Helium Leak Detectors

Mass Spectrometer Leak Detector (MSLD)

Helium Leak Detector, also known as the Mass Spectrometer Leak Detector (MSLD), is an instrument commonly used to find the location of the leakage and measure the rate of it in a system. Among several methods of leak detection, helium leak detection is the most accurate one.

Helium as a non-toxic, inert, non-flammable gas, is the most suitable choice for leak detection. It exists in our environment with a concentration of about 5 ppm. It is relatively inexpensive and is available in cylinders of different sizes. Because of its small atomic size, helium gas can easily pass through leakage pores. 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.

Ideally, the vacuum chamber should retain the vacuum after the pumps have been turned off. But in real systems, over time, system pressure increases. This increase in pressure can be due to the penetration of wall molecules (Outgassing) or due to leakage and penetration of molecules from outside into the system.

Accordingly, there are two types of leaks, residual leaks and virtual leaks. A residual leak is a real leak due to incomplete sealing, the existence of holes or micro-cracks in the system. A virtual leak is caused by the outgassing of chemicals trapped or adhered to the interior part of a system.

In fact, no vacuum system can completely be without leak. What is important is that the leakage should be small enough that the final pressure and gas balance in the chamber do not disturb the vacuum. And, determining the allowable limit of the leak for each vacuum system is essential. Therefore leak detection is an important step in creating a vacuum system and guaranteeing the desired final pressure.

Helium Leak Detection Methods

There are various methods for leak detection using helium gas, and anyone can be used usually depending on the operating conditions of the vacuum system. In general, determining the location of the leak and the amount of leakage are two important issues that are followed in all procedures. Two main methods for detecting leakage using helium gas are vacuum tests (outside-in) and pressure tests (inside-out).

Vacuum Test Method

In the vacuum test method, the system first appears to be vacuumed by an external pump or pump mounted on the system itself. The helium is then sprayed in places suspected of leakage. If there is a leak, helium molecules penetrate into the chamber and the helium leak detector sensor which is inserted into the vacuum chamber detects them. This will determine the location of the leak (Figure 1).

 
Leak Location Detection by Vacuum Test Method
Figure 1: Detection of Leak Location based on the Vacuum Test Method

To determine the overall leakage value of the system, the test object is placed in a chamber under a special helium gas pressure. In this case, all leak sites are detected by a helium mass spectrometer (Figure 2).

Total Leak Detection by Vacuum Test Method
Figure 2: Detection of Total Leakage based on the Vacuum Test Method
Leak Location Detection by Pressure Test Method
Figure 3: Detection of Leak Location based on the Pressure Test Method

Pressure Test Method

In the pressure test method, the test object is first filled with helium gas or a mixture of helium gas and air. Leak suspected sites are then scanned by the leak detector. If there is a helium gas, the leak detector starts to sound the alarms, and so the location of the leak is determined (Figure 3).

To determine the overall leakage rate of the system, the test object is filled with helium gas and is placed inside a pumped chamber with a leak detector. The amount of helium gas passing to the vacuum pump is measured by a leak detector; thus, the total amount of system leakage is determined (Figure 4).

Total Leakage Detection by Pressure Test Method
Figure 4: Detection of Total Leakage based on the Pressure Test Method

Principle of Helium Leak Detectors

The principle performance of a leak detector is based on the ionization of the helium gas by an electron beam. The electron beam is produced by a filament in the analysis tube. The ions are accelerated by applying a voltage of about 400 to 1200V, and then pass a slit and move through the magnetic field generated by the analyzer. The radius of deviation of the ions varies depending on their mass-to-charge ratio in a magnetic field. So, the ions under the magnetic field are separated by their mass-to-charge ratio and those with larger mass have a greater radius of deviation.

Since the circular trajectories of the ions depend on their mass, the collector can catch only the helium ions and detect helium. Then the flow of ions collected by a detector is converted into electrical current and display on a screen. The measured electrical current is monitored by a leakage display unit on a screen. The measured electrical current is directly proportional to the concentration of helium and therefore equals the amount of leakage. For very low leaks, very small currents of about 10-15 amps is detected which is equivalent to a sensitivity of 10-12 mbars.L/sec.

It should be noted that the path of the ions in the mass spectrometer from the ion source to the collector is approximately 15 cm. Since the ions must travel this path without colliding with the gas molecules, the mean free path should be at least 60 cm (in the high vacuum range). Therefore, the mass spectrometer operates in high vacuum conditions.

Helium leak detectors usually include the following:

  • Helium Mass Spectrometer Leak Detector
  • Control systems and valves that control various stages of testing including measurement, vacuuming, testing, and venting
  • Rotary and turbomolecular pumps
  • Vacuum pressure gauge
  • A device that connects the test unit to the leak detector
Principle of the Helium Mass Spectrometer Leak Detectors Operation
Figure 5: Principle of the Helium Mass Spectrometer Leak Detectors Operation

Leak Detection of Vacuum Coating Systems

All vacuum deposition systems manufactured by Vac Coat Ltd. are required to be checked for the possibility of any leaks in these systems. In the first step, one must look for the real leaks, which in most cases can be found and fixed with an eye inspection. Most of these leaks are caused by physical damage to the coating system (e.g. cracks in the head of the vacuum chamber).

In other cases, while the location and cause of leakage in vacuum coating systems are not known, it is necessary to use the leak detection mechanisms of vacuum deposition systems like helium leak detectors. Vac Coat Ltd. uses helium leak detection systems for detection of any leaks in vacuum coating systems made by this company and is not a manufacturer or importer of such devices, and in this regard, it does not provide any services.

Resources

  1. Ahmadi, Afsaneh; Moenie, Mahdi; Kowsari, Mohammadreza, Leak detection of vacuum systems using helium leak detector.
  2. N. Hilleret. LEAK DETECTION. Geneva, SwitzerlandCERN. 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”
  3. Robert Brockmann. “UST method”. researchgate
  4. https://www.iitk.ac.in/ibc/HLDr.pdf
  5. https://www.fukuda-jp.com/en/leak/
  6. https://odishaexpo.com/global-helium-leak-sensor-market-2020-growth-analysis/

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