Military-grade optical sighting devices are an excellent example of how determining the right leak test for the job isn’t just a matter of desired test cycle time and target leak rate. We must always consider the application for the device and the environmental extremes in which it will be used.
Take a rifle scope. These devices are typically charged with argon or nitrogen to prevent penetration by moisture or oxygen. O-ring seals are crucial to the device’s functional operating life and also represent a potential leak path. Penetration by oxygen can cause internal corrosion. Any moisture penetration can cause fogging in the near-term and mold/mildew growth over the long-term. Mildew over time can actually etch the inside of the lens, resulting in a permanently blurred image.
The swings in ambient temperature, humidity, and atmospheric pressure that result from changes in elevation and between day and night create the perfect conditions under which water vapor can penetrate the seals and for the charged gas to leak out. The risk of penetration is greater in this case versus a device that is only ever exposed to water in liquid form. Under these circumstances, testing to an IP67 standard with a volumetric fill pressure decay leak test may not be completely sufficient.
For laser sights, the key element – the diode – is highly susceptible to damage from heat, humidity and contaminants if its enclosure doesn’t maintain an adequate seal. So, how do we test to ensure such a device either won’t leak its charge of gas or has a leak path that will allow unwanted entry of water vapor, as its ambient operating conditions change?
This calls for vacuum leak testingInstead of looking for evidence that pressurized air is leaking into the part, using vacuum leak testing, we instead test for evidence that the part is losing its state of internal gaseous equilibrium.
The test part is placed in a sealed chamber with minimal volume around the part. We then evacuate the air from the test chamber to create a vacuum instead of pressurizing the chamber with air. A vacuum serves to simulate the pressure changes that can occur with higher elevations, etc.
We then test using a two-step approach; first a gross leak test and then a fine leak test.
The two-step approach
A sealed part with a gross leak (large hole) will pass a fine leak test. That’s because the evacuation of the test chamber will empty the part of any internal pressure or gas charge at the same time. The result is that no pressure change will be measured during the actual test. We must first determine if the part has a gross leak before we can carry out the critical fine leak test that will determine pass/fail. This is done with a pressure decay test using a reference volume tank.
If the part passes the gross leak test, we then proceed with the fine leak test (step two), which 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 a tracer gas method. Since a device like a scope is charged with argon or helium mixed with nitrogen, we then sniff for any evidence of these gases leaking into the test chamber with a mass spectrometer.