Usually the first decision to make is whether traditional leak testing methods using air as a test medium are suitable for the application. As these methods are very mature and, in most cases, can be performed with off-the-shelf instrumentation with only tooling representing a challenge, they are the first choice whenever possible.
Unfortunately, there are cases when air leak testing reaches its limits. The rate of pressure change observed using a pressure decay test largely depends on the volume of the test part and the leak rate. If either the leak rate becomes very small or the part volume is very large, the rate of pressure change can get so small that minute changes in pressure caused by environmental changes (typically temperature) can make measurements highly unreliable if not downright impossible.
This is where trace gas based methods have an advantage.
Trace gas leak testing methodsNon-trace-gas-based methods of leak testing measure leaks indirectly, through the effects of the leak (as is the case with pressure decay leak testing).
Trace gas methods, whether with helium or a different gas/gas mixture, measure leaks directly – they aim to determine the quantity of material escaping from the part under test as a direct measurement.
Once it is determined that a trace-gas based method is the best (perhaps only) choice, one is immediately faced with the question of which gas works best for the given application.
A couple of choices are:
- Low concentration hydrogen in an inert gas
- Various refrigerants
- Sulphur hexafluoride
In an ideal situation, the trace gas selected must:
- Be inert relative to any materials it may come in contact with (i.e. it cannot enter any reactions with wetted components)
- Have a very low presence in ambient air
- Be easily and widely available, and
- Be cost effective
Let’s have a look at various options in this light:
HeliumHelium was one of the first trace gases used for leak testing in the mid-40s and has been the gas of choice ever since. A monoatomic noble gas, it is inert in typical test environments and its presence in the Earth’s atmosphere is around 5ppm, making it an ideal candidate from a physical/technical standpoint.
Using helium, it’s possible to detect leaks as low as 10-10 std.cm3/sec and even lower when using the so-called hard vacuum method. This method involves placing the test part inside a vacuum chamber and measuring helium concentration outside the part using a mass spectrometer while charging the part with helium. Using helium in vacuum is one of the most sensitive and repeatable leak test methods.
When such high sensitivity is not necessary, helium can be used to test parts at atmospheric pressure, either by measuring the rate of rise of helium concentration in a closed chamber around the part or by using a so-called sniffer probe with which leaks can be located with a high level of accuracy.
With all these near ideal physical properties the challenge (as with most good things life) is on the commercial side.
Availability and price became a limiting factor in the last decade as the Bureau of Land Management in the US started disposing of the Federal Helium Reserve (to be completed by September 2021). Most of the non-BLM sourced supply comes from certain types of natural gas found in a relatively few areas of the planet. Prices have seen wild swings as various producers encountered production and storage issues. There have been numerous times when the amount of helium available for certain types of facilities has been severely limited, leading to rationing.
HydrogenMost modern helium leak detectors are capable of detecting hydrogen as well as helium.
Even though its presence in ambient air in gaseous form is lower than that of helium (about 0.6ppm), its reactive nature makes it difficult/unsafe to use in higher concentrations.
Mixed with an inert gas like nitrogen or argon, it can be safe to use at concentrations up to 5% by volume.
Using hydrogen in a vacuum system has its own challenges: most materials used in vacuum chambers outgas hydrogen for very long amounts of time. Hydrogen from water vapor in the air and condensed water on the surface of vacuum components also adds to the hydrogen concentration and makes it too unstable to form a baseline.
Because of the problems with outgassing, hydrogen is mostly used in atmospheric tests (accumulation and sniffing)—as long as proper care is taken to keep the test area free of residual trace gas (e.g. by removing trace gas from the part at the end of test as much as possible).
Other gas optionsOf the practically useable gases, traditional helium mass spectrometer-based leak detectors can only sense hydrogen and helium. Other gases such as refrigerants or SF6 can be used when the leak detector is a Residual Gas Analyzer (RGA).
These are full mass spectrometers capable of analyzing gases in a wide range of AMUs (atomic mass units). They are very good at providing a qualitative analysis of the gas inside a vacuum system but getting quantitative information can be a challenge.
The challenge is that most of these analyzers are most used in either the semiconductor industry or in research. As systems meant for mostly controlled environments, adapting them to the factory floor can be a challenge from both an operating and a maintenance standpoint.
RGAs have been traditionally used mostly on hard vacuum testers, but there are also atmospheric models available to be used as sniffers.
In closingDespite there being some alternatives with promise, a replacement for helium that is universally acceptable in all cases and scenarios has not been found for trace gas leak testing yet.
The key to mitigating those occasional supply and pricing issues is to employ an efficient helium/gas reclaim/recovery method to ensure as little of the gas as possible is lost during the venting phase of the test cycle.
In the next post, we will talk about trace gas testing methods, including alternatives to a hard vacuum leak test, and when they are required.