maintain the same pressure conditions during the test as will exist during the actual use of the part. For example vacuum systems should be tested using the vacuum mode, whereas an air conditioning unit would normally be tested using the pressure mode.
Vacuum Testing - Outside-in
In vacuum testing, the part is evacuated with the leak detector’s roughing pump. Once a suitable vacuum level has been achieved within the product, the test valve of the HLD would be energised. At this point helium is administered to the suspected leak sites of the part using a spray probe with an adjustable flow.
Pressure Testing - Inside-out
In pressure testing, the part is pressurized with helium or a mixture of helium and nitrogen or air. To locate a leak, the potential leak sites of the part are scanned using a sniffer probe connected to the sniffer inlet of the leak detector.
As the name suggests, is a combination of the above two methods. Here, the part under test is pressurised with helium while being place into a vacuum chamber or bell jar. This method has the advantage over pressure testing in that it will measure the total cumulative leak rate of the part under test.
Leak Testing Vacuum Systems and Pressure Systems
Vacuum systemsand pressure systems should be leak tested under the same conditions as their operational conditions. Vacuum systems are tested with a portable leak detector. The leak detector is generally connected to the systems vacuum pump. Helium is applied to the potential leak site using a spray probe. If a leak exists, helium enters the system and quickly diffuses through it. The leak detector should respond within seconds. Pressure systems can be charged with helium or a mixture of helium and nitrogen. The leak testing is performed by using a sniffer probe.
Helium Leak Detection for Production Applications
Quality control of production parts and assemblies using helium leak detectors can help assure the integrity of the production process. Typical examples include: hermetically sealed semiconductor packages, valves, manifolds, seals, vacuum vessels and systems, medical devices, brake lines, fuel lines, hydraulic lines, refrigeration assemblies, radiators, heat exchangers, condensers, storage tanks.
Helium Leak Detection for Maintenance of Systems
Industrial process tools that use vacuum systems or pressure systems must be tested to check for occasional leaks. This can be part of preventative maintenance or in the event of an unexpected failure. Typical examples of vacuum systems include: Vacuum furnaces, vacuum coaters, electron microscopes, glove boxes, linear accelerators, electron beam and ion beam process equipment, semiconductor process equipment, laser process equipment. Typical examples of pressurized systems include: power plants, gas handling systems, bioreactors, liquid gas facilities, underground tanks, underground cables and pipes.