Chemical Sciences: A Manual for CSIR-UGC National Eligibility Test for Lectureship and JRF/Helium mass spectrometer
A helium mass spectrometer is an instrument commonly used to detect and locate small leaks. It was initially developed in the Manhattan Project during World War II to find extremely small leaks in the gas diffusion process of uranium enrichment plants. It typically uses a vacuum chamber in which a sealed container filled with helium is placed. Helium leaks out of the container, and the rate of the leak is detected by a mass spectrometer.
The leak detection technique depends on the separation of helium from other gases in a vacuum. It is accomplished by ionizing a gas sample containing helium, 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. The current produced by the helium ion flow is used to drive a ammeter. Often audio alarms, and visual display on leak detection system to give information and warnings about leak levels. Ionization, separation and collection takes place within a spectrometer tube, which is the heart of the system.
Helium is used as a tracer because it penetrates small leaks rapidly. Helium has also the property of being non-toxic, chemically inert, inexpensive to produce, and present in the atmosphere only in minute quantities (5 ppm). Typically a helium leak detector will be used to measure leaks in the range of 10−5 to 10−12 Pa·m3·s−1.
- A flow of 10−5 Pa·m3·s−1 is slightly less than 1 ml per minute at Standard conditions for temperature and pressure (STP).
- A flow of 10−13 Pa·m3·s−1 is slightly less than 3 ml per century at STP.
Internal spectrometer tube operation
In the spectrometer tube, the heart of the helium mass spectrometer, the electrons produced by a hot filament enter an ionization chamber under vacuum, and collide with gas molecules, creating within the chamber ions quantitatively 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 gases, as gases 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.
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.
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 be 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 additional vacuum pumping system may be required to reach low enough pressure in a reasonable length of time. Once operating pressure has been reached, the 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 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.
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 sprayed with a localized stream 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, the 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 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. Thus correlation between maximum leakage signal and location of helium spray head will allow the operator to pinpoint the leaky area.
Helium charged - vacuum test
In this case the part is 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 components that will operate under pressure: airbag canisters, evaporators, condensers, high-voltage SF6 filled switchgear.
Bombing test (overpressure) - vacuum test
This method applies to 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 part is leaky, helium will be able to penetrate the device.
Later the device will be placed in a vacuum chamber, connected to a vacuum pump and a mass spectrometer. The tiny amount of gas that entered the device under pressure will be released in the vacuum chamber and sent to the mass spectrometer where the leak rate will be measured.
This test method applies to implantable medical devices, crystal oscillator, saw filter devices.
This method is not able to detect a massive leak as the tracer gas will be quickly pumped out when test chamber is pumped down.
Helium charged - sniffer test
In this last case the part is pressurized with helium. The mass spectrometer is fitted with a special device, a sniffer probe, that allows it 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 operation is frequently used to locate a leak that has been detected by other methods, in order to allow for parts repair.
- 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."