Have you seen the bubble? - The disadvantages of bubble leak testing

We've all had a flat bike tire at one time or another but couldn't see the leak with the naked eye. The bubble test is the remedy. However, what is often sufficient for private use is not suitable for industrial applications. Although the disadvantages are sufficiently known, I still discover a water bath in the production environment with some customers.

Today I would like to show the disadvantages and effects in the context of quality assurance as an example.

How the bubble test works

In a water bath leak test, the component to be tested is pressurized and immersed in a water bath. Tools like holding frames are often used during the immersion process. By means of rising bubbles, the tester can now determine whether the component has a leak or whether the tightness requirements are met.

But is this method reliable and reproducible? - No!

What is the problem?

Several factors make this method not meet the requirements of today's leak testing solutions, be it the too low sensitivity or the human factor. I will discuss these factors in the following sections.

Sensitivity

Theoretically it is possible to detect small leaks with the water bath method, this is given by the literature with 10-3 mbar l/s, but this can only be achieved under laboratory conditions, but not in a production environment.

To make it a little more illustrative:

At a leak rate of 10-3 mbar l/s, 1 bubble per second escapes from the test part. However, this must be perceived correctly by the operator.

Another way to increase the sensitivity is to add softeners to the test medium, which allows a leak rate of 10-4 mbar l/s to be detected under optimum conditions. But here, too, the human factor is the greatest source of error.

What influence does the component geometry have?

Not every component is the same, often the geometries differ greatly from each other, let's compare a heat exchanger and a wiper water tank.

If the leak is on the upper side of the test part, the bubbles can rise unhindered to the surface of the water bath.

However, if the leak is on the bottom of the component, the air escaping from the leak is prevented from rising to the top by the test part. A considerable amount of air must accumulate before the accumulated bubble reaches a sufficient size to move to the side and then rise up the side of the test part.

The speed of bubble formation is greatly increased in this way and can easily be extended by a factor of 100, i.e. the time between two bubbles becomes 100 times longer. In the above example, a 10-3 mbar l/s leak (without plasticizer) will now bubble only about every 100 seconds.

This applies not only to the bottom of the test specimen, but also to any downward facing surface of the component under test. A perfect example of such a geometry is a heat exchanger that is pressed under water, with the fins running parallel to the water surface. The air escaping from the leak first collects in a large bubble that is stuck between two fins; this must reach a critical size before it escapes to the side and then rises up the side of the evaporator. Again, the time between bubbles is significantly increased, by a factor of 100 to 1000.

Not all leaks are the same

The above limits do not apply to every leak, however, because each leak is different from the other.

In the case of a capillary-like hole geometry, the above limit values can be used as a basis, but this leak geometry rarely occurs compared to a porous spot.

In the case of a porous spot, other limit values apply again. Such a spot is composed of several needle leaks, which in total can result in a leakage rate of 10-3 mbar l/s, but which are not noticeable in bubble form due to the surface tension of the water.

Therefore, if a limiting leakage rate of 10-3 mbar l/s has been defined, a component with a larger leakage rate will be considered to be within the leakage rate and will therefore not meet the requirements!

Summary

The leak test in a water bath seems to be a cost-effective solution at first sight, but it is not suitable for industrial use and ensures that components are put on the market that do not meet the specifications.

If you are still using a water bath for leak testing, now is the time to reconsider.

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