Leak Testing

What is leak testing necessary for?

Leak testing is a non-destructive test method, which is used to verify the tightness of specimens or the location of leaks. Tightness tests play an important role, especially in production, because a wide variety of products and systems require a certain level of tightness. Not only installed systems, such as pipelines such as gas and sewer pipes or vacuum and cooling systems, must be leakproof, but also components and systems in series production. Electronic assemblies, pump housings, fuel tanks or engine components are just a small selection of affected products. Therefore, the leak test, also known as leak test  , must be used in a wide variety of industries and areas. It is a particularly important test method for quality control, especially in the medical technology, plastics and automotive industries.

Due to the different applications, various leak testing methods have become established. Depending on the process, different leak testers are available, which can be integrated directly into the production process. In this way, defective parts can be detected early on, and consequential errors and unnecessary costs can be avoided.

Leak testing in serial production

Increasing automation and humanization of the workplace, with simultaneous increase in quality, are demands that must also be realized in quality assurance. While workpiece dimensions are fully automatically monitored during machining on turning, drilling and transfer machines and tools are corrected accordingly, in the leak testing of series components and assemblies often methods are still in use, which are far from the above requirements. Although a number of reliable test methods have been developed, integration into the production line, fast and accurate operation and clear test results are not given in many leak testing methods.

The most common test methods are:

• Immersion test
• Pressure measurement
• Flow Measurement
• Vacuum Fluorescent test
• Ionization test
• Halogen sniffer test
• Mass spectrometer test
• Radioactivity test

In the pneumatic leak test method, the workpiece to be tested is subjected to a pressure difference to determine if pressure escapes from the workpiece. Most of these methods can be used in both overpressure and negative pressure tests.

In the meantime, a wide variety of leak testers with different test methods exist for series production, which can be individually integrated into the production process of series parts. In order to ensure a consistent quality, the devices must be checked for their functionality at regular intervals only by means of a test leak or a leak calibrator.

The question of leak testing: tight or leaking?

The term tightness is not a defined statement. Absolutely tight parts do not exist. The requirement for the tightness of a component depends on the later operating conditions of the component.The leakage test procedures shall be determined according to this requirement with regard to the level of pressurisation and permissible leakage. The definition of such a limit usually causes difficulties, since there is no universal calculation formula, but only empirical values from similar applications.

Permitted leakage? The workpieces must be tight!

There are e.g. components on combustion engines that must be water or oil-tight, others must not have leaks for fuel or gas.Whether a workpiece has the required tightness must be checked as early as possible in the course of the manufacturing process. This will ensure that it will also be tight later in use under the specific conditions. Further processing or even installation in a final product causes costs that can be avoided with early control.

In pneumatic tightness tests, either air or a special test gas is used as the test medium. This is true even if a workpiece must be liquid-tight. The test medium air has a significantly lower viscosity compared to liquids and thus allows a considerably higher test accuracy. There are leakage points through which practically no liquid can escape - but the gaseous test medium. This allows a measurement.The essential prerequisite for a functioning leak test is the definition of a permissible leak.

Absolutely tight - impossible downright

Absolutely tight components do not exist. Therefore, an absolute tightness can not be determined with any test method. This also applies to the underwater method. In general, the requirement is that no bubble may rise. However, strictly speaking, this only means that in the course of a certain period of time no bubble that is just visible is recognizable.

Is a work piece tight or leaking when a 1mm diameter bubble rises every 3 seconds? Setting this means naming a maximum allowable leakage below which a work piece may still be considered "leak tight". This means that this leakage is still tolerable in later use without endangering the functional safety. The leak test should now enable the detection of a small leakage within a very short time.

