Helium leak testing: from airbags to air-conditioning units and fuel rails, from anything implanted in the body to hermetically sealed enclosures for integrated circuits, there are some instances where the acceptable leak rate is so small it’s practically zero. Since air leak test methods such as pressure decay don’t have the necessary detection range to measure such small leaks, these situations call for a trace gas-based test method.
Why Should I Use Trace Gas-Based Leak Testing?
Most air leak test methods measure leaks indirectly, through the change in a physical quantity (e.g. pressure change caused by a leak). The most sensitive detection range possible with this approach is approximately 1×10-3std.cm3/sec for very small parts.
Trace gas methods, on the other hand, measure leaks directly. This allows for a much more sensitive detection range as little as 1×10-9std.cm3/sec – roughly 1 millionth of the low limit of an air leak test.
Let’s have a closer look at a leak exposed to trace gas:
The charge side is pressurized with a trace gas while the concentration of the same gas will be measured on the detection side. In many cases, the charge side is the inside of a part, but there are some cases, especially when checking hermeticity, when the outside of the part is charged.
On the charge side, it is extremely important to ensure the highest possible concentration of trace gas at the inlet of the leak as the gas exiting the leak will have the same trace gas content. If the concentration is not high enough, the leak will have a very poor chance of being detected because the gas analyzer will only detect a concentration of leaking trace gas.
In direct contrast with the charge side, the detection side should have as low a concentration of trace gas as possible before the test begins. Why? Because leak detectors can’t differentiate between the trace gas exiting a leak and the concentration of the trace gas that is already present in the atmosphere.
Using the Hard Vacuum Method for Leak Testing
This method, which most often uses helium as the trace gas, allows for the detection of the lowest leak rates. The detection side in these systems is typically at less than 1Torr (mmHg) pressure, which is achieved using a vacuum chamber.
At the end of test, the chamber is under vacuum and needs to be brought back to atmospheric pressure. Whatever gas is present during the venting of the chamber will be the one outgassing in the subsequent test. Sound chamber design practices, such as minimizing surface area, eliminating trapped volumes and reducing complexity in general, are crucial to avoid trace gas build-up problem that can skew the accuracy of future tests.
Nitrogen Purge Technology with Leak Test
This method uses an inert gas (usually nitrogen) to “carry” the leak signal from an isolated test area into a leak detector probe that samples the trace gas concentration. Because the inert gas replaces the atmosphere inside the chamber, background trace gas levels are minimized. Instead of using seals to isolate the test area from the outside world, another technology uses a gas seal created by a very small flow of nitrogen around the perimeter.
The nitrogen purge approach creates a test zone around the entire part, or a particular area of interest (e.g. joints, ports, etc.). It allows testing for leaks at a level that is comparable to a mid-range hard vacuum system without the need to create a vacuum around the test area.
Important Considerations for Using Atmospheric Methods
Atmospheric methods have gained market share over the past decade due to their lower cost and complexity. Improvements in background suppression/elimination techniques have also made these methods feasible alternatives where hard vacuum testing was once the only reliable option.
Using signal processing (zeroing) to minimize the effect of the background is very common. It is a useful feature when trying to find small leaks in a trace-gas rich environment, but it does come with a few caveats. When the zeroing function is activated, the leak detector stores the leak rate as the tare value and begins to subtract it from the readings. If the background was steady, the proper leak rate would be indicated. But because the background level is dropping, AND the measured leak rate never got below the level measured when the leak detector was zeroed, the leak detector will show a dropping leak rate, eventually no leak at all!
Due to the potential difficulties described above, many manufacturers and quality engineers prefer physical background reduction techniques. These vary by test method and can include using sniffer leak detectors or accumulation methods.
All trace gas-based test methods depend heavily on sound gas management practices. Paying attention to these details when designing a test system can save many headaches during the commissioning of the system and in production.