How to Avoid False Leaks

An Introduction To Sealing In Leaks

A successful leak test relies heavily on the selection and application of the sealing method, in addition to the leak testing equipment. In this article we will discuss how to avoid false leaks and introduce sealing in leaks.

Using The Proper Fixture

Every pressure decay leak test uses some type of fixture, and the fixture will seal and contain the part. A fixture that progressively squeezes the part will cause the internal pressure of the part to rise, which will reduce the observed leak rate or, worse, mask the leak entirely. If the component is clamped too tightly in the fixture, a similar effect might be observed as leaks could be closed.

Similarly, if the fixture relaxes as the part is pressurized, the part volume may increase and a decrease in pressure might be observed thereby creating a false leak.

One might think that a threaded part to tester interface would be the best choice. Not so, even with PTFE tape air can creep around the thread and create a false leak.

Effects of Temperature Change on Leak Test Results

Placing the fixture and part where changes in temperature occur will also make the part behave in unexpected ways.

A longtime customer in Brazil reached out to us for help reducing their cycle times when testing steering gear assemblies.  The test looked simple, but it was complicated by a flexible seal inside the part, which increased the part volume when a certain pressure was reached. We experimented with several stabilization times but struggled achieving the desired cycle time. We switched to a differential Qualitek mR and started the tests again. Once more, the results were confusing. The differential tester was more sensitive, and the results were widely variable.

Then we noticed that when the air conditioning in the factory came on, the results changed dramatically. Even holding a piece of paper above the fixture to keep the cool air away was helpful. We also noted a difference depending on whether brass or steel quick connectors were used. The false leaks we observed were due to temperature changes. Once we had compensated for the temperature change, the test was successfully setup using a differential pressure Qualitek mR to meet the desired cycle time. To help the customer we converted their gauge decay Qualiteks to differential pressure.

Hands-Off During Test Cycle

The component should not be handled more than is necessary, and never during the actual test cycle, especially if the component in question has a large surface area-to-mass or volume ratio. I have seen radiator assembly technicians get a radiator through the leak test by just placing their hands upon it momentarily during the test.

The most effective seals are metal-to-metal seals around a retained O-ring in a groove. Consistently applied clamping force by virtue of controllable pneumatic cylinders is desirable to ensure that the part is controlled in an accurate and repeatable way. If pneumatic control is not an option then correctly set up manual toggle clamps can be used.

Sealing Is A Science

This is just a brief excursion into the subject of sealing, mostly as it relates to the subject of false leaks. The subject of sealing is a science in itself and is one anyone involved in leak testing should pay particular attention to since it is as crucial to the test as the selection and specification of the most effective leak tester.

Uson Can Help

This article showcases what we do best. Of course, we sell some of the best leak testing equipment available, but it’s our expertise and years of experience helping customers through unique challenges and complex applications that sets Uson apart from the competition.

Contact us and let’s find the right leak test equipment and solution for your application.