Testing systems

Quality through precision

Leak test

What is it?

Leak testing and leak testing systems for series production

Various sensors are used for leak testing in series production. An extremely important point is that the detection limit of the sensor is not infrequently mentioned in leak testing methods and not the detection limit or detectable leak rate of a leak testing system or device. The determination of the leakage rate of a leak detection system always depends on the particular application and test object. In the following you will first find an overview of the most frequently used sensors and their detection limit. Subsequently, the individual procedures are considered with regard to their use in a leak testing system. There are a number of leak test procedures. Which one is right always depends on what type of use case it is. In the following, we will show you which different processes are available and for which application they are suitable. We will also show you the definition, calculation and conversion of leakage rates. When you have any further questions about the various processes and applications. Write to us or call us directly. Together we will find the optimal solution for your specific application.

Procedure
Helium vacuum
Helium atmosphere
Ultrasonic
Sensor

mass spectrometry

e.g. Wise TechnologyTM

Ultrasonic probe

What is measured / detected?

Current 10-15 A.

2 x 10-10 A

Air bubble < 0,1 mm Ø

Detection limit

10-12  mbar x l/s

10-6 mbar x l/s

10-8 mbar x l/s in 60s

Procedure
Sensor
Helium vacuum

mass spectrometry

Helium atmosphere

e.g. Wise TechnologyTM

Ultrasonic

Ultrasonic probe

Procedure
What is measured / detected?
Helium vacuum

Current 10-15 A

Helium atmosphere

2 x 10-10 A

Ultrasonic

Air bubble < 0,1 mm Ø

Procedure
Detection limit
Helium vacuum

10-12  mbar x l/s

Helium atmosphere

10-6 mbar x l/s

Ultrasonic

10-8 mbar x l/s in 60s

Procedure
Sensor
Hydrogen

Semiconductor sensor

Pressure difference

Pressure sensor

Worker examination

Eye

Procedure
What is measured / detected?
Hydrogen

Electricity

Pressure difference

1 – 0,1 Pa

Worker examination

Air bubble 1 mm Ø

Procedure
Detection limit
Hydrogen

10-6 mbar x l/s

Pressure difference

10-4 mbar x l / s at 1 Pa, 0.1 l volume and 10s

Worker examination

2,6 x 10-5 mbar x l/s in 20s

Procedure
Helium vacuum
Helium atmosphere
Ultrasonic
Sensor

mass spectrometry

e.g. Wise TechnologyTM

Ultrasonic probe

What is measured / detected?

Current 10-15 A

2 x 10-10 A

Air bubble< 0,1 mm Ø

Detection limit

10-12  mbar x l/s

10-6 mbar x l/s

10-8 mbar x l/s in 60s

Procedure
Hydrogen
Pressure difference
Worker examination
Sensor

Semiconductor sensor

Pressure sensor

Eye

What is measured / detected?

Electricity

1 – 0,1 Pa

Air bubble 1 mm Ø

Detection limit

10-6 mbar x l/s

10-4 mbar x l / s at 1 Pa, 0.1 l volume and 10s

2,6 x 10-5 mbar x l/s in 20s

Procedure
Hydrogen
Pressure difference
Worker examination
Sensor

Semiconductor sensor

Pressure sensor

Eye

What is measured / detected?

Electricity

1 – 0,1 Pa

Air bubble 1 mm Ø

Detection limit

10-6 mbar x l/s

10-4 mbar x l/s bei
1 Pa, 0,1 l Volumen
und 10s

2,6 x 10-5 mbar x l/s in 20s

All about the Leak rate

Leak rate according to DIN EN 1330-08

The leak rate is the pV throughput per unit of time of a certain fluid through a leak under defined conditions:

Ql:
P:
V:
T:
Leak rate
Pressure
Zeit
SI unit: Pa · m3 / s
Common unit: mbar · l / s
1 Pa 1 m3 / s = 10 mbar l / s

The defined conditions necessarily include:

pi: Internal pressure (absolute)
pa: External pressure (absolute)
Fluid: e.g .: air, He (100%), water, etc.
T: Temperature

Why this definition?

