That first fact about leak testing is that everything leaks—even if it’s to a very small degree.
The real question isn’t “how do we keep this part from leaking” but rather, “what is an acceptable rate of leakage, and what is the best way to achieve a reliable, repeatable leak test for that target rate?”
The answer depends on the size, materials, and other unique physical characteristics of the part in question and the conditions under which it will be used.
Despite these variables, the basis of a “good” leak test remains largely the same, provided you know what questions to ask. Below are 5 common mistakes to avoid on your leak test to achieve a reliable, repeatable test every time.
1) Not using the ideal leak test technology for your application
The ideal technology for your leak test application will depend on a variety of factors including, the acceptable leak rate, the ideal test pressure, the size and function of the part, the testing environment, and more.
Let’s do a quick overview of the basic test options and when they are best used:
Dunk Testing: This is the most basic of leak tests. A test part is pressurized, submersed in a tank of water, and the operator looks for any evidence of bubbles. The frequency and size of the air bubbles are directly proportional to the size of the leak. Dunk testing is fast and cost effective. But the results are not measurable and remain dependent on a subjective determination by the operator.
Pressure Decay Leak Testing: Fast and highly accurate leak testing using pressure drop over time correlated to an air flow rate. it is easy to use this technology to seal a part pressurized for testing, calibrated to a leak standard, or set up as a pressure drop over time test to identify accept and reject parts. However, it does not identify the source of leaks and can be slow for large parts with low leak rates.
Vacuum Decay Leak Testing: Similar to pressure decay, vacuum decay leak testing evacuates air from the test part to detect leaks. Vacuum decay is commonly used with parts that could have leaks from external sources, such as underwater sensors, pipes, or outdoor electrical housings. It’s also less susceptible to temperature effects of compressing gas in a test volume but is still affected by temperature changes associated to part temperature and environmental conditions.
Mass Flow Leak and Functional Flow Testing: Ideal for applications such as medical catheters or tubes, where you want to measure the volume of flow based on leaks or blockages. With this form of test, the part is pressurized with air. Then, the rate of flow going into the part is measured while the part is held at a constant pressure. The sensitivity of this test is sometimes too low for smaller leaks. Also, the accuracy of the readings are highly dependent on the flow meter, air temperature, and system pressure.
Tracer Gas Leak Testing: For parts and systems with extremely low leak rate requirements, you can use tracer gas tests such as sniff testing, nitrogen purge leak testing, accumulation leak testing, and hard vacuum helium leak testing. The sensitivity of these tests can be negatively affected by uncontrolled atmospheric trace gas that creates background noise.
2) Lacking technical capabilities required for the test
When it comes to an effective leak test, it’s not just about the test setup and environment, but having the right technology with the right capabilities required for the test.
First, your leak tester must be equipped with digital sensors and gauges that can provide you with at least 100 times the higher resolution with which test measurement demands.
Second, it’s not just about being able to detect the smallest of leaks, but also, being able to identify and filter out those variables, and any noise, that can sabotage test accuracy. The culprits may be temperature, humidity, volume differences and/or contaminants.
The right leak test technology for your application is key to manage the conditions and the variables that can affect test accuracy and speed.
3) Wrong seals and poor connections
The next step is to choose the right seals and connectors. If you can’t properly seal a part to the test stand, the test results could be just as erratic as if someone were pinching an air supply line.
Seals and connectors must be held firmly in place during the entire test cycle. Whether a seal is mechanical or pneumatic, your setup should provide feedback to indicate that proper seating and orientation has been achieved before the test cycle begins. This can be accomplished through an electronic sensor (pressure switch), a proximity switch, or a simple travel stop on manually actuated seals.
A related point involves calculating and applying the ideal safety pressure ratio.
The force holding the seal is suggested to be 1.5x to 2x or more the force that is acting against the seal from the test pressure within the part. This prevents the seal from moving while under pressure. A helpful formula is Force = Pressure X Interior Area.
Don’t forget durometer
The ideal hardness of the seal also depends on the environment, the part type involved, the test pressure, and how many test cycles the seal must endure in a production shift. If too soft, the seal material will wear too quickly. Too hard and it will not compress correctly to make a quick and reliable seal.
4) Being unsure about test cycle time
What time is right?
Well, it depends—beginning with your desired rate of production. How many test cycles do you want a leak station to handle during a production shift?
In addition to what I have already said about the ideal leak test for your needs, depending on the desired test pressure and leak rate, cycle time is also vitally important. You want the shortest possible test cycle without compromising the repeatability and reliability of the test.
Depending on your test scenario, the ideal test method and cycle time may simply not be able to keep up with the pace of production with a single station. A line may need two or more parallel stations. This will drive up costs. On the other hand, that may be less expensive than the scrap and rework rates that can result from a leak test with cycle times that are too short to be reliable.
5) Uncalibrated equipment
A reliable, repeatable leak test cannot be expected to remain so without periodic calibration.
One standard method, common with pressure decay leak tests, is to use a non-leaking Master Part (given that everything does leak, “non-leaking” simply means a part with a leak rate that’s as close to zero as possibly manufactured).
During a calibration process the Master Part is tested twice; first test sets the zero flow limit (Master) and the second test sets the Hi Limit leak (Master + Leak Loss). During each of these tests the part is filled to a specified test pressure, then isolated from the source air. Test pressure stabilizes, and the pressure decay test is run. The instrument measures the delta pressure loss and records the results. At the end of the calibration process the instrument displays a Performance Factor which estimates the quality of the calibration.
This process employs a “cal ratio x pressure loss penalty x time penalty x 100” formula. If you have a performance factor greater than 50, then the test stand can be considered calibrated.
Unsure how to improve your leak test? Contact the team and Cincinnati Test Systems to discuss the best setup for your leak test >