Leak Size In Perspective

Determining Realistic Leak Specification

In this post I thought we might try to put hole sizes, leaks, and leak rates in perspective.
Much continues to be written about determining realistic leak specifications and the chestnut of the “no leakage permitted” type of specification is still commonly encountered.

Looking through some old training material I thought the “no leak” specification was interestingly compared to “no tolerance” in manufacturing. For example dimensional measurements are usually specified as X +/- Y if the value of Y = 0 then we would have zero tolerance. Is it not the same when specifying a leak rate?

Consider:

• A human hair is 70-100 microns in diameter.

• Our limit of visibility with the naked eye is around 40 microns

• A single white blood cell is in the region of 25 microns

• A grain of talcum powder is 10 microns

• A red blood cell is in the order of 8 microns

Why is this relevant? You may ask. In our white paper “ When is a leak not a leak?” we talked at some length about viscosity and its effect on leakage. Let’s look at the chart below and see how significant the hole size and leakage rate are, comparing water and air :

So you can see that at a test pressure of 2 bar or 30 psi air will leak through a hole of something larger than 5 microns whereas water will not leak until the hole is at least 20 microns.

In fact the second chart shows that air leak rates of significant sizes can be measured through very small holes :

Even at 70 microns there is no measurable water leak.

The first step to selecting a leak tester is to determine the appropriate specification for your leak test.  Next, calculate the fill time, stabilization time, test and vent times and then find the leak tester that can deliver the results with the required repeatability. It can’t be stressed enough that there is no value added to your leak test by over tightening the leak specification. In fact, specifying a leak rate that is too tight is likely to make your life way more interesting, in an unhappy kind of way.