RR454 - Probability of Detection (PoD) curves Derivation, applications and limitations
There is a large amount of 'Probability of Detection' (PoD) data available (eg National NDT Centre (UK), NORTEST (Norway), NIL (Netherlands) and in particular NTIAC (USA)). However, it is believed that PoD curves produced from PoD data are not very well understood by many who use and apply them. For example, in producing PoD curves, a certain material and thickness may have been used and yet one can find the same PoD quoted for a range of thicknesses. In other cases, PoD curves may have been developed for pipes, but they have been applied to plates or other geometries. Similarly, PoD curves for one type of weld (eg single sided) have been used for other welds (eg double sided). PoD data are also highly dependent on the Non-Destructive Testing (NDT) methods used to produce them and these data can be significantly different, even when applied to the same flaws and flaw specimens. It is often assumed that the smallest flaw detected is a good measure of PoD, but there is usually a large gap between the smallest flaw detected and the largest flaw missed. Similarly, it is often assumed that human reliability is a very important factor in NDT procedures, and yet it is usually found not to be as important as other operational and physical parameters.
It is important to question the validity of how PoD curves are applied as well as their limitations. This report aims to answer such questions and in particular their relevance to fitness for service issues involving PoD.
The overall goal of this project is to provide clear, concise, understandable and practical information on PoD curves, which will be particularly useful for Health and Safety Inspectors when discussing safety cases involving PoD curves.
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