///Leak testing – What is it
Leak testing – What is it2018-04-17T12:27:16+00:00

Leak test system – What is it?

Leak test systems for industrial serial production

For leak testing in industrial serial production different sensors are used. Often the detection limit of the sensor is mixed-up with the detection limit of the leak test system, the so called detectable leak rate. To determine the exact leak rate that can be detected by a leak test system depend on the specific application and test object.  The following table shows an overview about the sensors that are most commonly used and their detection limit as well as the “real” detection limit. In connection with the table we  look at the different methods regarding the use in leak testing. There are a number of methods and techniques for leak testing.  Which of the different methods is the right and correspondent one always depends on the  application and test object. We Show the different methods and for which application it is suitable. Furthermore the definition,  calculation and conversion of leak rates are listed. Do you have more issues or questions regarding the diverse methods and applications. Please send us an email or give us a call.  Together we will find the ideal solution for your specific application.

Operation Helium-Vacuum Helium-Atmosphere Ultrasonic
Sensor mass spectrometer z.B. Wise TechnologyTM ultrasound transducer
Was is measured/detected?  10-15 A (current) 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

Operation Hydrogen Differential pressure Underwater visual testing
Sensor semi-conductor sensor pressure sensor eye
What is measured/detected current 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

Definition leak rate2018-07-18T15:28:17+00:00

Leak rate according to DIN EN 1330-08

Leak rate is the throughput of p·V per unit of time of a certain fluid thru a leak under specified conditions:

leak rate
SI-unit: Pa · m3/s
common unit: mbar · l/s
1 Pa · 1 m3/s = 10 mbar · l/s

Zu den definierten Bedingungen gehören unbedingt:

pi: pressure inside (absolute)
pa: pressure outside (absolute)
fluid: e.g.: air, He (100%), water, etc.
T: temperature

Why this definition?

The relation between volume and mass (number of molecules) for incompressible fluids (liquids) is clearly given by the density:


For compressible fluids (gases) the volume is strongly depending on the pressure and the temperature by constant mass (number of molecules). Therefore a definition using the mass or the number of molecules is required. Using the gas equation this results in:


number of molecules
mass of molecules
gas constant
Calculation leak rate2018-07-18T15:21:32+00:00

Calculation of leak rate

This formula is valid for the throughput of a compressible fluid (gas) thru a leak in a pressurized vessel.
It is:


mass flow: Massenstrom
volume flow: Volumenstrom
leak rate: Leckrate


Α: leak area
Ψ: throughput function
pi: pressure inside
ρi: density inside
pa: pressure outside
ρa: density outside
R: specific gas constant  (air: 287, helium: 2078 [J/(kg*K)])
Τa: temperature outside


For the throughput function Ψ it is:
κ Adiabatic Exponent of test gas (1,4 air, 1,66 helium)

With correction due to friction:

mass flow:

μ: coefficient of friction

Example 1:

Water bath leak testing at 1350mbar (19,6psi) pressure inside and 1043mbar (15,1psi) pressure outside (considering the hydraulic pressure of the water too):

leak area in μm
leak rate in mbar · l/s.

Example 2: Formula counts for throughput of a compressible fluid (gas) through a circle leak.

mass flow: Massenstrom
leak rate Leckrate


m: mass
pi: pressure inside
pa: pressure outside
η: dynamic viscosity
l: length of leak channel
d: diameter leak
kR: Boltzmann-constant
mMolekül: mass of a molecule
T: temperature
R: universal gas constant
M: molar mass

Example 2:

helium(100%) leak rate of a leak of 1 mm with a diameter of 15µm at 300mbar inside pressure and 0mbar outside pressure (vacuum):

Beispiel 2
Beispiel 2

Example 3:

Air leak rate of a 1 mm leak rate with a diameter of 15µm at 1350mbar inside pressure und 1050mbar outside pressure:

Beispiel 3
Beispiel 3

Case 3:

Formula counts for throughput of a compressible fluid (gas) through a circle leak.

leak rate: Formel Leckrate


pi: pressure inside
pa: pressure outside
η: dynamic viscosity
l: length leak
d: diameter leak rate


Conversion of leak rate2018-07-18T15:20:44+00:00

Conversion of leak rate

For the most cases the leakage rates lie in the range of laminar flow (qL > 10-6 mbar · 1/s), then the conversion of the leak rate is the following:


qL: leak rate
pi: pressure inside (absolute)
pa: pressure outside (absolute)
η: dynamic viscosity

Practical tests for the leak rate show that the conversion can be properly applied in the area of gas-gas.

