Differential Pressure Decay Leak Testing

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Applying pressurised air or gas to a product is one of the most widely-used, accurate and convenient leak test methods, finding use across the manufacturing sector to ensure product quality and safety.

In its most simple form, pressure decay testing involves the use of a single sensor to monitor the pressure in the test item and detect any drop over time (absolute pressure decay).  A significant decrease in pressure indicates a leak, and therefore a fail.

However, the main drawback of the absolute method is that the sensor must be able to monitor the pressure as it increases during the initialisation phase, as well as be sensitive enough to detect small variations in the pressure during the test phase. The test pressure may be in the range of 8 bar, and a sensitivity of 0.001 mbar is required to detect a small leak. Finding a sensor with these characteristics is not possible.

A second drawback of the absolute pressure decay method is the effect of the Adiabatic Process; when the item is brought up to test pressure there will be an increase in temperature. Subsequent heat dissipation may lead to a pressure decrease during the test phase, which the instrument will incorrectly perceive as a leak.

The Differential Method

The use of differential pressure measurement to detect pressure decay in test items is one of the most sensitive methods of leak detection, and the one that is most commonly applied using the Furness Controls range of production line leak testers.  

The differential method uses two pressure sensors; the first an absolute sensor with a span to encompass the range between atmospheric and the required test pressure, and the second a highly sensitive differential pressure transducer (DPT) to detect leaks during the test phase.

The other feature of this approach is the pressurisation of a reference part or volume alongside the test piece, with the differential pressure transducer used to detect the relative change in pressure between the two items. Since the measurement is of the difference between a leak-tight volume and a leaking part, the pressure sensor&#;s measurement range is much reduced, and can therefore have much higher sensitivity to detect very small leaks.

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The second advantage of this approach is that the effect of the Adiabatic Process is cancelled out to some extent. Any warming and cooling effects occur equally in the test and references, and so cannot cause a difference in pressure.

Standard Phases of a Test

A standard differential pressure decay test has 3 main phases:

1. Fill.  The fill valve is open and air is introduced to the part (and reference) at the required pressure.
2. Stabilisation. The fill valve closes, isolating the test item and reference volume from the pressure source. Items which have large (gross) leaks will drop in pressure during this phase, and the test will not proceed. In addition, physical deformation of the part while under pressure may continue after the fill phase, and it is beneficial to ensure that this process has finished before the test phase begins.
3. Test phase. The test valve is closed and the differential pressure transducer monitors the relative pressure between the test item and reference volume.    

Choosing a Reference Volume

When testing parts using one of our Leak Detectors, there are three possibilities to consider for the reference side of the test:

1. The Leak Detector&#;s internal volume. For most parts it is sufficient to blank-off the reference port and conduct the differential leak test against the internal volume of the leak detector.
2. An external reference volume. A more accurate result is obtained if the volume on the reference side of the differential pressure transducer is not too different from the test side, so for larger test items an external reference volume may be fitted.
3. A known good item. In situations where creep and adiabatic effects take a long time to stabilise, a quicker and potentially more accurate result may be obtained by connecting another test item to the reference port. This item has to be verified as non-leaking by some other method.

Our Expertise

Furness Controls are experts in the manufacture of differential pressure transducers in the low-pressure range, with unparalleled sensitivity. As a result, our leak detectors can measure a pressure drop in a test item of only 0.1 Pa. This can translate into a leak rate of only 0.01 ml/min for real-world test items.

Furness Controls leak testers offer a wide range of pressure and leak ranges, are highly customisable, and can combine differential pressure decay tests with other test methods such as blockage tests, sealed-part tests, bell tests and ramp tests in complicated sequences. The wide range of available I/O and pneumatic outputs means that these leak detectors can provide a whole test station for your product, or integrate with a PLC-driven set-up.

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