General
What is a leak test method?
In metal and plastics processing industry you will find very often the requirement for a quality control with regards to the tightness of parts produced. Diverse leak test methods and leak test instruments are used.
Hollow parts of any kind (valves, pipes, cylinder heads etc.) which have to guarantee a media separation (air, water, oil etc.) have to be checked for a potentially existing leakage.
With regards to the various applications a number of leak testing methods have been developed over the years. It ranges from a dip tank filled with water to mass spectrometry to detect leakage rates in the order of 10-6 cm³/min of gas. The decision which method to use depends on the design of the part under test, the acceptable leakage, the required accuracy as well as the rapidness and the economic viability of the test device.
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Fundamentals
Fundamentals of leak test methods
For the automated leak test of serial production parts in the automotive industry the testing with compressed air has prevailed due the high cost-effectivness. In particular the differential pressure method has proven its worth. These leak test methods fulfill the most important requirements to testing device in productiong:
- Accuracy and reliability
- Objective quality evidence
- Economic efficiency
- Short testing cycles
- Careful treatment of the parts under test
- easy to integrate into concatenated systems
Certain operating conditions (short cycle time, big volumes, high testing pressure, temperature variations of parts under test or the testing air) may cause difficulties when using differential pressure test instrument. If for example the temperature of the testing air inside the part under test changes during the measuring phase the pressure changes proportionally. The test result therefore doesn’t represent the real leakage situation. Untight parts may remain undetected and cause customer complaints. On the other hand flawless parts would be a matter of cost-intensive and time consuming post treatment.
Tightness
What does tightness mean?
The term tightness is a relative measure. It depends on the requirements coming from part under test and its usage. As absolute tightness does not exist (e.g. diffusion through material) a limit according to the part under test needs to be taken as an assessment criteria.
Determinating such a limit very often causes difficulties because there is no universal calculation formula but just empirical values coming from similar applications.
This limit - also called limit leakage – is specified in different units according to the test method, usage or industry. In vacuum technology nowadays 1 mbar*L/s us used.
1 mbar*L/s represents the quantity of gas which cause a pressure change of 1 mbar in a volume of 1 L within 1 second.
Leak testing with gases (air) is based on the low viscosity of the test medium. Media with higher viscosity cannot (or only in marginal amounts) pass mirco porosities of the part under test.
The following table shows the common units for pressure and leakage and their conversion factors.
Conversion
Conversion table for pressure units
| Unit | Abbreviation | Conversion factor | |||
| Pa | bar | mbar | mmWC* | ||
| Pascal | Pa | 1 | 0.00001 | 0.01 | 0.101972 |
| Bar | bar | 100,000 | 1 | 1,000 | 10,197.16 |
| Millibar | mbar | 100 | 0.001 | 1 | 10.1972 |
| Millimeter Water Column | mmWC | 9.80665 | 0.0000980665 | 0.0980665 | 1 |
| Physical. Atmosphere | atm | 101,325 | 1.01325 | 1,013.25 | 10132.5 |
*not permitted by SI system
The methods of leak testing can be separated in two groups:
- Pressure test
- Vacuum test
The distinctive feature is the pressure inside the part under test compared to the atmospheric pressure around.
Methods
The different leak test methods
1.) Leak test using pressure
Bubble test
The bubble test is the simplest and oldest method for leak tests. The test equipment consists of a plunge basin and a pressure regulator.
Absolute pressure method
This method of testing is performed in three phases: filling, stabilizing, measuring. This technique can be used as pressure decay or pressure increase method.
Differential pressure method
The differential pressure method is quite similar to the absolute pressure method. The main difference is the measuring sensor. It is formed by two chambers separated by a membrane. Movements of the membrane and hence the pressure between the two chambers difference can be measured.
Pneumatic flow systems
For the leak testing following the flow measurement method the part under test is tightened by a fixture and charged with a test pressure. The pressure supply will be closed by a valve. A flow sensor measures an air flow in case air escapes via a leak.
Sniffing method
Instead of air a specific tracer gas (e.g.Helium) or a mixture of air and a tracer gas used to fill the part under test. With a sniffer probe and particular measuring methods the tracer gas can be detected when it leaves the part through a leak.
2.) Leak test using vacuum
Pressure increase method
With this method the part under test is evacuated using a vacuum pump or a vacuum generator. The part is then separated from the pump. Leaks lead to a pressure increase inside the part.
Vacuum leak detection
Vacuum leak detection is distinguished between local and integral detection. For both methods sniffer technology with gas specific detectors are used.
Together we will find the best solution for your leak testing task!
You can rely on the extensive experience gained in many successful leak testing applications in various industries. In addition to the automotive industry, medical technology and electronics manufacturing trust our exact leak testing equipment. Together with you we will find the solution! The versatile device program from HeMaTech offers many possibilities for your specific demands. For special requirements, we manufacture test equipment in various customised designs.
Contact
HeMaTech Prüftechnik GmbH & Co. KG
Siemensstr. 7
D-71409 Schwaikheim
Measurand Pressure
Tel.: +49 (0) 71 95-13 69 0
Fax: +49 (0) 71 95-13 69 29
eMail: verkauf@hematech.de
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