The use of helium gas as a medium for leak testing during the gas diffusion process dates back to the 1940’s. The mechanism that enables leak testing using helium is the helium mass spectrometer which, at its core, is simply an air sampling tool that takes accurate measurements, using a series of pumps, to quantitatively measure the amount of helium in, around or leaking from a closed vessel or system is completed. Using helium as the “tracer” gas enables the user to discover and measure extremely small leaks. With so many available gases in our ecosystem, what is it about helium that makes it the ideal tracer gas for leak testing?
In the case of leak testing, size really does matter. Helium is one of the smaller molecules known to man and boasts of an atomic mass of 4 making it “light and small”. In comparison, only hydrogen is lighter than helium but as the passengers of the Hindenburg can attest, hydrogen displays some other explosive properties that may make the average laboratory technician shy away from its use. Because of its size, helium has the ability to flow through much smaller openings and at a much faster rate than other gas compounds. In technical parlance, you will often see this referenced as helium displays a high sensitivity rate as low as 1 X 10-10 cc/s. In laymen’s terms, this simply means that the small molecule size of helium makes it easy to “quantitatively detect and measure”.
While we note above the major disadvantage of using hydrogen as a tracer gas due to its explosive nature, helium is on the other extreme. Helium has three key characteristics that make its use easy and ideal. It is non-explosive, non-toxic and non-destructive. These characteristics insure that helium will not interfere with, or in any way impact, the type of material or vessel that you are testing. In most instances, the system being leak testing is constructed of plastic, glass or some type of metal. The inert and odorless nature of helium makes it chemically inactive when it comes in contact with any of these material types.
The presence of helium in the atmosphere tends to hold in the 5 ppm (parts per million) range. This feature helps to insure that false readings and general atmospheric interference in a non-factor when using helium gas as a testing agent.
Collectively, the characteristics of helium gas outlined above make it a very user friendly tool for leak testing. Couple these features with the easy availability and relatively inexpensive acquisition cost, helium has become the “first choice” method for leak detection in a wide variety of settings. Advances in the development of better mass spectrometers have led to more superior and more robust techniques and methods.