The test phases in industrial leak detection: from filling phase to leak measurement

We discuss the principles, challenges and factors involved in choosing test parameters.
During the filling phase, a pressure difference is created between the ‘dense’ area under test and the outside environment. This can be done in two ways:

Positive pressure: This involves applying pressure from the inside or the outside, depending on the testing strategy.

Negative pressure (vacuum): This often helps to improve the tightness of a component, for example in containers or motor oil

The choice between positive pressure and vacuum is closely related to the type of component and specific testing requirements. In some applications, such as plastic soldered parts, a higher pressure can bring out defects better. At the same time, too high a pressure can adversely affect test performance by increasing the complexity of the filling phase and subsequent stabilisation phase.

After filling follows the stabilisation phase. This is a crucial period when the pressure or flow rates stabilise within the system. A stable environment is essential for accurate and reproducible measurement results. During this phase, additional effects may occur, such as temperature changes or mechanical expansion of the object under test. By allowing these factors to dissipate, you prevent the final leakage measurement from being affected by temporary fluctuations in pressure or flow rate.

Once the system is fully stabilised, the actual leakage measurement begins. During this phase, different strategies are employed to quantify the extent of leakage. The test method chosen can vary, depending on product requirements and desired accuracy. Common techniques include pressure decay, which measures the drop in pressure over time. This pressure decay provides a direct indication of the leakage current in the test object.

Test pressure and influence on measurement
The pressure value during the filling phase is of great importance. Typical test pressures range from -1 bar to 10 bar, but in fracture or explosion tests, the pressure can even exceed 40 bar. While higher pressure can increase leakage flow (and thus facilitate leak detection), it can also affect measurement accuracy. A lower pressure (less than 1 bar) can sometimes give more stable results.

Influence of the test object
Each test object has its own specific characteristics. Mechanical parts, such as cast-iron engine housings or transmissions, often have gaskets and seals that play into the test setup. In the case of gas or kitchen appliances, the test pressure is usually lower (around 150 mBar), which in turn better reflects actual operating conditions.

Choice of test gas

Many leak detection tests use filtered, dry and oil-free industrial compressed air.

In some cases, a gas with smaller atomic structures, such as helium, can increase the sensitivity of the test. For explosive or flammable substances, it is often prudent to use an inert gas, such as nitrogen.

Special strategies
For objects that expand under pressure, “pre-filling” at a higher pressure than the final test pressure can help. This allows the material to expand first and stabilise again, which can reduce the overall testing time. In addition, one can opt for a “bell filling” method, in which the pressure is controlled from outside (e.g. via a vacuum chamber) to create the most optimal test conditions.

Conclusionie
A successful leak detection test requires a good understanding of the different phases: from the initial filling (filling phase) to stabilisation (stabilisation phase) and finally the measurement itself (leak measurement). The right combination of test pressure, attention to the material properties of the test object and the choice of test gas all affect the accuracy and reliability of the results. By carefully optimising these factors, manufacturers and engineers ensure that their products meet the highest safety and quality standards.