Section 3 Methods of leak detection Section Title Slide © 2004. IUVSTA 3.01

Classification of leak testing according to test media used (a) Without additional test medium Pressure rise or pressure drop method Gas discharge excitation Accumulation with selective detection Acoustic or Ultrasonic testing Liquid level changes Weight loss or gain Notes 1/ This was taken from the 1st Edition slide 06-005 augmented by some special modern methods. © 2004. IUVSTA 3.02

Classification of leak testing according to test media used (b) With an alternative test medium Solid Filter paper, chemical indicator Liquids Water, oil, penetrating UV dyes Gases Air, H2, He, NH3, CO2 (pressurised), Hydrocarbons Freon, SF6, He, H2, Kr-85 (vacuum & pressure) Notes 1/ This was taken from the 1st Edition slide 06-005 augmented by some special modern methods. © 2004. IUVSTA 3.03

Test gas leak testing methods Vacuum method the object under test is evacuated and the test gas is applied externally (atmosphere) side with the test gas sensor inside the vacuum system. Pressure method the object under test is filled with test gas (or a gas mixture containing the test gas) under pressure and the sensor is applied at the external (atmosphere) side. [Note that liquids instead of gases may be used.] Notes 1/ This was taken from the notes to 1st Edition slide 06-007 © 2004. IUVSTA 3.04

Vacuum method: hood test for leak detection Test gas sensor Leak rate indicator Test item Vacuum system Hood filled With test gas Notes 1/ This slide was originally produced in the 1st Edition slide 06-007 without emphasis on depicting the test gas molecules within the Test item (considered leaky) and the vacuum system. The new picture is intended to depict high test gas molecule concentration in the hood compared with the vacuum system. The leak site in this case is near a flange port weld. Hopefully, in the case of a good part, the vacuum system will contain rather fewer test gas molecules than for a bad part, so that the leak rate indicator, which shows a linear response with the leak rate for a given test pressure, can be used for a simple go/no-go test or to give a quantitative measure of leak rate if calibrated with a known leakage. In production environments, using a good part (sometimes called the golden part) establishes zero level in testing, while a known bad part is used to set the reject level. This works well provided the reference part is really stable. 2/ This technique is reliant on the Test item being able to withstand the pressure differential applied to it during the testing, and on the Test gas sensor being sufficiently sensitive to the Test gas type when in the Vacuum system which is continually evacuated by one or more vacuum pumps. The Test gas reservoir is usually kept isolated from the Test item during the test, or after a preset filling time, so as to help avoid gross contamination of the Vacuum system and potentially saturation or poisoning of the Test Gas sensor which may reduce it’s sensitivity for next test or halted production line testing because of the necessary recovery time to return the system to normal conditions. Incidentally this slide also helps show why the Vacuum pump(s) must ensure they remove the test gas at a continuous rate without allowing a build up with exposure to the test gas: if they fail to do this the leak rate indicator will produce false backgrounds even with a good part under test. Zeroing is then often used but will be increasingly unable to compensate for time-dependent build ups if the pump performance also varies with time, since as the test progresses the background due to the vacuum pumps may fall while a true leak signal is rising, leading to the potential passing of a test piece that should rightfully have failed the test. The alternatives to this scenario are usually to lengthen the time between tests in order for the pumps to recover their performance, to reduce the amount of Test gas actually used per test, to incorporate pressure testing as part of the filling and part evacuation procedure (sometimes even using another gas for this purpose) and to suppressing the build-up of the test gas in the vacuum pumps (which can often be done by running the system’s backing pumps in viscous flow). Vacuum Pump(s) Test gas reservoir © 2004. IUVSTA 3.05

Pressure method: “sniffer test probe” leak location Test gas sensor Leak rate indicator Test item Vacuum system Test item filled with the test gas Notes 1/ This slide has been adapted from the previous slide of the 2nd Edition rather than the 1st Edition slide 06-10 to help emphasize the relationship between these two methods. However it may also provide a false impression of scales, since although more sophisticated probe test equipment is often necessary and may be of similar size as the foregoing vacuum hood test systems, many portable and self-contained hand-held probes exist in the market place, for a variety of different test gases: hydrogen, helium, SF6, freon (or rather the replacement HFA), ammonia, carbon dioxide, flammable gases and dilute hydrocarbon gas. 2/ In the slide the test item may well appear to leak from a place other than the real leak site since a sizable leak rate indication can result if the test gas rises from a leak below the probe. This emphasizes the practical importance of starting leak location work in the correct lace taking into account the known properties of the test site probe gas. This is typical of helium and hydrogen gases as test gas, since they diffuse rapidly but can be trapped underneath and constructional items such as walkways, equipment covers etc, and detected by such “sniffer” probe leak tests. The “heavier than air” gases such as CO2 and SF6 require the opposite approach, since leaks apparently sited at low positions can be due to accumulation from leakage occurring higher up the test item. 3/ Whereas the probe leak location test is slow and attempts to find the actual position on (the outside of) the test item, the previous slide is a total go / no-go test. In order that the test item as a whole be compared with the test specification, it may be necessary to combine individual leakage measurements each of which individually lies below the no-go threshold. This is a very subjective method, and is better approached by either rejecting the part for any detectable leakage within a (couple of) decade(s) of the no-go threshold, or by arranging to accumulate the leakage from all sites within a bag and using the probe to sample this collective resultant of all the leakage sites. This is of course throwing away the leak location capability, and makes the test subject to leakage from fittings and couplings that might otherwise be discounted by the leak location method, but it can often save considerable test time to try to accumulate any leakage from a whole section of the test item, especially if there are awkwardly sited flanges. Vacuum Pump(s) Test gas reservoir © 2004. IUVSTA 3.06

