Absolute leak-tightness does not exist. Nothing is absolutely tight. The necessary tightness of a part or a complete unit depends on the function of the part. E.g. Water Pump should be pressurized water proof or Fuel Tank should be Fuel Tight. In order to define leak tightness, we use the leak rate concept. The leak rate is a quantity of gas(Air) that flows through a leak (volume/time) at a defined pressure difference. Conventionally, leak testing of parts was done by wet bubble testing. In bubble testing, the operator pressurizes the part, submerges it in a water bath, and then watches for a stream of escaping bubbles signaling a leak. This involved Human dependency as well as Air Cleaning of the Part after Test.
In the automatic leak testing of mass produced parts, leak testing with air has become widespread. The Reasons for using this test method are:
In automatic leak tests with air, the work piece to be tested is subjected to a pressure difference and it is established whether air is escaping. The escaping air however, cannot be directly measured, only its effects, i.e. either the pressure variation which results from air leaving the work piece which is detached from the compressed air source (pressure measurement), or the quantity of air continuing to flow into the work-piece – which is connected to the compressed air source (flow rate measurement). There are different variants for both pressure and flow rate measurements.
In industrial leak testing, pressure measurement is the most frequently used method. With smaller test chambers, leaks can be detected from 0.1 cm3/min. The relative pressure or absolute pressure method allows a compact construction and the minimum possible volume of the measuring system itself. It is also distinguished by high operational reliability and a wide measuring range. The test signal definition depends on the test pressure used. In the differential pressure method at higher test pressures, a greater accuracy can be obtained than with the absolute pressure method, as the test signal definition is independent of the test pressure used. With measurement of the pressure drop with over pressurization of the test piece the usual operating conditions are simulated. In measurement of the pressure rise in the partial pressure test method the disturbing influences due to temperature variations or volumetric instability of the sealing device or of the work piece are smaller than in measurement of the pressure drop. Measurement of the pressure rise in the excess pressure method (capsule method) manages without a leveling stage. Moreover, in this case the test pressure used is not limited by the measuring range of the measuring element, as it is not subjected to the test pressure.
Whereas in pressure measurement, the test signal becomes smaller with increasing test volume, flow rate measurement delivers a test signal independent of the quantity of the test volume. This is an advantage in calibration: the test signal corresponds directly to the air flow rate flowing through the calibration leak. Here it is no longer necessary to know exactly the quantity of the test volume connected during the calibration process and to allow for determining the leakage rate. As a rule, measurement of the volumetric flow rate (e.g. pressure drop across a screen) is not used for leak testing, but for flow control such as for example, in monitoring free passage in gas systems which are tested by the pressure drop method with the same measuring element (differential pressure sensor) in a subsequent measurement for leaking. In measurement of the mass flow rate (thermal measuring methods) the test signal is not only independent of the size of the test volume, but also of the degree of test pressure. The test signal here corresponds directly to the leakage rate in standard cm3/min. The leakage rate does not as in pressure measuring methods have to be calculated.
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 can be measured. 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.
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
Nor must the test volume vary during the measuring stage 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.
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 measuring unit, valves, hoses and test pressure regulator.
Typical repeatability values of correctly set leak testers:
The accuracy of leak measurement naturally also depends on the accuracy of the leak simulator with which the measuring system was calibrated. For correct calibration of the test system, a master part (sealed work-piece) 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 work-piece. 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 about0.1 cm3/min. The air leaving the valve is measured with a calibrated calibrator with a definition of 0.01 standard cm3/min.
The measuring element of a leak tester cannot distinguish whether the leak stems from a porous area of the work-piece 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 test system
In automatic Leak Testing, the cumulative leakage of a work-piece 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 work-piece is thus wrongly classed as a “REJECT“. In fixing the permissible leakage rate, however, the worst case of an individual leak must be assumed.
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. Even a high pressure stage preceding the leak test could not blow this leaking area free. Therefore, dry work-pieces are essential and necessary for leak testing.
| Sr no | Product/Component | Test Pressure | Leak rate |
|---|---|---|---|
| 1 | WATER LEAKS/COOLING SYSTEMS | 1 to 2 bar | 4 to 10 cc/min |
| 2 | OIL LEAKAGE (various) | 0.1 to 7 bar | 6 to 15cc/min |
| 3 | FUEL/DIESEL | 1 to 10 bar | 0.1 to 15cc/min |