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DIFFERENTIAL PRESSURE DECAY METHOD

The decay pressure method is a quite widespread technique which allows to obtain a more accurate pressure resolution compared with the absolute decay method and a considerable reduction of test time.

The test cycle is divided into three phases: 
- FILLING (time T1): 
the product being tested is subject to the programmed rated test pressure.
- SETTLING (time T2):
the equipment waits for the pressure to settle in the product being tested.
- LEAK MEASUREMENT (time T3): 

is measured by the difference in pressure between the test piece and the reference sample: a drop of pressure inside the test piece, caused by a loss, causes an increase of the pressure difference measured.




The method is less affected by the environmental temperature variations. 

The system accuracy is about 0.1 cm3/min.

  • The test is performed by comparison between a reference master piece and an object to be tested.

  • Both objects are filled up at the same test pressure, then the equipment waits for the settling down of the pressures and the mechanical parts.The measurement of the leak is referred to the pressure difference between the two parts.

  • It is preferrable to use as master piece an object of the same volume and with the same features of the tested piece; moreover, it is better to use an identical piece, obviously good. In this case, the settling of the two objects will be very similar.

  • By measuring the leak for comparison between the two parts and behaving these two in a very similar way, it will be possible to obtain a significant settling time reduction.

  • Moreover, the use of a differential pressure sensor allows to increase the resolution of the pressure measures between the two pieces. It's like applying a magnifying lens on the leak in order to see the differences in a clearer way. The result is that, if a piece in test has a leak, it will be possible to measure it better and in less time.

Disadvantages

  • the equipment are generally more complex and expensive compared to those for relative pressure decay;

  • the test effectiveness is directly proportional to the measurement time and inversely proportional to the volume of the tested piece;

  • the test duration is in function of the piece volume;

 

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