Guide to the Fundamentals of Helium Leak Testing
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What is helium leak testing?
Helium is used to find small leaks or possibly larger leaks in
bigger volumes. The helium is used as a tracer gas and its concentration
is measured. .
Why use helium for leak testing?
Helium is one of the smallest gas molecules and is inert, (remember
your periodic table from Chemistry?). Being inert it is relatively
safe to use (rather than hydrogen) and will not react with any of
the materials within the part to be tested. In most helium leak
testing applications, one uses a mass spectrometer tuned to detect
helium although it is possible to use a residual gas analyser. Helium
leak testing can be generally be between one thousand and one million
times more sensitive than using pressure decay techniques.
What are the benefits of Helium
Using this technique you can leak test to find smaller leaks than
with other test processes, using a temperature stable, dry technique.
This should result in a longer product life.
There are 2 basic techniques; high vacuum testing which allows
leak test thresholds to be set down as low as 1x10-12mbar.l.sec-1,
or sniffing which is generally used for helium leaks down to 1x10-6mbar.l.sec-1.
For reference 1 cubic millimetre per second is approximately 1x10-3mbar.l.sec-1.
Most tests use readily available Balloon Gas (yes, this is what
is put in kiddies balloons!). On rare occasions certifiably pure
gas is used. On a safety note, please remember that helium contains
no oxygen and is therefore an asphyxiant.
It is worth remembering that leakage is a flow of fluid from a
higher pressure to a lower pressure through a fault in an assembly
or manufactured part.
The high vacuum technique requires that the test volume to which
the instrument is connected is at high vacuum i.e. less than 10mbar
absolute pressure. 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 arrange these pressures either side
of the leaking boundary. This may mean putting the part under test
inside a leak tight chamber.
When testing using helium it is possible to flood the mass spectrometer
with helium if there is a large leak. In most instances, where there
are many minutes between each test, this is not a problem, one just
waits for the instrument to clean itself up.
One can also either sniff or spray helium. Sniffing is used where
the part can be pressurised above atmospheric and a sniffing gun
is manually positioned round the part. Spraying is where the part
can be evacuated and helium is manually sprayed over the outside
of the part.
Background Helium concentration.
Helium can and will get everywhere if it can. It gets quite difficult
sometimes to determine where the helium is coming from.
There is approximately 5 ppm Helium in the atmosphere. If the part
under test is filled with helium it is important that the test charge
is taken away and not just released into the immediate area. For
just a few tests, the helium can be diluted in the immediate area.
For more frequent testing, this may mean piping the extracted test
gas away to the outside of the building on the downwind side and
well clear from doors or windows that could allow it back in.
To locate helium leaks one usually either sprays or sniffs, the
latter being the norm in high volume testing. When sniffing one
starts with 5 ppm and would usually look to detect an increase of
a further 5 ppm.
One can use this detectable rise in helium concentration to test
parts inside a shroud where the test pressure is at or near atmospheric
pressure. By circulating the air within the shroud and passing it
by a mass spectrometer in sniffing mode, you can set an alarm limit
at say 8ppm. Again it is important to flush or extract away any
contaminated air once the test has completed.
Helium leak testing at Lower
When testing at 1 x10-9 and below one may need to do additional
things to be able to complete a viable test. One may need to purge
away and clinging helium left in the part to be tested or the test
chamber. For example, with the chamber and part open to atmosphere
before the test, they are exposed to a helium concentration of 5
ppm. The helium can cling to various surfaces and affect the results
of the test. So, one may need to "wash" away any clinging helium
in the chamber and part in a gas containing no helium.
The sequence might be:
1. evacuate both the part and chamber,
2. back fill the test volume with certified clean dry nitrogen
4. fill the part with the test gas
Helium leak testing at high
When high volume production requires high vacuum, you must also
consider the time taken to pump down to the required level of vacuum,
this can be significant.
At higher production rates, having much shorter time to test a
part, large leakers may be a problem. The flooding of the leak test
instrument with helium may take several minutes for the helium to
reduce to a level where testing can recommence. To reduce the effect
of a large leaker one can; build up to the full test pressure in
stages, build up to the full concentration, flush the mass spec
with a gas with no helium present or pre-screen using an air decay
technique before helium testing. Air decay pre screening willallow
known larger leakers to be to be removed prior to helium leak testing.
When testing at high speed it is important to reduce the test time
to its minimum. To do this one may have to employ a number of techniques
to; reduce the test volume (by infilling voids and ensuring minimum
pipe run volumes), nitrogen flush, etc. Of course at high speed
the automatic handling of the product and automatic connection play
a large part.
Tooling & Pipework
If one is intending to helium leak test a part it is important
to note that the tooling and pipework used to create the test volume
and helium gas supply must be leak tight to higher degree than the
leak test threshold. This means that careful engineering of the
seals is necessary, particularly where complicated seals are required
(e.g. right angled seals for 2 perpendicular faces).
One must also use helium leak tight valves and pipework and pay
particular attention to the sizing of the vacuum pipework
Helium mixing, helium recovery
and helium re-use
When testing large volumes, at higher pressures and at high speed,
the quantity of helium being used may become significant. There
are a number of techniques to reduce the consumption of the helium
First of these is to mix the helium with another lower cost gas
either nitrogen or compressed air. This is only possible where the
sensitivity of the test is not compromised by the mixing process.
The second of these is to re-use the gas from one test by extracting
it from the device and then pushing it into the next device. This
can often be implemented by a combination of a vacuum pump and simple
air cylinder arrangement if the volume is not too large. Between
cycles it is possible to use the mass spectrometer to monitor the
concentration of helium that is being reused; when the concentration
falls below an acceptable level it is dumped and a new charge of
helium is used.
The third technique is helium recovery. Here one extracts the helium
into an intermediate holding vessel to be compressed back to high
pressure to recover the helium.
I hope you find the above a useful introduction into the area of
helium leak testing. If you have a project in mind or would like
a more detailed discussion on the possibilities of helium leak testing
your product please contact us at TQC.