Guide to the fundamentals of helium leak testing
What is helium leak testing?
Helium leak testing is used to find small leaks or larger leaks with larger volumes. Helium is used as the tracer gas and its concentration is measured. This helium leak test guide outlines the basics of using leak testing.
Why use helium for leak testing?
Helium is one of the smallest and inert gas molecules. Being inert, helium is relatively safe to use (instead of hydrogen) and will not react with any materials in the part under test. In most applications helium leak testing. Although, residual gas analyzers can also be used. Helium leak testing can generally be one thousand to one million times more sensitive than using the pressure decay technique.
What are the benefits of Helium Leak Test?
Using this technique, you can leak test for smaller leaks than other testing procedures, using a dry, mostly temperature-independent technique. This will lead to a longer product life.
There are 2 basic techniques: High vacuum testing allows the leak test threshold to be set as low as 1 × 10 -12 mbar.l.sec -1 , or used to get helium leaks down to 1 × 10 – 6 mbar.l.sec – 1 . For reference, 1 cubic millimeter per second is approximately 1 × 10 -3 mbar.l.sec -1 . or 1cc is about 1mbar liters per second.
In rare cases, certified pure gas may be used. As a safety note, remember that bottled helium does not contain oxygen and is an asphyxiant. It should be remembered that a leak is the flow of fluid from a higher pressure to a lower pressure through a fault in an assembly or manufacturing part.
The high vacuum technique requires that the test volume to which the device is connected is at high vacuum. Such as absolute pressure less than 3mbar. Can the part or assembly withstand this pressure?. It is possible to test a part at high pressure and high vacuum at the same time. You need to align these pressures on either side of the leak boundary. This may mean placing the part under test inside a sealed leaky chamber.
There may be a high vacuum connected to the Mass Spectrometer and a partial vacuum in the unit. This is very useful if you are looking to test a 60 liter car fuel tank that can only withstand 150mbar pressure. One only needs to evacuate both inside and outside, then backfill with 150mbar absolute pressure.
When testing using helium, it is possible to flood the mass spectrometer with helium if there is a large leak. In most cases there are many minutes between each check, this is not a problem, one just needs to wait for the unit to clean itself. Alternatively, and in higher throughput systems, one can also pre-test using pressure decay to screen for larger leaks before introducing helium that could flood the mass spectrometer. .
Background Helium Concentration
Helium can and will be everywhere if possible. Sometimes it is quite difficult to determine where helium comes from. There is about 5 ppm Helium in the atmosphere. If the part under test is filled with helium, it is important that the test charge is properly removed and not released into the immediate area.
For some tests, helium can be diluted in the area. For more frequent testing, this may mean that the extraction test gas pipeline goes to the outside of the building on the windward side and vents away from doors or windows that may allow air to flow. that’s back.
To locate a leak with helium gas is usually sprayed or inhaled, the latter being standard in mass testing. When using a helium leak tester, one starts with a background concentration of 5 ppm in the ambient air and will typically seek to detect a further 5 ppm increase.
This detectable increase in helium concentration can be used to test parts inside the shroud where the test pressure is at or near atmospheric pressure, this is commonly referred to as the volumetric test. capacitor. By circulating the air in the shroud and passing it through the mass spectrometer in sniff mode, you can set the alarm limit at 8ppm. Again, it is important to drain or exhaust the contaminated air after the test is complete. However, this process can take a considerable amount of time for larger parts.
Helium leak test at lower limit
When testing at 1 x10 -9 or less, one may need to do additional things to be able to complete a viable test. It may be necessary to remove any remaining helium in the part under test or the test chamber. For example, with the chamber and section open to the air before the test, they were exposed to a helium concentration of 5 ppm. Helium can adhere to various surfaces and affect test results. Thus, one may need to “wash off” any helium remaining in the chamber and partially in the non-helium gas.
The sequence could be:
- Evacuate both parts and compartments
- Refill the test volume with certified clean dry nitrogen
- Fill the unit with test gas
Helium leak test at high production rate
When high volume production requires a high vacuum, you must also consider the time taken to pump down to the required vacuum level, which can be significant. At higher production speeds, which have much shorter inspection times for a part, large leaky parts can be an issue. Immersion of a leak test device with helium can take several minutes for the helium to decrease to a level where testing can be performed.
To reduce the effects of a large leak, one can; stepwise increase to full test pressure, rise to full concentration, mass spec with helium-free gas or pre-screen by air decay technique prior to helium test. Air decay pre-screening will allow for larger known leaks to be eliminated prior to conducting helium leak testing.
To reduce the effects of a large leak, one can; stepwise increase to full test pressure, rise to full concentration, mass spec discharge with helium-free gas or pre-screen by air decay technique prior to helium test. Air decay pre-screening will allow for larger known leaks to be eliminated prior to conducting helium leak testing.
When testing at high speeds, it is important to keep test times to a minimum. To do this, one may have to use a number of techniques to; test volume reduction (by filling gaps and ensuring minimal tube run volume), nitrogen discharge, etc. Of course at high speed, automatic product handling and automatic connection play an important role.
Tools & Pipes
If one is intending to leak test with a helium gas component; It is important to note that the instrument and piping used to generate the test volume and the helium supply must be leak-tight to a degree above the leak test threshold. This means that careful sealing technique is required, especially when complex seals are required, such as right angle seals for 2 perpendicular faces. One must also use helium leak-proof valves and piping and pay special attention to the size of the vacuum piping.
Helium Mixing, Helium Reusing and Helium Recovery
The amount of helium can become significant when testing high volumes, at higher pressures and/or at high speeds. There are several techniques to reduce helium consumption.
The first is to mix helium with another less expensive gas, nitrogen or compressed air. This is only possible when the sensitivity of the test is not affected by the mixing process.
The second way is to reuse the helium from one experiment by extracting it from one device and then pushing it into the next. This can often be accomplished by a combination of a vacuum pump and a simple gas cylinder arrangement if the volume is not too large. The concentration of helium can be monitored between cycles using a mass spectrometer or another sensor. If the concentration falls below an acceptable level, the helium must be discarded and a new helium charge used.
The third technique is helium recovery. The helium is extracted into an intermediate container which is then compressed back to high pressure to recover the helium.