Why is helium chosen as a tracer gas?
Safety: Helium, being an inert noble gas, is non-toxic, non-flammable, and non-reactive. Therefore, no undesirable events will occur during leak detection using this gas.
Small Molecular Size: Due to its small atomic size and light weight, helium easily passes through leaks. Only hydrogen is lighter and smaller than helium, but unlike helium, hydrogen is reactive and flammable. Helium is also lighter than neon, the next inert gas, which is much more expensive.
Low Atmospheric Concentration: The concentration of helium in the atmosphere is only 5.24 × 10⁻⁴, or 5 ppm by volume. Helium leak detectors can easily be calibrated to ignore the atmospheric helium. Therefore, the helium detected by the instrument represents the helium introduced during testing.
Helium Detection Principles in Mass Spectrometers
Helium leak detectors operate based on mass spectrometry. Helium entering the device is ionized by electron beams from a filament inside the machine. The ions are accelerated by an increase in voltage and pass through a slit and a magnetic field generated by the analyzer. Due to the ions' motion being determined by their mass, the collector can only identify helium gas. Since helium ions are lighter compared to other molecules, they can enter the collector and be detected. By measuring the ion current and converting it into a leak measurement unit, the leak rate is displayed on the screen.
Four Methods of Helium Leak Detection
There are two general methods of helium leak detection:
- Integral testing (measuring the overall leak rate)
- Local testing (pinpointing the leak location)
The selection of either method depends on the location and final operational conditions of the equipment under test.
Integral methods determine the presence or absence of leaks (without specifying the number of leaks or their location), while local methods pinpoint the exact location of the leak (though accurately determining the leak rate and size is more complex). Both of these methods can be performed using either vacuum or pressure techniques, giving a total of four defined leak detection methods.
Integral Method
High-pressure integral method
In this method, the equipment under test is pressurized with helium and placed inside a vacuum chamber. By connecting the vacuum chamber to the leak detection system, all gases passing through leak points and entering the device can be identified, allowing for the total leakage of the equipment to be measured.
Vacuum integral method
Here, the equipment under test is also placed in a chamber, but in this case, the chamber is pressurized with helium while the equipment is kept under vacuum. The equipment is directly connected to the leak detector. If a leak exists, helium will pass from the chamber into the equipment and be detected by the helium leak detector, allowing the leak rate to be measured.
Localized Methods
Sniffer method
In this case, the equipment under test is pressurized with helium, and a sniffer passes over its surface. Helium entering the sniffer is directed to the leak detector, where the helium leak rate is measured.
Spray method
In this approach, the equipment is placed under vacuum, and helium is sprayed onto its surface. If there is a leak, helium will enter the vacuum chamber of the equipment and be detected by the helium leak detector, where the leak rate is measured.
Helitek Company is ready to provide all kinds of leak detection services for all industries.