Leak Testing: Helium Leak Testing - Using a Mass Spectrometer

TQC has considerable experience in integrating mass spectrometers into production systems for leak testing various parts.

This ranges from Robot Helium Sniffing to High Speed Helium Leak Testing and to High Vacuum Helium Leak Testing.

The helium mass spectrometer, although much more robust these days, is really a laboratory piece of equipment that can be used in production. They generally do not like knocks, shocks or being switched on and off repeatedly. However, these instruments can be applied successfully for the measurement and detection of very small leaks below the threshold that is considered acceptable for most manufactured parts and assemblies.integrated mass spectrometer

Today, most of the suppliers provide packaged Mass Spectrometers that consist of the sensing head, turbo pump, rotary vane pump, valves, pressure sensors and control system, although it is possible to buy all of these items as individual components and build your own system. Often it is more cost effective to purchase the packaged units and they are quick to change if a fault occurs.

TQC has supplied systems with Pfeiffer Vacuum (Adixen), Varian (Agilent), Leybold and Edwards Mass Spectrometers.

Mass spectrometers can be used for 3 basic techniques; sniffing, accumulation and high vacuum testing. One can also use these techniques in combination to first detect leakage from the part then sniff to locate the leak. The mass spectrometer measuring head works at high vacuum and can measure leakage down to 1x10-9 mbar.l.sec-1 easily, they can also be used in some instances and with special arrangements to detect leakage down to 1x10-12 mbar.l.sec-1. Sniffing and accumulation are similar in that the detector head is used at a pressure above full vacuum, typically atmospheric pressure. Sniffing and accumulation are used to detect leakage in the 1x10-3 to 1x10-7 mbar.l.sec-1 range.

High Vacuum Helium Leak Testing

It is important to switch on the system well in advance of the test being performed to allow the mass spectrometer to warm up and stabilise (typically about 1 hour). Secondly, it is important to detect the background concentration of helium. Helium makes up 5.2 parts per million by volume of atmospheric air if more than this is detected then there is a source of helium close by.

In high vacuum helium leak testing either the inside or the outside of the part has to be at full vacuum. For many rigid systems that can take pressures exceeding 1 bar, one can simply connect the inside of the part to the mass spectrometer and evacuate the part. However, leaks can be directional, so if you are concerned with the leakage from inside to outside of the part and it has to be tested that way, the part will have to be placed into a vacuum chamber. This is typically how automotive fuel tanks and other fuel systems are tested with helium as the seals can be directional. In the case of automotive fuel tanks, the pressure has to be limited internally to a partial vacuum as the part would rip itself open with the full 1 bar. In these cases, both the inside and the outside of the part is first evacuated, then the internal pressure back filled to say 150mbar absolute.

Before such a test starts, one has to charge the test part with helium. For larger leaks, this can mean just pressurising with a few psi from a bottle of balloon gas. However, it is important to realise that most test parts start with being full of air, so by introducing helium into this air it will be diluted. This will result in the measured leakage rate on the instrument being many times less than the actual leakage rate because only a proportion of the leaking gas will be helium, one would have to correct for this. To ensure that you have close to 100% helium in the part, one must first evacuate the air from the part, then the measured leakage rate will be the same as that shown on the instrument.

At all times, one has to take care about the pressure applied to the part being tested to ensure safety. The part being tested may come under the provisions of the Pressure Directive and arrangements need to be made to check the part is safe to pressurise.

One should also ensure that the part does not have a gross leakage by checking that the system can hold a pressure that is above atmospheric pressure, this can be achieved for most test volumes by using a simple gauge pressure transducer. One can also partially charge the part with a smaller pressure and detect leakage on the mass spectrometer before introducing the full test pressure and swamping the instrumentation system with helium. Once the system is swamped, it may take some time for the instrument to be available for the next test as it has to clean itself out of the helium.

TQC has also arranged a number of machines that switch the instrumentation from one fixture to a second and back. This fully utilises the equipment allowing an operator or automatic loading system to unload and reload one chamber whilst the other is testing.

Sniffing

The mass spectrometer system will need a sniffer probe fitted to the “K” fitting on the instrumentation system, the probe can then be moved round the most likely areas for leakage i.e. welded joints, gaskets etc. The sniffer probe can usually locate the leakage to within a centimetre. It is important to switch on the system well in advance of the test being performed to allow the mass spectrometer to warm up and stabilise (typically about 1 hour). Secondly, it is important to detect the background concentration of helium. Helium makes up 5.2 parts per million by volume of atmospheric air if more than this is detected then there is a source of helium close by.

Before such a test starts, one has to charge the test part with helium. For larger leaks, this can mean just pressurising with a few psi from a bottle of balloon gas. However, it is important to realise that most test parts start with being full of air, so by introducing helium into this air it will be diluted. This will result in the measured leakage rate on the instrument being many times less than the actual leakage rate because only a proportion of the leaking gas will be helium, one would have to correct for this. To ensure that you have close to 100% helium in the part, one must first evacuate the air from the part, then the measured leakage rate will be the same as that shown on the instrument. It is also necessary to check that the part can be evacuated without atmospheric pressure collapsing and destroying the part being tested.

At all times, one has to take care about the pressure applied to the part being tested to ensure safety. The part being tested may come under the provisions of the Pressure Directive and arrangements need to be made to check the part is safe to pressurise. Do not pressurise a part greater than its proof pressure.

One should also ensure that the part does not have a gross leakage by checking that the system can hold a pressure that is above atmospheric pressure, this can be achieved for most test volumes by using a simple gauge pressure transducer. Often this can be achieve with compressed air before helium is introduced into the part.

The sniffing technique relies on the operator to be vigilant and attentive to ensure that the probe is positioned round all of the potential leaking parts, so this is an objective test. Also, the quantative results can be affected by drafts and windy conditions increasing the dilution of the emerging helium.

Having tested your part, you then have to be careful about removal of the test gas from the part being tested if you are going to complete multiple tests. One does not want to release the test gas into the test area and then affect the results of the next part so either remove the helium to recycle on the next test or purge it outside your building away from any direct path back into your test area. If you shed is big enough and the testing infrequent it may be fine to vent the test gas within the test area.

Accumulation Testing

This is where a sniffing probe is arranged within an enclosed volume with the pressurised part under test inside. The enclosed volume does not necessarily need to seal, just shroud the part sufficiently to contain the bulk of the leaking test gas so that it will pass by the sensor head. Sometimes a small circulation fan is required to ensure that the leaking test gas is adequately detected.

The comments about pressurisation mentioned above still apply and there is an added element to removal of the test gas, as it has to be purged from the shroud.

Due to TQC’s experiences with a Helium Leak Test Systems, if you have a particular requirement contact TQC to discuss the options you may have.

To find out more about how we can help you with your leak testing requirements:

We build all machines in-house, applying our 25+ years experience in specialised test and automated handling machines experience to the engineering projects we undertake. We offer customer support, backup and service call-out for all projects, whatever the size.

Get in touch with us to discuss your requirements and we will be happy to offer our professional advice and visit you at your site.

TQC Ltd, Hooton Street, Carlton Road, Nottingham, NG3 2NJ, United Kingdom
Tel: +44 (0)115 9503561   |   Fax: +44 (0)115 9484642   |   E-mail: sales@tqc.co.uk