Thursday 5th May 2022

CHOOSING A MICRO-LEAK TESTER FOR BEVERAGE AND FOOD ENDS

Air or helium? All things considered

A micro-leak tester is used to identify leaks from defects and cracks in sampled easy-open ends/lids after they have been through the conversion press. When deciding which tester to install, it might seem obvious to choose one with the highest sensitivity. Systems that use helium as a ‘sniffer’ gas to find possible leaks are typically quoted as having a higher sensitivity than systems based on air pressure differential. Helium molecules are a lot smaller than air molecules, so this appears to make sense. However, the situation is more complicated than that both in theory and in practice as laboratory sensitivity is not the only issue that determines the real-world effectiveness of a micro-leak tester.

HELIUM VERSUS COMPRESSED AIR

It's only with extremely small holes (smaller than around 0.1 microns, under typical temperature and pressure conditions) that the size of helium molecules becomes the determining factor in how fast the gas flows through an aperture. Above that threshold, when the flow regime becomes "laminar" rather then "molecular", it is the viscosity of the gas that principally influences its flow rate through a leak. Air is less viscous than helium, so it actually flows slightly better (7.7%) than helium through the most common defects in metal lids.

In laboratory tests, helium is good at detecting extremely tiny flow rates (as small as 1 x 10^-3 cc/sec), yet there is no evidence from the canmaking world that such truly microscopic pinholes are significant on metal ends/lids. If they are present at all, they could easily be blocked by sugar molecules, bacteria or pollen grains, for example, all of which are considerably larger.

There is no practical difference between the performance of helium and air testers in use on beverage and food making lines, and air pressure testers like Sencon's MLT have been successfully used on most of the world's end  making lines for over 20 years without problems or customer complaints. There are good reasons for this.

Helium is generally more difficult to manage and store than compressed air. Precisely because it can flow more easily through extremely small openings, the seal around an end under test needs to be extremely tight on helium based systems. Also, helium that is voided from the test chamber after each test will tend to contaminate the local atmosphere, potentially misleading the ‘sniffer’ test itself. So helium requires a much more controlled test environment than MLT, which detects changes in air pressure either side of the end under test in order to detect leakers.

A FINITE RESOURCE

But most important of all, helium is a finite natural resource with a limited global supply that is now being heavily restricted. This also means that its already expensive cost is set to skyrocket in the near future. Despite being the second most abundant element in the universe, helium is quite rare on earth. Most of it is associated with underground deposits of natural shale gas, so it is mined from a fixed number of known locations around the world. As these stocks are running low, supplies are being rationed by various governments in view of the fact that helium is an essential component in medical imaging devices, aerospace systems, and in the manufacture of semi-conductors.

Micro-leak testing on can lids/ends is just as effective, far cheaper and easier, and more environmentally friendly using compressed air which is already freely available within the factory.

For more information on Sencon's Micro-Leak Tester contact your regional Sencon office or agent.