Leak testing is about more than identifying leaks in factory parts and assemblies, as it’s understood that in leak testing, everything leaks. The real question is: To what degree do parts leak? On top of simply identifying leaks, you also need to define the leak rate to determine if the size of a leak exceeds specifications. The only way to do that is to know how to evaluate your leak test data.
Using leak testing tools calibrated to specific parts under test can tell you how much a part leaks. But how do you apply the data? How do you make leak rate measurements meaningful?
To apply leak rate data, you need both real-time reporting and historical analytics. The real-time leak test data illustrates the performance of the part under test, and the historical data provides a benchmark to put the test results in context.
Applying a Data-Driven Approach to Leak Testing
When performing a leak test, you usually get a pass/fail indication based on whether the leak rate is higher than the maximum specified rate. Are you sure it’s the part that’s faulty, or could it be the test? Here are steps you can take to validate the test environment:
- Rerun the test on the part to ensure you are getting consistent results. If the results are inconsistent, you need to check the test environment.
- Test the seals to ensure they aren’t worn out and affecting the results.
- Check the test calibration using a master part to ensure the test is providing reliable results.
- Test the pressure regulators to ensure they are working properly.
- Restart the machinery and start again if the test results still seem inaccurate.
Although these steps will reset faulty equipment and potentially identify problems with the test itself, they don’t tell you what occurs during the test cycle. It could be something happening during the test cycle that is causing the test failure.
To assess the actual leak performance, you must measure pressure loss over time. That’s the only way to determine the leak rate. You can use a pressure decay test to pressurize the part, then measure pressure loss over time. You also can use a mass flow test, in which you measure the amount of air required to maintain a constant pressure. Whatever test you choose, you want accurate test data, so you need to know where to look for anomalies in the leak test data that indicate your test results may be inaccurate.
Choosing the Right Leak Metrics
To understand what is happening during a leak test, you must consider different measurements. Here are the four most meaningful metrics to help you understand leak rate:
1. Flow Rate
Whether you are using a pressure decay or mass flow test, you will get flow rate data. Measuring the flow rate against time using a waveform will reveal different things:
- Is the leak a gross leak or a small leak? Once the test stabilizes, you should see a constant leak rate.
- Is the temperature stable or fluctuating? Temperature changes will affect the flow rate.
- Is the pressure stable? Changes in pressure affect the leak rate over time.
- Are there any indications of interference with the test, such as a disruption in pressure due to a hose being moved or some outside factor?
- Is the test cycle time optimized? You need to make sure you run the test long enough to get trustworthy results.
2. Test Pressure
The test pressure must be reliable, so you should use a waveform to assess test pressure over time. You want to answer these questions:
- Is the pressure stable? Even small changes in pressure can have a significant impact on flow rate, especially as the part volume increases.
- Is the leak a gross leak or a small leak?
- Are there anomalies or “blips” in the data caused by fluid in the part, a stuck test valve, or some other factor?
- Is the cycle time optimized? Be sure the test pressure is stable and you have left sufficient fill time.
3. Supply Pressure and Pilot Pressure
In addition to the test pressure, you also want to make sure the pressure source is constant. It pays to use a waveform to assess supply pressure and pilot pressure against time. Look for answers to these questions:
- Is the air supply constant? Whether you are conducting a pressure decay test or a mass flow test, the supply pressure should fill the part quickly.
- Are there other tests using the same system? If other systems are drawing air, it could create pressure fluctuations.
- If there are pilot air return valves, are they latching quickly and completely? Faulty pilot valves will affect test results.
Temperature changes can have a dramatic impact on test results. Be conscious of the effects of both ambient temperature and part temperature as you answer these questions:
- Is the ambient temperature stable? You may run a test in the morning and get different results when you run the same test in the afternoon. Using a waveform, you can see if there is a correlation between the change in ambient temperature and test results.
- Is there a change in the part temperature that is creating variations or affecting repeatability? If there is a fluctuation in the part temperature, you may have to change the test process to compensate.
Your leak test data only has value if it’s trustworthy. Any variables that affect the outcome need to be evaluated and cleared to ensure the test release is trustworthy. Any leak test needs to be repeatable to be reliable, so you need to know where to look for anomalies and how to interpret them.
Have questions? Contact us to learn more about interpreting and making better use of your leak test data.
Want to learn more about improving your leak test using your production data?
Watch this on-demand webinar, presented in partnership with Sciemetric, which uses CTS Sentinel instruments to discuss and illustrate how to use leak test data to address common production challenges.