Applus+ can perform the various methods of leak testing for non-destructive testing (NDT) on new components, according to the specified codes and procedure, or work with a customer to assist in locating leaks within their operating systems and existing assets.

There are numerous methods of leak testing in NDT, with the most common being:
  • Direct-pressure bubble leak testing
  • Vacuum-box bubble leak testing
  • Halogen diode detector probe testing
  • Pressure-change testing
  • Helium mass spectrometer detector-probe, tracer-probe and hood testing
  • Thermal conductivity detector probe testing
  • Ultrasonic leak detector testing
These non-destructive leak test are used to either determine the location of leaks or to determine an actual leak rate.
Leak testing can be performed on systems in almost all industries, including liquid or gas-piping systems, heat exchangers, pressure vessels, tanks or numerous other system and plant components.
The advantages of leak testing for NDT are twofold:
  • First, it is prudent to perform a leak test on a component or system prior to service. For example, a welding leak test can deploy a helium leak test on a heat exchanger to verify the leakage rate across the tube-to-tube sheet welds, as well as determine if there is leakage in a tube itself. Another example is performing a vacuum box leak test on a tank floor. 
  • Secondly, if a leak is suspected in a system, performing one of the leak test methods, such as a vacuum box leak test on a tank floor, can help determine the location of the leak for repairs.

Leaks from installations and systems can adversely impact the environment, system performance and/or a company’s revenues due to the loss of product and significant downtime.


Bubble leak testing is used to find leaks in many different components. The two most common forms of bubble leak testing are the direct-pressure technique and the vacuum-box technique. The direct-pressure technique is conducted by pressurising a component with a gas and then either submerging it in a solution or applying a solution to the outside of the component. If a leak is present, bubbles will form on the surface because of the leaking gas passing through the solution. The vacuum-box technique is conducted on parts that cannot be directly pressurised or where access is not available to both sides of a component. The test is conducted by applying a solution to an area of a pressure-boundary surface and creating a differential pressure across the area, causing the formation of bubbles as leakage gas, such as atmospheric air, passes through the solution.

The Halogen diode detector probe test is a method to conduct a leak inspection by using a tracer gas and a detector probe to detect the presence of halogen. The detection of halogen across a pressure boundary would indicate the presence of a leak.

Pressure change testing is conducted to determine the allowable leakage rate across the boundaries of a closed component or system at a specific pressure or vacuum. By monitoring the change in pressure over a period of time, the leakage rate can be determined, either by the loss of pressure in a pressurised system or through the increase in pressure in a system under vacuum. The change in pressure can then be compared to a maximum allowable change in either pressure per unit of time, percentage volume, or mass change per unit of time.


Helium leak mass spectrometer test is a way to detect very small leaks across a pressure boundary. 

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