For incompressible fluids (liquids) there is a fixed relationship between volume and mass (number of particles) over the constant density:

V:
m:
ρ:
Volume
Mass
Density

For compressible fluids (gases), the volume varies greatly with pressure and temperature at a constant number of particles. A general definition of the amount of substance released (number of particles or mass) is therefore better. With the ideal gas equation then applies:

oder

p:
V:
N:
kB:
m:
R:
T:
Pressure
Volume
Number or particles
Boltzmann’s constant
Mass of particles
special gas constant
Temperature
The various methods of leak testing

Helium leak test

There are two different methods of helium leak testing. On the one hand the helium leak test under vacuum and on the other hand the helium leak test in the atmosphere. While the detection of the helium gas flowing out of the leak with a mass spectrometer always requires a high vacuum in the mass spectrometer (p <10-4 mbar), helium sensors are now also available that no longer require a vacuum. However, much smaller amounts of helium can be detected with the mass spectrometer than is the case with other sensors.

Helium leak test under vacuum

A mass spectrometer is required as a sensor for this method. The helium atoms are ionized with the help of an electron beam. The now electrically charged particles are directed onto a circular path in a magnetic field. The radius of the circular path depends on the mass of the particles. Only particles of a certain mass get through the slit in the diaphragm. These then generate an electrical current at the detector.

Measuring principle – test system / device
The vacuum test chamber and test part are evacuated. Then the test piece is exposed to helium or a He mixture. The previously existing vacuum in the test part ensures that the test gas is evenly distributed. After a certain vacuum has been reached in the test chamber, the mass spectrometer is switched on and the measurement is carried out. With a sniffer probe, the leakage can then possibly be localized in the atmosphere. Since only larger leaks can be detected with the sniffer probe, it may well happen that no leak is found. An alternative to locating the leak is to check it in a water bath.

Helium leak test under atmosphere

Measuring principle – WiseTM sensor
With the help of a maintenance-free helium sensor (e.g. T-GuardTM), an increasing helium concentration can be measured in a test chamber even at atmospheric pressure. The actual sensor consists of a quartz membrane that is only permeable to helium and a permanently evacuated glass tube (see tube television), as well as an anode ring and a cathode plate. At a concentration of 5ppm helium, a current of 2-10-10A flows.

Measuring principle – test system / device
With the help of fans or a circulation, in the event of a leak (helium flows into the chamber), the most homogeneous possible distribution in the chamber is created. This is necessary because otherwise leaks at different points would result in very different readings.

Reliable localization and detection of leaks

With the ultrasonic leak test, leaks can not only be detected, but also localized using the optional ultrasonic bubble detection system. This method is more reliable than the worker water bath test (the so-called underwater visual test), since even the smallest air bubbles can be detected and localized by the ultrasonic bubble detection system, which are barely or not at all visible to the human eye are.

Measuring principle – sensor
A differential pressure sensor is used as the sensor. Such sensors are now able to resolve pressure differences of up to 1 Pa, sometimes even up to 0.1 Pa. The differential pressure sensor is attached between the test part and a reference volume to which the test pressure is also applied.

Measuring principle – test system / device
Test pressure is applied to the test part and reference volume. After a calming phase, the pressure difference between the test part and the reference volume resulting from a leak is measured. Knowing the volumes in the test room (test part and lines) enables the change in pressure to be converted into a leak rate. If the requirements for a differential pressure test are well met, this test method is usually the most cost-effective and fastest leak test method.

Test volume
Since it is not the amount of gas emerging from the leak that is detected, but the indirect change in pressure, it must be taken into account that one and the same leak causes a large pressure change with a small volume, but almost no pressure change with a large volume. DPLT are therefore more suitable for smaller volumes or for high permissible leakage rates. Change in temperature or volume during the measurement time.
Unfortunately, a change in pressure cannot only be caused by a leak. A change in temperature or volume during the measurement period also causes a change in pressure. If a differential pressure test system is to function reliably, the pressure change due to the required limit leakage rate must be significantly higher than the pressure change due to the influences mentioned. Just inserting a test part by a worker can heat the test part. The result can be greatly falsified by the subsequent cooling.

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