FAQ-L.4: Can the monetary loss and the size of a compressed air leakage be determined?

The indication of values of leakage losses are for orientation, as they are influenced by many factors. In ultrasound testing, the acoustic radiation largely depends on the leak size, leak shape, surface condition of the material, pressure difference, outflow velocity and outflow profile as well as the temperature. An important role is also played by the measuring distance and the measuring angle. The multiplicity of factors creates a complexity that is difficult to model through simple analysis.

Traditional, narrow-band instrument technology around 40 kHz is very well suited for the localization of compressed air leaks. However, the validity of the sound pressure level in this narrow frequency range with respect to the determination of the amount of loss (quantification) has to be questioned, since the frequency maximum (amplitude) of the ultrasonic signal stochastically occurs in a wide frequency range. Therefore, the loss of a leakage should be evaluated in a wider frequency range. The range between 20 and 100 kHz has been proven to be practicable. Broadband digital instruments can provess this additional information and evaluate leakages on the basis of their frequency characteristics (integral sound pressure level).

However, the determination of the volumetric flow loss in l/min remains subject to statistical fluctuations as a result of the aforementioned factors. Therefore, it is recommended that a monetary estimate of leakage losses should always be made for the total loss of multiple leakages detected. When performing statistical analysis of many leaks, the measurements will get sufficiently accurate and reliable.

For the same reasons - and in particular the fact that leaks have various shapes - the mathematical derivation of a circular leak size from leak losses in l/min may be less meaningful. Nevertheless, for basic understanding and orientation, some hole sizes with estimated leakage losses should be shown here (basis: Leakage loss calculation by Postberg+Co. GmbH):