Testing Sealed Parts

Furness Controls provide testing for products with sealed parts, using our world-leading pressure decay leak testing instruments.

For non-sealed parts, the standard method is a differential pressure decay test, with the pressure applied through some sort of connector or port into the internal space of the item (for example, the inlet port of a pneumatic cylinder. For sealed-parts, the test carried out is known as a ‘Dump Test’, or ‘Dosing Procedure’ and can meet the key requirements of speed, accuracy and consistency for leak testing across a range of industries.

Dump Tests are used for a wide array of products that can’t be pressure-decay tested due to their sealed nature. Examples of sealed products include waterproof electronics (field radios, car key fobs, underwater cameras) and medical items such as pill packets and vials. A specialised test is required for these items because an external pressure source can’t be connected to them without compromising the item itself.

The Basic Method

The apparatus needed to carry out a sealed-part test consists of:

  • The test item
  • The leak detector
  • A charge volume
  • A leak-tight test chamber that full encloses the test item

Simplified pneumatic diagram for a sealed part test: The test item is enclosed in a chamber and a charge volume is used to dose the chamber with pressurised air. The pressure sensor will detect gross leaks, and the differential pressure transducer (DPT) will detect fine leaks.

Detecting Gross Leaks

The first stage of testing detects large (gross) leaks in the part, and comprises two steps:

Step 1: The reference volume must be brought to a specific pressure that is significantly different to the pressure inside the test item. At the beginning of the test, the air in the leak-tight test chamber is at atmospheric pressure.

Step 2: The dump valve is opened, starting the stabilisation phase. During this stabilisation, the pressure in the reference volume and the pressure in the test chamber converge to an average value. This value is dependant on the pressure difference between the two, as well as the difference in free volume between the two sides.

The volume of the system after the dump valve is opened in step 2 of the process will be distinctly greater if the product has a large leak. This is how we detect a Gross Leak and effectively means the product is open to the atmosphere. In terms of the pressure-detecting test equipment, it is expected that the pressure in the chamber will sit within a certain band of tolerance during stabilisation. If the pressure is outside of this tolerance, the test item has grossly failed the test and is not suitably sealed.


Diagram: (a) Charged volume pressurised, (b) Gross leak – low pressure reached, detected by the absolute pressure sensor

If the item does not have a gross leak, the stabilisation pressure will be higher, within the acceptable pass limits.

Diagram: (a) Charged volume pressurised, (b) Intact item – correct stabilisation pressure reached.

Detecting Fine Leaks

In order to continue to the next stage the product must have passed the gross leak test, and hasn’t allowed a significant ingress of pressurised air. There is now a pressure differential between the outside and the inside of the test item.

The second stage of testing is a standard pressure-decay leak test, and uses a high-accuracy differential pressure transducer to give a precise measurement of any further decrease in pressure in the test chamber.

Diagram: (a) Pressures at the end of the stabilisation phase. (b) Item has a fine leak, which is detected by the DPT.

Question: Why do we not simply apply a pressure to the chamber and then detect a decay over time?

Answer: Without the first stage of the test it wouldn’t be possible to distinguish between a completely intact item (a pass) and one with a very large (gross) leak. In both situations, if we had simply applied pressure to the chamber first, the pressure around the item would quickly stabilise to the test pressure and remain at this point throughout the test.

Practical Considerations

Expected Stabilisation Pressures

For the first stage of the Dump Test to be accurate, it is important to know the expected pressure of the system following stabilisation for a ‘good’ item. This could theoretically be calculated from: the pressure of the reference volume, the volume of the system on the reference side of the equalisation valve, and the volume of the system on the test side of the equalisation valve, minus the volume of a ‘good’ test item. In this case the volume of pipe/tubing on each side of the valve must also be included.

In practice, accurately calculating all of these values for a given test system is very difficult, and the expected stabilisation pressure is more easily determined through the use of a reference test item that is known to be good.

Similarly, the expected stabilisation pressure for a leaking item can be checked, and suitable pass/fail thresholds determined.

Hint: To ensure a reference volume is really ‘good’, an often-employed method is to fill it with some kind of resin. This guarantees that air can’t leak in.

Choosing a Test Pressure

Different products require different test pressures. Having access to Leak Detectors with diverse pressure ranges ensures that a high level of accuracy is available when testing sealed items.

Often, the chosen test pressure is a vacuum, which has the advantage of reducing leaks in the test chamber set-up by causing an inwards force. In addition, positive pressures around test items can have the effect of reducing leaks by tightening seals, which may mask leaks that would occur during real conditions. A vacuum around the test item may instead help to amplify problems with seals and increase the likelihood of detecting a leak.

Testing with Furness Controls

We can provide testing for a wide range of sealed items using our high specification devices.

Our leak detectors are built with world-class accuracy and reliability and meet the global gold standard for low pressure detection. The instruments operate at multiple pressure (vacuum to 30bar) and leak range options and form an invaluable aspect of any testing scenario.

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