A leak test that is reliable, repeatable, and fast enough to keep pace with production begins with how the test part is connected to the test instrument.
Let’s start with the obvious – the primary cause of misleading test results is leaking seals and fittings.
Choosing an ideal seal
The best seals have a mechanical component that physically locks them to the part and ensures proper orientation. This can come in the form of:
- A mechanical or pneumatically (air) activated seal that is physically deformed to provide the mechanical lock. (We recommend a mechanically compressed seal for port seals and single compression ferrule over the common push lock connector for test line connections.)
- A bolted together connection.
- An automated seal on the end of a ram or robot that is locked into position during testing.
Whether a seal is mechanical or pneumatic, it should provide feedback to indicate that proper seating pressure and orientation has been achieved before the test cycle begins. This can take the form of an electronic sensor (pressure switch), a proximity switch for automated rams, or a simple travel stop on manually actuated seals.
Controlling pressure
Fluctuating pressure in the air supply line can impact fill time and test accuracy. It can also impact seal performance. For example, if a pneumatic seal is used, supply pressure must be constantly measured and controlled to ensure the required air pressure is maintained for the duration of the test.
A general rule of thumb is that the opposing seal force (force to hold a seal in place compared to the test pressure force) should be 1.5 to 2 times that of the test pressure force from the part to prevent any false leaks through an opening. But no more than three times—excessive pressure can accelerate how quickly the seal wears out.
Protecting against seal wear
Speaking of seal wear, it’s a simple equation between hardness, softness, and durability. A softer seal material makes it quicker and easier to achieve a proper seal but will wear faster. A harder seal material will last longer but may require more time and diligence by the operator to ensure a good seal.
Durometer is a measure of hardness for rubber compounds or polyurethane. Choosing the right durometer seal depends on the production environment and the contact surface, the desired test pressure, and how many test cycles the seal must endure in a production shift. The goal is to strike the right balance to ensure the longest possible reliable life.
Using the right plumbing
The closer the part is to the test instrument, the more accurate the leak test and the shorter the test cycle. A shorter hose means there is less air volume in the test circuit—less air must enter the test volume and less air must reach thermal equilibrium within the part.
Hose diameter and material is also important. If you match the hose’s inner diameter with the fill port, it will optimize the fill time. Also, the hose must be constructed from a material that won’t deform under pressure. Thin PVC lines just won’t do. Consider instead seamless stainless steel tubing, and better because of its thermal resistive properties poly tubing rated to no less than 500 psig.
Using the right fixturing
Consider how the part will be held and controlled during the test cycle to avoid any movement that would compromise seal integrity.
With flexible parts that may be prone to ballooning, such as a battery pack, some method of nesting may have to be used to prevent this distortion. Even if ballooning doesn’t damage the part, it can easily skew the reliability of test results if left unchecked.
Other considerations
When a test or check is manually operated, there is always a risk of error. Proper training and oversight is crucial. Fixturing choices can also help mitigate the risk. For example, using an air-actuated connector, which allows for fast, safe, and repeatable connections.
Data is also important. Every byte of data generated at the test stand should be collected, archived, and indexed by part serial number. This way, you have a data trail to fall back on to troubleshoot quality issues. This will assist with calibration, its verification and root cause analysis to ensure your leak test is optimized.
Contact us to learn how to optimize your leak test station for reliable, repeatable results.