Pressure drop & flow measuring

Two leak testing methods with air

ZELTWANGER Dichtheitsprüfung is the specialist in all testing methods using the medium of air. It is possible to use our devices and systems to conduct measurements using the flow and pressure change methods.

The basic principle of leak testing or leak measuring with air is based on the two basic methods of flow measuring and pressure drop measuring.

With both methods, the test object is first filled with air up to an adjustable test pressure and then disconnected from the compressed air supply. Now the actual measuring begins.

Flow measuring

Flow measuring provides a measurement signal which corresponds to a possible leakage regardless of the size of the test volume and the measuring time. A conversion as with the pressure drop method is not necessary.

Mass flow method (MF)
Graphic of the mass flow method

The mass flow method works according to the calorimetric principle (thermal method).

The result is available directly in cm³/min and, after being offset against the volume factor (ratio between reference and test volume), already corresponds to the leak rate. Both the test object and the reference volume are pressurized for the test. After a calming phase, the mass flow and the direction of flow between the reference and test volumes are determined.

The test pressure can be relative to the current ambient pressure, i.e., in excess pressure or negative pressure (vacuum test). It is also possible to execute the test in absolute pressure based on the absolute zero point.

The ideal application of the mass flow method is leak testing for large-volume test objects.

Flow method (RD/DF)
Graphic of the flow method

The mass flow in the supply line to the test object is measured using the flow method. Pressure changes in the test object are directly compensated for by a precision pressure regulator when the filling valve is open until a constant air flow is established at the sensor. Flow measuring provides a measurement signal which is output directly in the units of the leak rate regardless of the size of the test volume and the measurement time. A conversion as with the pressure drop method is not necessary.

The test pressure can be relative to the current ambient pressure, i.e., in excess pressure or negative pressure (vacuum test). In addition, it is possible to conduct measurements using the relative pressure method in this setup.

This combination is therefore very well suited for testing tasks in which the tightness of the component needs to be verified using the relative pressure method and a minimal flow needs to be demonstrated. Application examples may be valve tests or flow tests on pressure equalizing membranes or filter units.

The flow method is characterized by very fast test times. However, this method is not suitable for measuring small leaks.

Pressure change measuring

With this method, the leak is identified by a change in pressure. If there is a leak, it causes a pressure change that can be converted into a leak rate using the test volume and the measurement time. This method can also be carried out as a vacuum test.

Relative pressure/absolute pressure (RD/AD)
Graphic of the relative pressure/absolute pressure method

The relative or absolute pressure method is the most common method for measuring pressure changes. In this case, the test object is pressurized and the change in pressure is determined over time. The result can be output as a pressure change or in the leak rate units.

With the relative pressure method, the test pressure is relative to the ambient pressure. This either involves an excess pressure test if the test pressure is greater than the ambient pressure, or a negative pressure test (vacuum test) if the pressure is less than the ambient pressure.

With the absolute pressure method, the test pressure is related to the absolute zero point (absolute vacuum).

The relative or absolute pressure method is characterized by high reliability as well as a wide measuring range. Two channels can be implemented per module thanks to its compact test setup.

The measuring accuracy of this method is sufficient for most tightness requirements. Due to its low volume, the setup is ideal for small-volume test objects.

Differential pressure (DD)
Graphic of the differential pressure method

The differential pressure method is an extension of the relative or absolute pressure method. A reference volume is pressurized in addition to the test object. If the pressure in the test object drops due to a leak, the pressure change between the reference volume and the test object is calculated using a high-precision differential pressure sensor. The relative or absolute pressure sensor is only used for monitoring the test pressure. The result can be output as a pressure change or in the leak rate units.

The test pressure can be relative to the current ambient pressure, i.e., in excess pressure or negative pressure (vacuum test). It is also possible to execute the test in absolute pressure based on the absolute zero point.

By using the differential pressure sensor, the measuring accuracy in the differential pressure method is independent of the test pressure and thus independent of the measuring range of a conventional sensor. The setup is therefore particularly suitable for stringent tightness requirements and for measuring small leaks at high test pressures.

Accumulation pressure method (SD)
Graphic of the accumulation pressure method

The fundamental difference to the other methods is that the accumulation pressure method does not regulate the pressure, but rather the flow.

This method does not verify the tightness, but rather the continuity of a component. For this purpose, a constant flow is established and the excess pressure accumulated at the test object inlet is measured – the accumulation pressure. The smaller the constriction in the component, the greater the resulting accumulation pressure.

A large field of application for this method is the identification of core breakages in foundry technology. Constrictions from 20% of the diameter can be reliably detected. Other areas of application can be found in medical technology or valve technology.

The accumulation pressure method is characterized by tests that take just a few seconds.

Bell testing (RD/GP)
Graphic of the bell testing method

The bell test is an extension of the relative pressure method. In some cases, components do not have openings that allow direct filling. In this case, it is necessary to perform an indirect test using a bell. To do this, a test bell adapted to the contours of the component is created into which the test object can be placed.

The test is divided into two individual measurements which are carried out automatically one after the other using the leak testing device:

The first phase corresponds to a relative pressure method. The required pressure is applied to the test bell. If the test object is leaking, air penetrates into the component. The pressure change is registered by the measuring device and can either be output as a leak rate or as a pressure change.

In the second step, the amount of air in the test bell is checked to rule out that the test object has already been filled by a large leak in the first step. For this purpose, the air quantity is flowed into a volume of a known size.

The overall result of the sequence is displayed in the leak testing device.

The test pressure can be relative to the current ambient pressure, i.e., in excess pressure or negative pressure (vacuum test).

The right devices for testing methods using air

No matter which test method you are considering, we offer a variety of measuring devices for all of them.

Do you want to find out more or need some advice?

We can help with analysis and design

Leak testing with detection gases

For the localization of small leaks

Leak testing dictionary

We explain the most important terms

Contact us
Still have questions?
Melissa Quintus, customer service at ZELTWANGER Dichtheits- und Funktionsprüfung
Lena Rüdiger, sales at Zeltwanger Dichtheits- und Funktionsprüfung