Helium leak detectors

Helium Leak Detector, also known as Mass Spectrometer Leak Detector (MSLD), is used to find the location and measure the leakage rate of a system. 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.

Helium gas is the most suitable choice for leak detection. It is non-toxic, inert, non-flammable, and also scarce in the air around us, with a concentration of about 5 ppm. Because of its small atomic size, helium gas can easily pass through leakage pores. The only molecule which is smaller than helium is hydrogen, which is not an inert gas. Helium is also relatively inexpensive and is available in cylinders of different sizes and purified form. There are other methods of leak detection, but none of them are as accurate as helium leak detection and quantification.

Ideally, the vacuum chamber should retain the vacuum after the pumps have been discharged. 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. Typically there are two types of leaks: residual leaks and virtual leaks. A residual leak is a real leak due to an imperfect seal, a puncture, or some other hole in the system. A virtual leak is the semblance of a leak in a vacuum system caused by outgassing of chemicals trapped or adhered to the interior of a system that is actually sealed. As the gases are released into the chamber, they can create a false positive indication of a residual leak in the 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. Therefore, determining the permissible leak limit for each vacuum system is essential. Therefore leak detection is an important step in creating a vacuum and guaranteeing the desired final pressure.

There are various methods for leak detection using helium gas; usually depending on the operating conditions of the vacuum system one of them could be selected. Maintaining pressure conditions similar to the actual operating conditions of the system is one of the most important issues. 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 test (outside-in) and pressure test (inside-out).

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 suspect of leakage. If there is a leak, helium molecules penetrate into the chamber and the helium leak detector sensor inserted into the vacuum chamber detects its presence. This will determine the location of the leak (Figure 1).

Figure 1: Detection of leak location by vacuum test method

To determine the overall leakage value of the system, the system is tested under a special pressure helium gas chamber. In this case, all helium gas leak sites are entered into the test system and measured by a helium leak detector (Figure 2).

Figure 2: Detection of total leakage by vacuum test method

In the pressure test method, the test chamber 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).

Figure 3: Detection of leak location by pressure test method

In order to determine the overall leakage rate of the system, the test chamber is inserted into a pumped chamber with a leak detector after being filled with helium gas. 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 (Fig. 4).

Figure 4: Detection of total leakage by pressure test method

Basis of Helium Detector Performance:

The basis of these sensors is that the helium gas that is leaked first is ionized by an electron beam. The electron beam is produced by the filament in the ion chamber in the analysis tube. The ions are accelerated using added voltage (about 400 to 1200V) and move out through a slit and then pass through the magnetic field generated by the analyzer. The ions in this magnetic field are separated by their mass-to-charge ratio because the radius of deviation of ions varies depending on the mass-to-charge ratio in a magnetic field. The ions 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. The 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. The basis for detecting helium gas is based on the same mass-to-charge ratio difference. 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 average path free length should be at least 60 cm (in the high vacuum range).  Therefore, its mass spectrometer operates in high vacuum conditions.

Figure 5: Basis of operation of helium mass spectrometer leak detectors

Helium leak detectors usually include the following:

  • Helium Mass Spectrometer Leak Detector
  • System and control valves that control various stages of testing including measurement cycle, vacuum, test and venting.
  • Rotary and turbomolecular pumps to reduce pressure sufficiently
  • Vacuum Pressure gauge to measure vacuum
  • A device that connects the test unit to the leak detector

Sources:

  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