How Tight Is Your Welded Joint?

Leak testing a welded joint with tracer gas

For welded components that should be gas or liquid tight, it is important the welded joint be free from leaks. Traditional leak test methods, which usually include soaping, water bath and pressure decay, are very common but they have many drawbacks. The most sensitive method consists of filling a product with a tracer gas and detecting escaping gas with the help of a leak detector. This way you can easily see if there is a leak, and where and how large it is.

Water Bath and Soaping

Water bath and soaping are done by pressurizing the component with air. The product is subsequently submerged in water and air bubbles will appear. Alternatively soap is applied on the product surface and bubbles will emerge on the leak site. If the operator carefully and patiently observes the tested object through the whole test cycle, this may give a good result. Other advantages of these simple methods are that they are cheap and easy to understand.

But they also have some drawbacks. Reliability is highly dependent on the operator's accuracy. On irregular structures, the bubbles can hardly emerge from the surface where they are formed. In case of small leaks, the bubbles may not overcome the surface tension of the water and the object will then not leak under water. When immersing the product in a water bath, bubbles can follow into the water and then rise to the surface which can lead to unnecessary rejection of products. After the test, the product needs to be dried before proceeding to the next step. In worst case, the water can cause corrosion and discoloring. The water and soap can also make the workplace slippery and pose a serious safety risk.

Pressure Decay

A dry method consists of measuring air pressure changes. Pressure decay measurement is based on filling the product with air to a given pressure and monitoring pressure during a certain time interval. An alternative method is measurement in vacuum and works the other way around, which means air is evacuated from the product and pressure increasing over time indicates the presence of a leak. These methods are simple, reliable and both can be automated.

Pressure decay, however, works poorly if the product has a large internal volume, because the flow through a leak becomes small in relation to the large volume. The pressure difference is simply difficult to measure. Another disadvantage is that temperature variations affect pressure. If the temperature rises during the test, the leaks will not be discovered. If the temperature drops, false leak indications will be given. Temperature changes lower than 0.1°C can lead to incorrect measurement results. This means that the product must cool down completely after welding before it can be further tested. If welding is part of a production flow, additional actions and storage space are required. Additionally, pressure decay does not give any indication about the leak position. Inspection and repair can therefore be a time consuming process.

Tracer Gas

Leak detection with tracer gas gives more accurate results in terms of leak size and position. The object is filled with a tracer gas that can easily be detected. The amount of escaping gas is then measured. Tracer gas tests are not temperature or volume dependent and the method can also be used for leak localization.

The most common tracer gases are helium and diluted hydrogen. The hydrogen gas used is a standard, safe mixture of 5% hydrogen in nitrogen.

The welded component is filled with gas immediately after welding. The joint is then tested with help of a leak detector. If the instrument gives a leak indication, the joint can be repaired and tested again directly after repair. Tracer gas also allows for test of the whole product in a measuring chamber. This is done at atmospheric pressure or in a vacuum depending on the leak limit and test time requirements.

INFICON leak detectors give high sensitivity and reliability to meet the professional welding and joining standards. Learn more about our broad range of instruments on our website.

For more detailed information about leak detection on weld joints, contact the closest INFICON office.