Leak Testing with Tracer Gas (Helium)
Leak testing is a branch of the non-destructive inspection methods aimed to guarantee the tightness of the components under test.
The most common leak test methods are able to determine the value of the leak (typically measured in cc/min or mbar/s) by using:
- Pressure decay (with or without reference volume)
- Flow inspection (for big volumes)
- Tracer gas inspection (high performances)
Few technologies are able to identify the position and the value of the leak by using:
- Air bubble (under water inspection)
- Tracer gas (using sniffing probes)
The use of helium as tracing gas for leak detection is a consolidated system in the main industrial sectors, mostly where there is a high technological content, but it is quickly expanding to other sectors due to the increasing requirements of the market for high quality.
Helium is the second lightest element after hydrogen: it can penetrate small leaks rapidly (where heavier elements could not) up to 10−11 mbar*L/s.
Is a noble gas: it is inert and does not react with other gases or with the part under test.
Is ecological: it is colorless, odorless, tasteless, non-toxic and does not create any problem in case of dispersion in the environment.
Is present in the atmosphere only in small quantities (5 ppm) and so the background noise for the leak detection instrument is very low.
How does it work?
The usage of helium as tracing gas to find and quantify leaks requires an instrument able to detect its presence.
The instrument is the mass spectrometer, that transforms the measurement of the mass of a gas into an electric signal.
To do this, the air that enters the spectrometer is ionized and guided into a curved tube immersed in a magnetic field.
For a given value of the magnetic field only helium ions go through, while the others are destroyed. The electric current produced by the helium ions flow is measured to quantify the leak.
Helium charged - vacuum test
The part is put into a chamber where vacuum is created. The part is then pressurized with helium and the helium leakage is measured through a mass spectrometer. The test is objective, since does not depend on the operator. Detectable leak range goes from 5.0 x 10-3 mbar*L/s to 5.0 x 10-8 mbar*L/s. This method is used in automatic systems to test components as evaporators, condensers, tubes, hoses, pipes, airbags, canisters, fuel tanks, thermostatic valves.
Helium charged - sniffer test
The part is pressurized with helium. The mass spectrometer is fitted with a sniffer probe that is moved over the areas of possible leak to detect its presence. The test can be performed manually (by operator) or automatically. The detectable leak range is between 5.0 x 10-3 mbar*L/s and 5.0 x 10-5 mbar*L/s. This test is used for parts that cannot be put inside a vacuum chamber (refrigerators, refrigerated food displays) or to locate a leak that has been detected by other methods, in order to allow for parts repair.
Helium spray - vacuum test
The part to be tested is directly connected to a mass spectrometer and put inside a chamber. Vacuum is created inside the part and the helium concentration in the chamber is raised to 100%. If a leakage is present, the molecules of helium migrate into the part and the leakage is measured by the spectrometer. The possible leak range to be detected is from 5.0 x 10-3 mbar*L/s to 5.0 x 10-8 mbar*L/s. This test is frequently used to check porosities in cast parts or for parts that operate under vacuum, as water pumps and brake pumps.
This method applies to parts that are supposed to be sealed, as implantable medical devices and bulbs. The component under test will be first exposed for an extended length of time (e.g. 10 minutes) to a high helium pressure (40-50 bar) in a “bombing” chamber. If the part is leaky, helium will be able to penetrate the device. Then the component will be placed in a vacuum chamber. The amount of helium that entered the device will be released in the vacuum chamber and measured by the mass spectrometer. This method is not able to detect small leaks. Usual leak range to be detected is from 5.0 x 10-2 mbar*L/s to 1.0 x 10-4 mbar*L/s.