Tricky Leaks: The disappearing leaks

Different types of leaks behave differently. This can make leak detection tricky, unless you understand the reasons for what you experience. The geometry of the leak, the pressure, material, humidity, temperature and flow are all factors that influence the behavior of a leak.

Type #1: The weather dependent leak

Small leaks, typically gas leaks in the refrigeration industry, can in some climates seem like they come and go. You might have experienced that the process tends to miss some smaller leaks when weather is cold and humid. This can be due to condensed moisture forming droplets that find their way into leaks and with the help of capillary forces, form an efficient “cork” in the leak. It may sometimes not disappear even if you apply gas pressure on the inside of your test object due to these capillary forces. It is especially difficult if there is a drop sitting on the inside of the object that results in a volume of water to press out to clear the leak. Another reason for this behavior can be that the parts have been treated with some chemical or electrochemical process without being properly dried afterwards.

To remove the liquid “cork” you may have to wait for a sunny day with dryer circumstances or heat your test object in order to make it dry. Another possibility is to pump vacuum down to below 5 mbar to boil off the water. This will, however, add time to each test cycle.

Type #2: The disappearing refrigerant leak

A refrigerant system contains a refrigerant gas that is not supposed to leak out. One day you are tasked with locating a leak on a refrigerant system that has lost all refrigerant gas during use. You test with a tracer gas (helium or diluted hydrogen) and are unable to find the leak. You then re-fill the system with refrigerant and use a refrigerant leak detector to check the integrity. To your surprise you can now locate a leak on the system, which could not be located previously by using either helium or diluted hydrogen. The possible reason for this situation is that compressor oil has migrated through the system and is now effectively filling the leak.

Why can it then be located by the refrigerant detector and not with helium or hydrogen? The difference lies in the interaction between the oil and the different gases. The oil has been specifically selected to be miscible with the refrigerant gas. In practice this means that the refrigerant can diffuse through the oil.

Helium and hydrogen are not readily dissolved in the oil and therefore, cannot pass the oil “cork” in the leak. Remember that tracer gas leak testing in a production situation should be made on clean products.

Do you have a case you wish to discuss with our leak detection experts? Have you also experienced some tricky leaks? Contact INFICON and tell us!