Leak Testing: High Vacuum Methods

TQC has a depth of experience applying high and ultra-high vacuum testing to a range of parts.

TQC has been involved in 2 types of test using high vacuum testing.

  • The first consists of pulling almost complete vacuum of below 5 x 10-3 mbar absolute and looking for a detectable pressure increase.
  • The second is used when trying to detect the presence of liquid or moisture. It uses the physical property of liquids (and some solids) that vaporise at room temperature and very low pressure. In the case of water (or saline solution) the water will vaporise at just below 3 mbar absolute pressure. The boiling of the water and production of large amounts of vapour can be detected with special pressure transducers when plotting pressure against time as vacuum is applied to the test part in a vacuum chamber.

There are some specific design issues to overcome when using these techniques. Clearly, the parts must be dry and clean, if they are not, the outgassing from the volatiles within the chamber are significant. One also has to be careful about any assembly or sealing greases being used. Vacuum grease has to be used as it is specially formulated not to vaporise at such low pressure.

In the case of trying to test leakage below 5 x 10-3 mbar absolute it was clear that any outgassing needed to be removed quickly, this required the system being engineered with an additional turbo pump backed up by the rotary vane pump to take the pressure well below the leakage pressure threshold of 5 x 10-3 mbar absolute. This also required special instrumentation for measuring pressure. Here is an example of a standard helium leak test bench adapted to also do a high vacuum test.
Many medical devices contain or are packaged within a liquid; examples include some diagnostic units, disposable contact lenses and gel dispensers. The problem here is to find a leak-test technique that is not affected by the liquid plugging the leak path because liquid can easily plug a hole of 10–15 µm in size. A hole of this size in smaller parts is detectable by using air-pressure decay; but a leak can be detected only until liquid comes into contact with the opening of the hole.

There are two other potential techniques that can be used to detect a leak path in these types of medical device. One employs helium leak detection because often the helium will find its way through the liquid. The other uses high vacuum and boils off liquid as it emerges from the leak path. Even when there is a small amount of liquid, this will create a massive amount of vapour if the pressure is sufficiently reduced. Typically at approximately 98% vacuum, water, alcohols and some oils will boil at ambient temperatures. By monitoring the pressure as the vacuum is applied, the corresponding graph (see below) will show that the curves are different for sealed and leaking parts. This has been demonstrated to work on liquid-filled blister packs containing contact lenses where the leakage of the fluid to the outside of the pack is the only way to detect leakage.

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