The Bubble Test
The bubble test seems to be the oldest leak detection method at all. A container is filled with pressurized air and dipped under water. From a leak there will be some bubble emission, an easy method for leak location. To measure the leak rate is not impossible but needs collection of the bubbles, measure their volume and divide this volume by the measuring time. As easy it sounds, the bubble test needs some precautions when it comes to higher filling pressures. A container with gas at high pressure can burst or explode and injuries are possible. So a safety wall between test and operator and inspection via a mirror may be necessary.
Some containers to be tested are too large to be dipped under water. Here an alternative bubble test is used. The critical areas of the container (for example the welds) are moistened with a foam developing liquid. The use of liquids manufactured for just this purpose is recommended. They are delivered in spray cans and they develop a very fine and white bubble mushroom. A large leak may blow away any bubbles and therefore the test may be carried out with first a very low overpressure and only second with the specified pressure.
A variation of the leak test with a foam developing liquid is the leak search on a test object which is not a container and therefore does not have an internal volume (for example the bottom of a large refinery tank) where the welds must be tested for leak tightness. Here the welds are wetted generously with the liquid. Then a small bell jar with a strong window is placed on top of the wet weld. With a small pump the bell jar becomes evacuated. Some rubber gaskets keep the bell jar sealed to the bottom. Inspection of the weld through the window (having proper lighting) will show eventually developing bubbles.
One must be aware that these tests, as all visual leak detection methods, have a weak point. That is the human eye. It is not an exact measurement device and is due to make errors. Even an experienced operator, working with high concentration can miss a bubble.
The sensitivity limit of the bubble test is difficult to determine. Looking at a small area say 10 cm2 with a magnifying glass for the time of a quarter of an hour a bubble developing from a leak of 10-7mbar l/s can be detected. But if an operator has to inspect the welds of 500 tube ends of a heat exchanger, one can expect to see only bubbles coming form a leak of 10-4mbar /ls. Also the sensitivity changes with changing pressures.
On the seminar about tightness measurements and leak testing methods ( see under products) these methods are described in more details.