Military-grade optical sighting devices are a great example of how determining the right leak test for the job is more than a matter of desired test cycle times and target leak rates. We should always consider the application of the device and the environmental factors in which it will be used.
Get a rifle. These devices are usually charged with argon or nitrogen to prevent the ingress of moisture or oxygen. O-rings are critical to the functional operating life of the equipment and also present potential leak paths. Ingress of oxygen can cause internal corrosion. Any ingress of moisture can cause fog in the short term and mold growth in the long run. Mold over time can actually corrode the inside of the lens, resulting in permanently blurred images.
Variations in ambient temperature, humidity, and barometric pressure due to changes in altitude and between day and night create the perfect conditions under which water vapor can penetrate the seals and allow the gas to charge and leak out. In this case, the risk of penetration is higher than with equipment that is only exposed to liquid water. In these cases, IP67 testing with volumetric pressure decay leak testing may not be entirely sufficient for military industry.
For laser scopes, the critical element – the diode – is very susceptible to damage from heat, moisture, and contaminants if its outer housing is not kept sealed. So, how do we check that such a device does not leak gas charges or have a leak path that allows unwanted water vapor to enter when ambient operating conditions change in military industry?
This calls for vacuum leak testing
Instead of looking for evidence that pressurized air is leaking into a part, use vacuum leak testing, we instead check for evidence that the part is out of balance with internal gas.
The test portion is placed in a closed chamber having a volume around the smallest part. We then sucked the air out of the test chamber to create a vacuum instead of pressurizing the chamber with air. Vacuum is used to simulate pressure changes that can occur with higher altitudes etc.
Then we experiment with a two-step approach; first an overall leak test and then a minor leak test.
The two-step approach
The overall leak-sealed part (large hole) will pass the leak test well. That’s because vacuuming the test chamber empties the internal pressure or air intake at the same time. As a result, no pressure change will be measured during the actual test. We must first determine if the part has an overall leak before we can perform the critical minor leak test to determine pass/fail. This is done with a pressure decay test using a standard volumetric flask.
If the part passes the gross leak test, we then proceed with the fine leak test (step two). It measures a pressure change in the test chamber over an extended period of time. With a vacuum, any pressure change during the fine leak test indicates that gas is leaking into the chamber from the test part.
Another approach is to use the tracer gas method. Since a device like a scope is charged with argon or helium mixed with nitrogen, we will then evaluate any evidence of these gases leaking into the test chamber with a mass spectrometer.
Let’s talk about the right test for your leak test application
This is a brief introduction to how we can trust leak test scopes, laser scopes, and any other electronic or sealed device that must meet military-grade performance standards against the ingress of steam. If your components need to function under these extreme circumstances, we can help design a test solution to ensure they function at their best.
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