What is helium leak testing?
Leak testing using helium is used to detect small or large leaks in larger volumes. It is used to measure the concentration of helium and as a trace gas. This guide will explain how to use the helium leak test.
Why should you use helium to test for leaks?
Helium is one the smallest gassed molecules and is inert. Helium, being inert, is much safer than hydrogen and won’t react with any materials. A mass spectrometer can be used to detect helium in most helium leak test applications. It is possible to also use a residual gas analyser, The sensitivity of helium leak testing is generally between one thousand to one million times greater than the pressure decay methods.
What are the benefits to Helium Leak Testing
This technique can be used to detect smaller leaks than other methods. It is largely temperature-independent and dry. This will result in a longer product lifespan. There are two basic methods: high vacuum testing, which can set leak thresholds as low as 1×10-12 mbar.l.sec-1; and sniffing, which is used to detect helium leaks as low as 1 x 10-6 mbar.l.sec-1. Referring to the above, 1 cubic metre per second is roughly 1×10-3 mbar.l.sec-1. 1cc equals approximately 1 mbar liter per sec.
Most tests are performed using readily available Balloon Gas. A certifiably pure gas is sometimes possible, but this is rare. Please note that bottled Helium is not able to contain oxygen, making it an asphyxiant. Remember that leakage refers to fluid flowing from a higher pressure into a lower pressure due to faults in an assembly or part.
High vacuum techniques require that the volume of the instrument to which it is connected be at high vacuum. This means less than 3 mbar absolute pressure. Is the part or assembly capable of withstanding this pressure? You can test the part simultaneously at high pressure and high vacuum. These pressures should be placed on either side of any leaking boundary. This could mean that the part is tested inside a leak-tight chamber.
You can have a high vacuum connected to your Mass Spectrometer, and a part with a partial vacuum. This is helpful if you need to test a 60-liter car fuel tank that can only withstand 150 mbar of pressure. You simply need to evacuate both the inside and exterior, then fill up to 150 mbar absolute pressure.
If there is a significant leak, you can flood the mass-spectrometer with Helium. This is usually not an issue as the instrument will clean up after each test. In higher throughput systems, it is possible to pre-test with pressure decay to filter out larger leakers, before injecting helium into the mass spectrometer.
You can also spray or sniff helium. Helium Sniffing can be used when the part is pressurized above atmospheric. A helium sniffer gun manually positions around the part. Spraying is used when the part can evacuate and helium is manually sprayed on the outside.
Background Helium Concentration
If it can, helium can and will travel anywhere. Sometimes it can be difficult to identify where the helium comes from. Helium is found in the atmosphere at a level of approximately 5 ppm. It is crucial that any helium-filled parts are properly removed and not released into the surrounding area. The helium can be diluted in the immediate vicinity for a limited number of tests. This may be used for more frequent testing. The extracted gas should be piped to the exterior of the building, on the downwind side. It should also be well clear of any doors or windows that could allow it in.
One can either spray or sniff helium leaks to find them. This is the preferred method for high volume testing. Using a helium sniffer one would start with 5 ppm ambient air concentration and then look for an increase of 5 ppm.
This detectable increase in helium concentration can be used to test parts within a shroud whose test pressure is close to atmospheric pressure. This is also known as accumulation testing. You can set an alarm limit of 8ppm by circulating the air inside the shroud and passing the mass spectrometer through sniffing mode. Once the test is complete, it’s important to flush out or remove any contaminated air. For larger parts, however, this can take a while.
Lower Limits Helium Leak Testing
To be able complete a valid test at 1×10-9 or below, you may need to do some additional work. You may need to remove any helium clinging to the test chamber or part. The chamber and the part are exposed to atmospheric air before the test. They will be exposed to a concentration of 5 ppm. The test results can be affected by helium adhesion to surfaces. One may have to “wash” the chamber of any clinging Helium and then use a gas that contains no Helium.
This sequence could be:
- Both the chamber and the part should be removed
- Fill the test volume with certified dry nitrogen.
- Re-evacuate
- The test gas should be used to fill the part
High production rates require helium leak testing
High volume production can require high vacuum. This is why it is important to consider the time required to get to the desired level. Large leakers could be a problem at higher production rates. This is because it takes longer to test the part. It may take several minutes for the helium in the leak test instrument to decrease to a point where testing can resume. You can reduce the effects of large leakers by building up to the full pressure in stages. After that, flush the mass spec using a gas without helium or pre screen with an air decay technique. Pre-screening with air decay will allow for larger leaks to been identified and removed before helium leak testing.
It is crucial to minimize the time taken to test at high speeds. You may need to use a variety of techniques to reduce the test volume, such as nitrogen flushing or infilling voids to ensure minimum pipe run volumes. The automatic handling of product and connection are crucial at high speeds.
Helium Bombing to Test Hermetically Sealed Containers
It would be a good idea to inject some detectable Helium into the assembly process before sealing the container. Bombing is a method that forces helium into a part if it is impossible to insert helium within the part.
Tooling for Helium Leak Testing and helium bombing
By pressing urising the device in a helium gas chamber, helium bombing can be achieved. The chamber is usually evacuated first to remove any barometric pressure air around the parts, and then pressurize with bottled balloon glasses.
To force the helium into any leaking parts, the device is soaked in pressurized helium. The chamber is then vented, and the components are removed from the chamber.
Next, the parts are tested using a combination of a gross and a high vacuum leak test with a helium mass-spectrometer.
Tooling & Pipework
It is essential to ensure that the pipework and tooling used to create the test volume as well as the helium gas supply are leak-free to a greater degree than the threshold for the part being tested. It is important to carefully engineer seals, especially where complex seals are needed, such as right-angled seals for two faces. You must use helium leak-tight valves and pipework, and pay special attention to the size of vacuum pipes.
Helium Mixing, Helium Re-use and Helium Recovery
When testing large volumes at higher pressures or at high speeds, the amount of helium can become significant. There are many ways to reduce the consumption of helium gas.
The first is to mix the Helium with a lower-cost gas such as nitrogen or compressed air. This can only be done if the test’s sensitivity is not affected by the mixing process.
The second is to reuse the helium from a test by removing it from the device, and then pushing it into another device. If the volume is small, this can be done with a simple vacuum pump or air cylinder arrangement. You can monitor the concentration of helium between cycles by using a mass-spectrometer or other sensor. If the concentration drops below an acceptable level, it is best to remove the helium and replace it with a new charge.
Helium recovery is the third method. To recover the helium, it is first extracted into an intermediate container. Then it is compressed to high pressure.
