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Measurement methods

Leak measurement attempts to quantify the flow of the fluid over a period of time. There are three commonly used fundamental techniques for leak measurement: a pressure change within a known volume, a direct measurement of flow, or measurements of changes in concentration.

Pressure measurement

Absolute pressure method

A transducer or gauge is used to measure pressure change within the device being tested. In general, this technique is difficult to apply because small changes in pressure are being measured. The sensitivity of the transducer as a percentage of the full scale becomes an issue because the pressure differences can be extremely small even when trying to detect a large leak.

Differential pressure method

This technique involves locating the test and reference volumes either side of a transducer diaphragm. Ideally, these volumes should have similar pneumatic characteristics, and they are arranged to be as stable as possible and at the same pressure. Subsequent time-related changes in the pressure of the test volume are used to measure the value of any leakage. A differential pressure transducer is placed across the two volumes. The sensitivity of the transducer is not related to the actual pressure in absolute or gauge pressure terms, but to the sensitivity of the differential pressure transducer at the test pressure. For example, it is possible to detect 0.0005% of the test pressure using this technique by testing at 1 bar gauge (2 bar absolute).This may be 100 times more sensitive than using the absolute measurement technique mentioned above.

Differential pressure component dosing method

This technique involves investigating sealed components. In this technique, twin tandem pressurization and reference volumes are used together with the sequential application of a differential pressure transducer. It is designed to identify gross leaks in sealed parts. The differential pressure technique alone will not differentiate between a good part and a gross leaking part; dosing is used with the differential pressure technique to detect the gross leaking parts.

Inter-stream testing method

Two test pressures and three interrelated differential pressure transducers are used to simultaneously test two adjacent volumes. This technique is used when there are two fluid volumes in a single part, which could leak to the outside or between each other.

Flow measurement

Mass flow rate method

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.

Volumetric flow rate method

This method 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.

Gas trace

Gas tracer systems have the ability to detect smaller leakages, but tend to require higher capital investment.

Helium

This requires enclosing the test piece within a sealed chamber and evacuating the chamber and/or test piece. After charging one or the other with helium, the gas loss from the respective volume is monitored with a mass spectrometer that detects the helium.

Hydrogen

This gas is used in the same way as helium. It is also used in a technique that employs a sensor to detect and measure levels of, in this case, hydrogen in a continuous ambient or low pressure gas flow, which allows the use of lower cost enclosures and ducting. The test gas is a mixture of 95% nitrogen and 5% hydrogen, which is non-explosive and relatively inexpensive.

Gas bombing

This involves exposing a sealed component to a gas-charged environment and subsequent transfer to a test chamber that is monitored by gas sensing instrumentation. It is essential that the possibility of a gross leak is eliminated before fine leak measurement is attempted.

Various halogens can be used in all of the above techniques, but they are avoided if possible. In addition to toxicity, they are heavy gases that tend to accumulate readily in low recesses. As a result, contamination occurs and the test area is difficult to clear; this is made worse by the tendency of these gases to adhere to surfaces. All gases will stick to the tooling surface to some degree, most particularly when flooded from a leaking component. However, hydrogen and helium are lighter and they are more easily removed or dispersed by extraction, flushing or natural diffusion.



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