How many people and organisations are involved in leak testing?

One of the NDT methods listed in BS EN ISO 9712:2012 is leak testing (hydraulic pressure tests excluded). Currently, there are two draft standards being worked on relating to leak testing: BS EN ISO 20486 Non-destructive testing – Leak testing – Calibration of reference leaks for gases and BS EN ISO 20485 Non-destructive testing – Leak testing – Tracer gas method. These in turn refer to more standards that relate to the method, including:
ISO 20484 Non-destructive testing – Leak testing – Vocabulary, ISO/IEC Guide 98-3 Uncertainty of measurement – Part 3: Guide to the expression of uncertainty in measurement (GUM:1995), EN 1779 Non-destructive testing – Leak testing – Criteria for method and technique selection and EN 13625:2001 Non-destructive testing – Leak test – Guide to the selection of instrumentation for the measurement of gas leakage. While this demonstrates how mature leak testing as an NDT method is, I very rarely hear about it and have never been asked about training and certification for it.

From the draft standard ISO/DIS 20485:2016(E) there are two basic techniques used for the generation and detection of tracer gas flow:
  • Tracer gas flows into the object (Group A techniques)

A pressure difference across the wall is obtained either by evacuation of the object, for example through a connection, or by placing it in a pressurised chamber. Usually the test object is evacuated. Tracer gas is then applied to the external surface using a probe jet or by enclosing the object (totally or partially) in a hood or chamber filled with the tracer gas. Tracer gas leakage into the test object is detected by a sensor within or connected to the internal volume.

  • Tracer gas flows out of the object (Group B techniques)   

The object is filled with a tracer gas. A pressure difference across the wall is obtained either by pressurisation of the object, for example through a connection, or by placing it in a vacuum chamber. The tracer gas is collected on the outside surface by a sampling probe, a carrier gas flow or by accumulation into a hood or chamber. Tracer gas can also be detected by chemical reactions.

A special technique (bombing) may also be used. This involves pressurisation of a sealed object to force tracer gas into its internal cavities, if a leak exists. The object is then placed in a vacuum chamber and escaping tracer gas is detected (this procedure is usually only used with helium-4). This method is applicable to specimens with small free internal volumes (in the order of a few cubic centimetres). I was amused to find the term ‘bombing’ being used as this has also been used in radiography and comes from the days of the BIX Pots that held the isotope and were shaped like a bomb.

We then come to the calculations. The total leakage rate of the object, in molecular flow conditions, is calculated using the formula below:


where qG is the total leakage rate, qCL is the leakage rate of the calibrated leak (pure tracer gas), SL is the leak signal, SCL is the signal generated by the calibrated leak, RL and RCL are the background signals associated with signals SL and SCL, respectively, c is the volume fraction of the tracer gas in the gas mixture and p is the total pressure in the auxiliary enclosure.

It appears that the method is complex and requires a level of mathematical ability. How many people and organisations are involved in leak testing and does industry have significant use for it?

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