Helium mass spectrometer

A helium mass spectrometer (often called a leak detector or helium leak detector) or improperly sniffer, is a scientific instrument, used to detect, locate and measure very small leaks, typically using a vacuum and injecting helium around a chamber or cavity.

The helium mass spectrometer was initially developed in the Manhattan Project during World War II to find extremely small leaks in the gas diffusion process^[1].

The leak detection method uses helium (the lightest inert gas) as a tracer and can detect (not measure), with today's technology, a flow of 5x10^-12 mbar.l/s. The helium is selected primarily because it penetrates small leaks readily. Helium is also non-toxic, non-hazardous, plentiful, inexpensive, food compatible and present in the atmosphere only in minute quantities (5 ppm).

Typically an helium leak detector will be used to measure leaks in the range of 10^-4 mbar.l/s to 10^-11 mbar.l/s.

• A flow of 10^-4 mbar.l/s is approx less than 1 ml per minute
• A flow of 10^-12 mbar.l/s is approx less than 3 ml per century

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Process

The Helium mass spectrometer leak detection technique depends on the separation of helium from other gasses in a vacuum, which is a relatively simple procedure. It is accomplished by imparting an electrical charge to a sample of gas, pushing the sample through a magnetic field, and collecting the helium ions as they emerge. Since helium ions exit along a different path from all other ions, collection of helium is reasonably simple; and because they have a charge of electrical energy, helium ions can be counted. The current is used to drive a meter, actuate an optional audio alarm, and illuminate a display. Ionization, separation and collection takes place within the spectrometer tube, which is the heart of the system.

Internal spectrometer tube operation

In the spectrometer tube, the heart of the helium mass spectrometer, the electrons produced by a hot filament enter an ion chamber under vacuum, and collide with gas molecules, creating within the chamber ions quantitively proportional to the pressure in the ion chamber. These ions are repelled out of the ion chamber, under vacuum, through the exit slit, by a repeller field. The combined electrostatic effect of the repeller, exit slit, focus plates, and ground slit collimates the ion beam so that it enters the magnetic field as a straight "ribbon" of ions.

Types of leaks

Typically there are two types of leaks in the detection of helium as a tracer for leak detection.

• Residual leak: A residual leak is a real leak that may be gross, or small, according to the sensitivity setting of the leak detector.
• Virtual leak: A virtual leak is the semblance of a leak in a vacuum system caused by slow release of trapped gasses, as gasses may adhere as pockets to the interior sides of a chamber. This can cause confusion to the operator as it may be a false indication of a present leak.

Uses

Helium mass spectrometer leak detectors are used in production line industries such as refrigeration and air conditioning, automotive parts, carbonated beverage containers, food packages and aerosol packaging, as well as in the manufacture of steam products, gas bottles, fire extinguishers, tire valves, and numerous other products including all vacuum systems.

Test methods

Global helium spray - vacuum test

This method requires the part to be tested to be connected to a helium leak detector. The outer surface of the part to be tested will located in some kind of a tent in which the helium concentration will be raised to 100% helium.

• If the part is small the vacuum system included in the leak testing instrument will be able to reach low enough pressure to allow for mass spectrometer operation.
• If the size of the part is too large, an addition vacuum pumping system may be required to reach low enough pressure in a reasonable length of time. Once operating pressure has been reached mass spectrometer can start its measuring operation.

If leakage is encountered the small and "agile" molecules of helium will migrate through the cracks into the part. The vacuum system will carry promptly any tracer gas molecule into the analyzer cell of the magnetic sector mass spectrometer. A signal will inform the operator of the value of the leakage encountered in mbar.l/s.

Local helium spray - vacuum test

This method is a small variation from the one above. It still requires the part to be tested to be connected to a helium leak detector. The outer surface of the part to be tested is being gently sprayed with a narrow shower of helium tracer gas.

• If the part is small the vacuum system included in the instrument will be able to reach low enough pressure to allow for mass spectrometer operation.
• If the size of the part is too large, an addition pumping system may be required to reach low enough pressure in a reasonable length of time. Once operating pressure has been reached mass spectrometer can start its measuring operation.

If leakage is encountered the small and "agile" molecules of helium will migrate through the cracks into the part. The vacuum system will carry promptly any tracer gas molecule into the analyzer cell of the magnetic sector mass spectrometer. A signal will inform the operator of the value of the leakage encountered in mbar.l/s. Thus correlation between maximum leakage signal and location of helium spay head will allow the operator to pinpoint the leaky area.

Helium charged - vacuum test

In this case the part is being pressurized (sometime this test is combined with a burst test, i.e. at 40 bars) with helium while sitting in a vacuum chamber. The vacuum chamber is connected to a vacuum pumping system and a leak detector. Once the vacuum has reached the mass spectrometer operating pressure, any helium leakage will be measured. This test method applies to a lot of component that will operate under pressure: airbag canister, evaporator, condenser, high-voltage SF6 filled swithgear.

Bombing test (overpressure) - vacuum test

This method is dedicated to the objects that are supposedly sealed.

First the device under test will be exposed for an extended length of time to a high helium pressure in a "bombing" chamber.

If the parts is leaky, helium will be able to penetrate in the device.

Latter the device will be placed in a vacuum chamber, connected to a vacuum pump and a mass spectrometer. Again, when there is a leak path, as the part is filled with helium. This tiny amount of gas will be released in the vacuum chamber and forwarded in the mass spectrometer where the leak rate will be measured.

This test methods applies typically for implantable medical devices, crystal oscillator, saw filter devices.

This method is not able to detect massive leak as tracer gas will be quickly pumped out during the pumping of the test chamber.

Helium charged - sniffer test

In this last case the part is being pressurized with helium. The Mass spectrometer is being fitted with a special device, a sniffer probe, that allow to sample air (and tracer gas when confronted with a leak) at atmospheric pressure and to bring it into the mass spectrometer.

This mode of operationis frequently used to localise leakage that have been detected by other methods, in order to allow for parts repair.

See also

• Mass spectrometry
• Tracer-gas leak testing method
• Test of medical devices (FDA)
• Leak Detection

1. ^ LEAK DETECTION by N. Hilleret of CERN, Geneva, Switzerland. "At the origin of the helium leak detection method was the ”Manhattan Project” and the unprecedented leak-tightness requirements needed by the uranium enrichment plants. The required sensitivity needed for the leak checking led to the choice of a mass spectrometer designed by Dr. A.O.C. Nier tuned on the helium mass. Because of its industrial use, the material choice (originally glass) turned out to be unbearably fragile and after many complaints by the users, a new metallic version was developed and constructed. The sensitivity of the apparatus was in 1946 ~10-7 Pa. m3.s-1 and it increased to ~10-10 Pa. m3.s-1. by 1970. Nowadays the quoted sensitivity of the most sensitive detectors is ~10-13 Pa .m3..s-1, a factor 106 gain within 50 years."