Helium
Leak Testing
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
Leak Testing?
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 Limits
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
3. re-evacuate
4. fill the part with the test gas
Helium leak testing at high production
rates.
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 gas.
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.
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