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It has already been mentioned that in automatic leak testing with air, the escaping air cannot be measured directly, only its effects on the pressure in the test chamber. According to the gas law, however, apart from leaking, temperature and volume variations also have effects on the pressure in the test chamber. To carry out precise leak testing, therefore, it must be ensured that the variation measured is exclusively due to a leak.

1. The Accuracy of Measurement can be impaired by the following effects:

1.1 Temperature Effect
The test air temperature must not change during the measuring stage, or any temperature variations must be precisely repeated in each measuring process. In order to be able to keep to this requirement, all the components involved in testing, namely test piece, test air and sealing device, must have the same temperature, or always the same temperature difference for each measurement. Under normal production conditions this can be achieved only as an approximation. In the meantime, there are repeatedly proven methods of compensation for the temperature effect (JW FROEHLICH Patent No. DE-C-3106981). The need for this depends on the ratio of the test signal variation due to the permitted leakage rate to the test signal variation due to the temperature effect.

1.2 Variation of Volume
Nor must the test volume vary during the measuring stage. An increase - e.g. caused by test pressurisation - simulates a leak, while a decrease - e.g. caused by the contact pressure of the sealing rams - compensates for any leak which may be present. In order to avoid "breathing" of the sealing elements, the rubber seals and the sealing forces should be dimensioned such that in the sealed state, there is metal contact between sealing element and workpiece. Because in leak testing by the pressure measuring method, the size of the test volume enters into determination of the leakage rate, the tolerances of the test volume act as an error of measurement.

1.3 Repeatability of the Leak Tester
The quality of leak testers for production is determined not first and foremost by their absolute precision, but rather by repeatability. The repeatability of a leak tester is composed of the tolerances of various components such as, measurement transducer, electronic measuring unit, valves, hoses and test pressure regulator.

Typical repeatability values of correctly set leak testers:

Absolute pressure or relative pressure leak Testers: 1 to 10 Pa (depending on the test pressure)
Differential Pressure Leak Testers: 1 Pa

1.4 Calibration
The accuracy of leak measurement naturally also depends on the accuracy of the leak simulator with which the measuring system was calibrated. Frequently an adjustable valve fitted in the leak tester with a float-type flowmeter is used for leak simulation. This leak simulating method is of course easy to use, but the accuracy of the flowmeter is only 3% of span, also, the float is sometimes difficult to detect, and the accuracy of reading is low.

For correct calibration of the test system, a master part (sealed workpiece) is inserted in the sealing device and the permissible leak is simulated under serial conditions. This is done with a valve which is connected to a branch of the air inlet pipe to the workpiece. The valve must be calibrated to the permitted leakage rate. There are fixed calibration leaks as well as adjustable precision valves, which can be set to a leakage rate from about 0.1 cm³/min. The air leaving the valve is measured with a calibrated calibrator with a definition of 0.01 standard cm³/min.


2. The Leak Tester cannot differentiate a leak

2.1 Leaking Seals
The measuring element of a leak tester cannot distinguish whether the leak stems from a porous area for the workpiece or from defects of the seals. With rejects continuous one after the other, it can only be assumed that the cause lies in a worn rubber seal, or in a leakage in the system.

2.2 Cumulative Leakage
In automatic Leak Testing, the cumulative leakage of a workpiece with its sealing points is measured (integral measurement). In this case, it may be that several individual leaks each with a leakage rate still below the permissible limit together exceed the maximum value, and the workpiece is thus wrongly classed as a "REJECT". In fixing the permissible leakage rate, however, the worst case of an individual leak must be assumed.

3. Moisture at the surface of the Test Chamber
A leak can be sealed by moisture at the surface. It then cannot be detected at the time of leak testing. In the subsequent operative state, however, it can reappear as a leak. Tests on a grey cast iron cylinder block have shown that air leaks on the dry workpiece of upto 80 cm³/min were no longer detectable in the wet state (cutting fluid). Even a high pressure stage preceding the leak test could not blow this leaking area free. Therefore, dry workpieces are essential and necessary for leak testing.