Test specification of the leak test

For the specification of a leak test two parameters are needed:

• max. permissible leakagre
• test pressure

The size of the permissible leakage depends on many factors. Namely there are:

• Workpiece material
• Design constraints
• Intended conditions of use

The trick is now to set the rate of leakage with air that does not leak on a work piece under operating conditions.For the ratio of air and liquid leakage, the ratio of the viscosities is crucial. However, factors such as wall thickness, porosity or surface finish also play their part.The indication of a leakage value must always be stated with a corresponding test pressure. For the leak test, the test pressure is usually designed so that the conditions are traced during later use. This applies to the amount of pressure as well as its effective direction.

As the test pressure increases, so does the leakage. This increase is approximately proportional to larger leaks. Small leaks often do not behave as consistently as described. They may also change under the impact of the exam.

Finally, the profitability of the test is decisive. The leak test only needs to be as accurate as necessary. If the leakage rate is set too low, there is a risk that an increased amount of rejects will be "tested" and thus increase the manufacturing costs. Carrying out leak detection tests is a tried and tested means. Often there are empirical values for similar components.

For leakage testing with air (pressure or flow measurement), the leakage is usually expressed in Ncm³ / min. Another commonly found size is mbar * L / sec. This claim is often used when using a special test gas (e.g., helium).The conversion of the two sizes is easily possible:

60 Norm cm³/min = 1 mbar*L/sec

1 sccm is the amount of gas that fills the space of 1 cm³ in the physical standard state (0 ° C and 1013 mbar). 1 standard ccm is not a room measure but a gas quantity, expressed in room dimensions. The specification sccm/min is thus a mass flow and no volume flow.

Defining the leak rate in sccm/min will result in the leakage test being either measured in the mass flow method or the pressure measurement signal corrected for the influence of the barometric pressure and the ambient temperature.If this is not done, the leakage in ccm/min or ml/min must be specified, which is also common in practice. The difference between standard condition and test condition is on average about 10%.

The methods of leak testing

Immersion method

The immersion test (visual inspection under water) is still a frequently used test method, which also provides a very accurate result with careful execution by the examiner and good observation of the workpiece.Other advantages including the relatively low cost and the ability to locate the leak.

It is the simplest and oldest method of leak testing, which is not only applied industrially. By the same method, for example, holes in bicycle tubes are determined, thereby reaching a simple bucket with water.The commercial testing device consists of a dip tank, a device for sealing the workpieces and a pressure regulator. Disadvantages of this method, however, are the lack of automation possibility, the dependency on the operating personnel (low inspection reliability) and the additional effort for rust protection, as well as the drying of the workpieces. In addition, it takes a long time, which is why automatic leak testing systems are frequently used today.

Leak test by pressure measurement

During pressure measurement, the amount of air escaping from the test specimen is not measured directly. The effect of the leakage quantity on the test pressure confined in the test specimen is measured.

The drop or increase in pressure depends on the size of the volume to be tested.

Relative pressure / absolute pressure method

The relative pressure and the absolute pressure method are a very commonly used method in leak testing.

In the relative pressure method, the difference is measured against the ambient pressure. Pressures above ambient pressure are therefore considered to be overpressure testing and at pressures below negative pressure testing.

In the absolute pressure method, the pressure is determined against the absolute vacuum and is therefore always positive.

For the leak test, the test piece is impinged with pressure or vacuum. The change in pressure resulting from the leaks is then measured and analyzed.The test setup for this leak test is relatively simple and the possibility of automation is given. The disadvantage of this method is the dependence on the test pressure at the measured value resolution and the strong temperature influence on the test result.

Differential pressure method

This test method can be easily automated and used in leak testing systems, making it particularly suitable for use in series production.In this process, the test specimen is filled with a certain pressure and compared with the pressure of a reference volume whose tightness is established.The measured value resolution is independent of the test pressure. The measuring accuracy is determined by the measuring range of the differential pressure transducer. Temperature influences act on both sides of the measuring system, the influence is thus less than in the aforementioned method.

Leak test by flow measurement

In the flow measurement, the amount of air flowing through the measuring element is measured. A distinction is made between the mass flow method and the volume flow method.