How to choose a leak testing method that’s best for your needs
When deciding on the best solution for a leak testing challenge, many factors need to be taken into consideration, including the risk, suitability and cost of each method. Various technologies are available and it’s critical to choose one that is optimal to your application requirements.
In recent years, huge strides have been made in electronic leak testing methods that use air or tracer gases, resulting in new techniques, enhanced sensitivity and faster test cycles. These changes have resulted primarily as a response to cost pressures as well as the need to adapt to a changing environmental and safety regulatory environment.
Before making any decision on which technology offers the best solution for your specific application, it’s best to discuss your options with a professional equipment supplier to get the most practical and cost-effective solution.
Here, we look at two main technologies, namely air leak testing (especially on micro-flow technology) and helium leak testing (with mass spectrometry).
Air leak testing with micro-flow sensor
In this method, the test part is pressurised from a pressure source with the regulator and valve isolating the test part from the source once pressurisation is complete. The pressure sensor then monitors and measures the pressurised test part and if the pressure measurement decreases, air is leaking out of the component or assembly under test. As air leaks, the micro flow sensor replenishes the lost air to maintain a constant pressure, with the loss causing an electrical signal that is proportional to volumetric or mass flow. This method has a sensitivity of 5x10-4mbar*l/s. Easy-to-use, sensitive pressure sensors are available for this type of testing method.
Air leak testing with mass extraction
This method is an extension of the micro flow sensor one detailed above, and operates on the basic principle of rarefied gas flow. The test is done under a vacuum to achieve a higher sensitivity, with sensor designs operating either in a shallow vacuum (continuum/slip flow conditions) or a deeper vacuum (transitional/molecular flow regimes).
This method can be done in a number of ways. The closed container under test can be placed into a vacuum chamber (with a vacuum as low as 1 mbar or less) and the leak rate determined by measuring the remaining flow between the chamber and the vacuum reservoir once the item has been removed. The vacuum can also be applied inside the part under test and the barometric air leaking in can then be measured.
Mass spectrometer detectors are widely used for leak detection of tracer gases. These robust and fairly simple-to-use instruments are typically tuned to detect mass of 4 amu (helium) and have a sensitivity of 5x10-12mbar*l/s (for helium). The most sensitive test method is when the part is first evacuated and then filled with helium while inside a vacuum changer.
These mass spectrometers can also be used to identify the location of a leak. The test object can either be connected to a leak detector and vacuum drawn whilst being sprayed with helium from the outside or the sniffer test technique can be applied. In this method, the part under test is pressurised with helium and scanned with a sniffer probe connected to the leak detector.
Choosing the right method
Several factors need to be considered when it comes to choosing a leak detection method. The first question to ask is ‘what are the tightness criteria that need to be detected?’ Highly sensitive helium leak detection is the best way of detecting leak tightness in the range of 1x1-7mbar*l/s or lower, but if your required leak rate is above this value, then there are numerous other possibilities.
Another question to ask is ‘how important are cycle/throughput times?’ If your application involves testing of 100% of components or parts in a production line, then speed is an important consideration. If your leak testing challenge is limited to sample testing and laboratory applications, then throughput times aren’t such a critical factor in your choice of leak testing technology.
The design of the part under test – and specifically the design of the seals – should also be considered. Leak testing often involves pressurising a part to its maximum operating pressure, and some seals will perform better under higher pressure and vacuum than others.
Leak test methods can also be affected by environmental conditions. As an example, let’s look at pressure decay testing, which relies on stabilisation times as well as temperature control in order to get reliable, accurate measurements. These requirements make it unsuitable for leak testing in a number of different environments, e.g. when items are very hot (such as a part that has just been welded) or when there are rapid and extreme temperature swings. In these circumstances, vacuum air testing or helium testing are preferable to pressure tests.
Timing is important. The time required for the test is directly associated with the capacity of the test process – and different methods have differing test times, stabilisation time and/or drying time. For example, helium leak testing is quickest for medium- and large-sized components, whilst mass extraction technology is faster than other air testing methods such as pressure decay. Cost is another crucial factor – and that’s where your equipment supplier has a key role to play. It’s imperative to look at the life-cycle costing of the test method and its overall suitability and not just the cost of acquiring the product. What are the other costs associated with the method? Where can this technology be applied elsewhere in the operation? Does it meet a short- or a long-term requirement? Does it provide flexibility to meet changing needs?
Sensitivity must be considered too. In applications where high levels of sensitivity are required (e.g. medical products where leaks can cause a risk of contamination) or that have stringent water ingress or safety specifications, including semiconductor tool applications, airbag activators, nuclear facilities, etc, the only option is helium leak detection.
However, in most applications such as heat exchangers, brake systems, welded assemblies, electronic enclosures, etc, both air or helium test methods can
be used.
A combination of both types of test methods is generally only applied when the parts under test are high-cost items such as those found in the aerospace industry.
Choosing the right leak testing method is critically important and it all starts with your choice of equipment supplier. It must have an extremely high level of expertise and experience but it also needs to really understand the intricacies of your business, how it works and what your long-term goals are, so that it can recommend solutions that do more than just meet technical needs. One example of an expert in this field is Nexxis, a leading technical equipment supplier in Western Australia that has a reputation for service delivery that goes beyond expectations.
For more information visit www.nexxis.com.au