Different operations for leak testing

Helium leak test2018-07-18T15:23:13+00:00

There are two different methods for helium leak tests. On the one hand helium leak test under vacuum and on the other h and helium leak test in atmosphere. While the detection of a leakage with exhausting helium needs to be detected by a mass spectrometer with a high vacuum (p < 10-4mbar), are currently existing helium sensors that do not need a vacuum. However by means of the mass spectrometer even lower amounts of helium can be detected which is not possible for helium sensors.

Helium leak test under vacuum

For this method a mass spectrometer is the necessary sensor.
The Helium Atoms are ionized by an electron beam. The now electric charged particles are directed on a circle path within the magnetic field. Thereby the radius of the circle path depends on the mass of the particles.  Only particles with a specific mass pass the slot of the cover. By the Slot of the cover only particles with a specific mass. These particles generate an electric flow at the detector.

Measurement principle – Leak test system
Vacuum test chamber and test object are evacuated. Then the test object is applied with helium or a helium mixture. By the existing vacuum within the test object the test gas is distributed homogenously. When a specific vacuum is reached the mass spectrometer is connected and the measuring process is started.
By means of a sniffer probe the leak can be eventually localized in atmosphere. As the sniffer probe only can detect bigger leaks, it can happen, that no leak is found. An alternative for leak detection can be the control test in water bath.


Helium leak test in atmosphere

Measurement principle – WiseTM-Sensor
By means of an maintenance free helium sensor (e.g. T-GuardTM) even at atmospheric pressure an increasing helium concentration can be detected in the test chamber.  Atmodruck Der eigentliche The real sensor consists of a quartz membrane that is only permeable for helium and a permanent ecacuated glass tube ( cf. a ray tube television) and a anode ring as well as a cathode board.  During a concentration of 5ppm Helium flows a current of  2-10-10A.




Measurement principle  – leak test system
By means of  ventilators or recirculation in case of a leakage (helium gets into the chamber) a highly homogenous distribution within the chamber is reached. This is indispensable because otherwise leakages at different points would produce different measured values.


Ultrasonic leak test2018-07-18T15:23:55+00:00

Detecting and localizing reliably Reliable detection and localization of leaks

By using ultrasonic leak test leaks can not only be detected, but also be localized via ultrasonic-bubble-detection System. This method is more reliable compared to the bubble test ( the so called under-water visual testing) because even tiny air bubbles, that are merely or not visible for humans,  are detected and localized by the ultrasonic-bubble-detection System.


Differential pressure leak tester2018-07-18T15:22:48+00:00

Differential pressure leak tester

Measurement principle – sensor
For differential pressure leak testing a differential pressure sensor is used. These kind of sensors are now able to detect  pressure differences up to 1 Pa or sometimes up to 0,1 Pa. The differential pressure sensor is attached between the test object and a reference volume applying pressure to it.

Measurement principle – leak test system 
Both test object and reference volume are put under pressure. After the setting phase the differential pressure of the leak between test object and  reference volume is measured. By means of the volume within the test area ( test object and pipes) the pressure difference can be converted to the real leak rate. If the conditions are fulfilled for a differential pressure leak test, this method generally is the most cost-effective and fastest one.

Test volume
As the released gases out of the leak are not detected, but the indirect pressure difference, it has to be considered that the same leak can generate an enormous pressure difference related to a small volume and almost no pressure difference related to bigger columes. Generally speaking the differential leak test system is rather suitable for smaller volumes or higher leak rates.

Amending of temperature and volume during measuring phase
Unfortunately a pressure difference can not only  be generated by a leak. Amending of temperature or volume during the measuring phase can also generate a pressure difference.  For a reliable operation mode of a differential pressure leak tester, it is crucial that the pressure difference has to be higher due to the required leak rate limit than the pressure variation of the mentioned effects.  Even inserting a test object by the worker can heat the test object. Due to the cooling down afterwards the result can be considerably distorted.


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