Pressure method – Chamber test Test gas sensor Chamber Leak rate indicator Test item Vacuum system Test item filled With test gas Notes 1/ This slide is adapted from the previous slides of the 2nd Edition rather than the 1st Edition slide 06-009 to emphasise the technique. 2/ The test item is depicted leaking from the top corner weld. When the chamber is under a reasonable vacuum, the test gas molecules can migrate around the whole chamber rather than flow directly towards the vacuum system pumps and/or past the test gas sensor. However the chamber test is a complete go/no-go test with little ambiguity and can be checked by using “golden part” and known leaks (very commonly attached to an otherwise good item). Vacuum Pump(s) Test gas reservoir © 2004. IUVSTA 3.07

Vacuum method – spray probe test Test gas sensor Test item Leak rate indicator Vacuum system Spray probe for test gas Notes 1/ This slide is adapted from the previous slides of the 2nd Edition rather then the 1st Edition slide 06-008 2/ By using this set-up the spray gun is run slowly over the whole external surface of the test item, whereby particular attention has to be paid to welds or brazed joints and to all kinds of mechanical joints, flanges in particular. 3/ This method is useful to find the exact location of any leaks present and also to determine their sizes if calibrated instruments are used. Vacuum Pump(s) Test gas reservoir © 2004. IUVSTA 3.08

“Bomb” method – Chamber test Test gas sensor Chamber Leak rate indicator Test item Vacuum system This slide has been created from slide 3.07 to show the very important variant on the pressure test –chamber test Which is relevant to testing of hermetically sealed items by the “bomb” test method. The product is exposed to high pressure helium in a separate pressure chamber for a preset long period of time before being removed and then placed into a vacuum test chamber and test gas sensor for any leakage of the tracer gas back out from the product into the vacuum test chamber. This method is used for small level leakage and is only suitable for products that do not absorb the helium “bombing” gas onto external surfaces. Nevertheless this is an important method for sensitive helium leak testing of hermetic IC packages and pacemakers not normally containing helium. Test item previously “bombed” with high pressure test gas in a separate chamber Vacuum Pump(s) © 2004. IUVSTA 3.09

Filled product – Chamber test Test gas sensor Chamber Leak rate indicator Test item Vacuum system Air pressure sensor Slide 3.10 is another important test method because the test item has been previously filled with helium gas, and then is tested in a vacuum chamber. The logical problems with such testing in production of validated products such as air-bag inflators is that the product may have leaked nearly all of its helium tracer gas content before the vacuum leak testing commences, especially in leak test systems that have to pump the chamber down to medium or high vacuum levels before they are sensitive to the helium tracer gas, and that of course they rely on helium being added to the product in the first instance. Therefore there may be some additional sensors and valves between the test chamber and the helium leak detector, which are not shown above. An air pressure sensor is always used, however, to measure the atmospheric pressure prior to the leak test and just after a known evacuated volume is attached to the test chamber. A viscous flow probe may be attached to the test gas sensor system to provide a cost effective method for checking the helium tracer gas fill percentage. [These methods are described in BOC patents by the author.] Vacuum Pump(s) Test item previously filled with helium gas © 2004. IUVSTA 3.10

Equivalent leak rates for applications Definition kg air/h mbar l/s Water tight 10-5 10-2 Vapour tight (sweat) 10-6 10-3 Bacteria tight 10-7 10-4 Fuel – and Oil tight 10-8 10-5 Virus tight 10-9 10-6 Gas tight 10-10 10-7 Technically tight 10-13 10-10 Conversion: 1 mbar l/s = 4.3 x 10-3 kg/h air at 20° C and the same test pressure © 2004. IUVSTA 3.11

Leak test methods, limits & principle Methods detectable principle Leak rate Pressure drop 10-2 mbar l/s Overpressure Pressure rise 10-4 mbar l/s Vacuum Bubble Test 10-4 mbar l/s number of Bubbles with defined  He – Sniffing 10-6 mbar l/s change of Helium concentration He – Integral 10-10 mbar l/s change of integral Leak rate Other Methods © 2004. IUVSTA 3.12