Helium is the best choice of tracer gas to find leaks for a number of reasons.  It is non-toxic, inert, non-condensable, non-flammable and not normally present in the atmosphere at more than trace amounts (5 ppm).   Due to its small atomic size, helium passes easily through leaks.  The only molecule smaller than Helium is Hydrogen which is not inert.  It is also relatively inexpensive and is available in various size cylinders.

Helium Leak Detection

A helium leak detector, also known as a Mass Spectrometer Leak Detector (MSLD), is used to locate and measure the size of leaks into or out of a system or containing device.   The tracer gas, helium, 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 meter.

Helium leak detectors consists of the following components:

• A spectrometer to detect the mass of helium
• A vacuum system to maintain the pressure in the spectrometer
• A mechanical pump to evacuate the part to be tested
• Valves which enable the various stages of detection: evacuation, test & venting
• An amplifier and readout instrumentation to monitor the output signal
• Power supplies and controls
• Fixturing that attaches the part to be tested to the detector

Methods of Leak Testing Parts

There are two main methods to leak test parts using helium: vacuum testing (outside-in) and pressure testing (Inside-out).  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.  Vacuum systems should be tested with a vacuum inside the chamber.  A compressed air cylinder should be tested with high pressure inside the cylinder.

Vacuum Testing (Outside-in)

In vacuum testing, the part is evacuated with a separate pumping system for larger volumes, or within the detector itself for smaller volumes.  To locate a leak, 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 air.  To locate a leak, the potential leak sites of the part are scanned using a sniffer probe connected to the inlet of the leak detector.

Leak Testing Vacuum Systems & Pressure Systems

Vacuum systems and 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 connected to the line of the vacuum pump.   Helium is applied to the 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 Applications

Quality control of production parts and assemblies using helium leak detectors can help assure the integrity of your production process.  Typical examples include: hermetically sealed packages, valves, manifolding, seals, vacuum vessels and systems, medical devices, high purity piping, brake lines, fuel lines, hydraulic lines, refrigeration assemblies, radiators, heat exchangers, condensers, storage tanks.

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.