Pressure decay leak testing is a common and popular method of leak testing for manufacturers across the globe. The test set up for an absolute pressure decay leak test can be fairly simple, using pressure decay leak test instrumentation to measure the drop in pressure of a test part to identify the leak rate and identify pass/fail results.
A similar leak test method, the differential pressure (DP) decay leak test is also becoming popular among manufacturers, as it can deliver improved resolution and faster cycle times than absolute pressure decay, depending on the application.
To learn more about Absolute Pressure Decay vs. Differential Pressure (DP) decay, and which parts are best suited to each test, read our last blog on this topic →
Differential pressure decay leak testing uses similar parameters as absolute but makes use of a differential pressure transducer, which tests for leaks by measuring a pressure change between a reference volume of air and the test part volume.
In this blog post, we will discuss how to set up an accurate, reliable differential pressure decay leak test, including several variables that can affect your test, how to manage these variables, and the details on how to set up your test for success.
Variables to consider with setting up a pressure decay test
There are a variety of variables that can affect the accuracy of your pressure decay leak test. These variables must be considered and controlled in the set up of your test to ensure accurate leak test results. Learn more about each of these factors below.
Test pressure
Since pressure decay testing measures changes in pressure, controlling the factors that can affect or change test pressure during a test is of the utmost importance to achieve accurate measurements and pass/fail results. Aspects of part design and part volume can also impact the correct test pressure required for accurate leak results.
Leak rates
If you’re testing around 10 sccm/10 psi and you don’t need to cut cycle time, there may not be a major benefit to using differential pressure decay instead of absolute pressure decay. However, if your goal is to improve your cycle time and repeatability, particularly at higher test pressures (>30 psig) and/or lower leak rates (<1 sccm), differential pressure could present a wide array of benefits on your line. Learn more here →
Volume
Part volume is a major factor that must be considered when setting up your leak test. Large volume parts, in particular, can introduce several challenges that must be considered and controlled during your differential pressure decay leak test setup, including:
- Part and test system volume variances
- Part elasticity
- Internal part movement
- Absorption rate of porous part materials (like in cast housings often present in large volume part designs)
Temperature
Temperature can drastically affect leak test results across many methods, including differential pressure decay. Your test set up must consider and compensate for temperature issues such as adiabatic temperature effects, ambient temperature effects, and part temperature (commonly affected by processes like weld or washing stations that the part may be going through prior to the leak test station).
How to manage these variables for accurate differential pressure decay testing?
For an accurate, repeatable result, you need to identify and isolate each of the above variables during the test sequence.
First, we bring the part up to pressure during the fill cycle time; long enough to bring it to test pressure. Careful testing and consideration needs to be given to porous and flexible parts during this phase to ensure the fill time is long enough to push pressure into the porosity of the part, and/or extend the flexible part to maximized expansion (while carefully avoiding over pressurization).
You also need to ensure that your stabilization time is long enough to allow for the thermal effects of compressing gas, so that the temperature rise caused by the compressed gas has time to dissipate. If you are dealing with porosity or part expansion, this time will also allow the part pressure to become stable.
How to set up differential pressure decay leak test
The two main factors that will make a difference in how you set up a differential pressure test compared to an absolute pressure test are 1) the use of a differential pressure transducer, 2) the resulting changes required to the stabilization phase of your leak test.
Add isolation stabilization sequence
With differential pressure decay testing, you need to add an isolation sequence after the stabilization time. After stabilizing the whole system (instrument and test volume), you will isolate the test part from the reference volume. In some cases, this doesn’t even need to add to over cycle time. You could simply cut the stabilization time into two halves, using 50% of the time for stabilization, as in the example below.
Above screens show difference in test set up between an absolute pressure test (left), and differential pressure test (right). You can see the 50% isolation time added to the overall stabilization time in this case.
Identify proper test time
Ultimately, the test time needs to be long enough to see the difference between known good and known reject parts (long enough to obtain a measurable and resolvable pressure change). A reasonable minimum target is a conservative 0.02 psi difference between known good and reject parts, and/or a 2:1 difference between that pressure change of good part vs. reject part.
Ideal gas law can be used to estimate ideal cycle time required to ensure an accurate, reliable leak test. Download CTS Leak Rate app to help calculate estimated cycle time →
Note: when using the CTS Leak Rate app, you can choose between absolute or differential pressure technology to help identify the resolution differences and test pressure loss measurement differences of 0.02 versus 0.009 psi drop (ultimately a repeatability study dictates the correct test time).
Need help with pressure decay or differential pressure decay leak testing? CTS can help!
Whether you’re interested in differential pressure decay leak testing, or any other method of leak testing, CTS leak test specialists are here to make sure you set up an accurate, reliable leak test for any application. We will help you make sure you are selecting the right method for your application and the best leak test instruments for the job. Contact us to discuss your application today!
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