Dunk testing remains a common leak test method due to its simplicity and relative affordability. This traditional approach, however, can be highly subjective and prone to error, leading manufacturers to look for ways to achieve a more objective and reliable leak test—but what’s the best method?
First, let’s clear up a longstanding misconception about dunk testing: The theory is that if you put a test part under water and it has a leak, that leak is going to form a visible bubble. In a dunk test best-case scenario, these bubbles can be correlated to a volumetric measurement of air loss over a set period of time. This volumetric measurement tracks standard cubic centimeters per second or per minute, depending on the allowable leak rate.
However, this is not always the case. When you do see a bubble, it’s because water is displacing air. The leak has to be relatively large for the water pressure to displace enough air to form a bubble. You may have an undesired leak, but not so large that it will form a visible bubble.
Dunk testing also doesn’t suit test parts with materials prone to corrosion, or electronics that can be damaged. Another issue is that the water pressure in the dunk tank may be sufficient to squeeze close an undesired leak path, resulting in a false pass.
For a reliable leak test that doesn’t rely on a natural bubble or a subjective judgment call by the operator, you need to transition to a more automated, objective, and repeatable approach.
Dry alternatives to the dunk tank: Pressure decay, vacuum decay, mass flow
So, what are your best dry alternatives to test for a volumetric leak rate?
Pressure decay, vacuum decay, and mass flow are the three primary test methods available. Each method uses pressurized air as its media and conducts measurements that are correlated to a volumetric flow of air as the tested part leaks. Resolution and repeatability are based on a pressure differential caused by the leak.
Repeatable flow rate measurement depends on test pressure and part characteristics. The test range for mass flow can be as high as hundreds of SL/min or as low as 0.5 SCC/min. Test capabilities for pressure and vacuum decay can range from 0.5 SCC/min up to thousands of SCC/min. Both can go as low as 0.1 SCC/min, depending on the test part and the control of variables such as temperature.
Targeting lower leak rates? Consider tracer gas leak testing
For even lower target leak rates, we turn to a tracer gas test method. The part is charged with a trace gas in a sealed chamber and a detection device such as a spectrometer is used to detect leakage of the trace gas from the test part.Due to its inertness, low parts per million concentration in atmosphere and small atomic size, helium is the ideal trace gas for the most minute of target leak rates. Helium does, however, suffer from cost and supply issues.
Another option is to use forming gas. With forming gas, hydrogen and nitrogen are mixed in a five and 95 percent blend. Hydrogen in this case is the trace gas and its five percent concentration prevents it from being flammable.
Helium has the advantage that it can be used as a trace gas in concentrations from five to 100 percent – this provides you with a lot more flexibility in terms of test pressures and test cycle times versus forming gas, to best optimize your test for the application at hand.
In all cases, precision control is critical for a reliable, repeatable leak test
Whether your testing needs are best served by a volumetric air-based method or a trace-gas method, what’s important is that the test instrument be a modern digital device with the sensors and data analytics to deliver fine granular control of all test parameters and variables. This is crucial to achieve a reliable and repeatable pass/fail result quickly enough to keep pace without slowing down your production line.
Download our latest guide, Leak Testing 101: A Primer, or contact us to learn more about the best leak test methods and instruments for your application.
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