How to Use Reference Volume When Differential Pressure Decay Leak Testing

Reference Volume Do’s & Don’ts

This article outlines what you should know about using reference volume when differential pressure decay leak testing, as well as what factors/parameters are taken into consideration on whether or not to use a reference volume for differential pressure decay tests.

The differential pressure decay leak test works very much like a set of scales, both sides are filled to the same pressure and if one side changes the balance moves in a direction to show there is less on one side than the other.

When You Should NOT to use a test volume

Like the scales, both sides of the transducer should have the same quantity of air on both sides, which in an ideal world would mean that the reference volume should be the same size as the test volume, and therefore we should always have a reference volume fitted. So why don’t we? There are several reasons:

1. Pressure change is affected by temperature. P.V/T =k
2. If the reference volume is in a different place to the test volume it will have different ambient conditions
3. Reference volume may not have the same thermal mass and therefore take up temperature change at a different rate to the test volume.
4. The reference volume is a permanent installation and will therefore adopt a background temperature that has been driven by repeated pressurization and therefore behave differently to the test part.
5. Pressure change is affected by volume.
6. Test and reference volume may not expand due to pressurization effects in the same way and therefore there will be dissimilar changes in pressure due to changes on volume.
7. Two volumes take longer to fill than one.

The design of the differential transducer allows us to get away with a small reference volume for most of the applications we see. The internal volume with a combination valve is about 5 cc’s (with the Stallion it’s much bigger) and as we have a taught diaphragm, the displacement due to a leakage on one side is small. The response to a leak will be non-linear. As the leak increases so the rate of movement will decrease because the driving force, that is the pressure on the reference side, decreases with increase in volume. This is not usually an issue. Leak rate calculations use 10 cc’s as an internal volume which compensates for the non-linearity.

When You SHOULD Use a Test Volume

So, when DO we use a reference volume?

1. When we are testing very big volumes for small leaks. We will be looking for very slow changes in pressure and possibly be in the test phase for 20 seconds or more to get discrimination. 4 cc/min in 100 litres on the test side can easily be masked by a 0.0002 cc/min leak on the reference side.
2. When testing flexible parts. It’s useful to have a master part (a sealed-up version of a production unit) which will behave similarly to the test part and cancel out the effects of expanding test volumes.
3. A reference volume identical to the length of the test line run parallel to the test line (usually taped to it) to cancel out effect of drafts on the test line.

The reference volume and differential pressure decay leak tester combination can help increase the range of applications air decay leak testing can handle and can improve your ability to find small leaks in large volumes quickly.

Uson can help!

If you are unsure whether you need to use a reference volume our team of experts is here to help.  Contact us to speak with one of our Application Engineers for guidance.