Avoiding uncertainty

The statement at the end of this and all previous articles states that the views expressed are “the author’s own personal ramblings for the purpose of encouraging discussion within NDT News”. Personal ramblings they most certainly are! But encouraging discussion within NDT News? Not so much. The evidence is there. The 100 articles archived on the BINDT website (www.bindt.org/forums/on-the-job) have attracted only two comments and neither of these prompted further feedback.

Although a little disappointed with this, no one likes failure, I have had sufficient encouragement and direct feedback to know that my articles are read and people do enjoy them. I also understand that people do not necessarily have the time nor the inclination to put their views into the public domain. Other than these ramblings, I have never responded publicly to anything that I have read, heard on the radio or watched on TV and nor have I had the desire to do so. Until I read May’s issue of NDT News.

There were a number of fascinating articles, but ‘News from the West of England Branch’, written by Tony Dunhill, was of particular interest and made me want to sit down immediately, pick up my pen and write a letter in response. (I should point out for younger readers that letters were what we had before the advent of emails and social media.) I was not driven by a sudden urge to complain or criticise the article, but to commend the raising of an important topic that too often is ignored: the topic of uncertainty in NDT results. The only reason there is no letter to the Editor from me in this issue is because I am privileged to have this forum at my disposal.

I will mention a few of the issues raised, in reverse order of their appearance in the Branch report:

• How will a measured result with a defined uncertainty be used? Non-destructive testing (NDT) results are always inputs to a decision on what, if anything, needs to be done with the component going forward. For critical components, these are important decisions with safety and economic consequences if they are wrong. Anyone responsible for making such decisions should have the complete information available, including the probability of detection (POD)/detection capability and the sizing uncertainties. It should be pointed out that when looking at wall loss or crack growth between two sets of measurements, the uncertainty in both sets needs to be considered.
  
  
• In light of the importance of the decisions made on the through-wall measurement of crack sizes, why is it rare to use a +/- uncertainty value in the report? Exercises to establish the uncertainty do incur a cost, which could explain why it is not carried out. NDT can be a grudge purchase and there is pressure to keep costs low, but if NDT results are going to be used in plant decisions, turning a blind eye has the potential to cost a lot more than estimating the uncertainty. There is now more information on uncertainty in NDT available in the literature (see below), so the cost need not be onerous.
  
  
• Full matrix capture images and new technology may improve the situation but lack of knowledge on the part of the user can also make it worse. The analysis of images is one step removed from the practicalities and hence the inherent uncertainties of the practical application of non-destructive testing techniques.

Back in January 2022, I wrote an article on uncertainty (see: www.bindt.org/forums/on-the-job/we-need-to-talk-about-uncertainty/). It references two available documents that provide assistance on addressing uncertainty in NDT:

• The National Association of Testing Authorities (NATA), Australia, documents on ‘Measurement Uncertainty in NDT’ are available at: https://nata.com.au/files/2021/05/Infrastructure-and-Asset-Integrity-Measurement-Uncertainty-in-NDT.pdf 
• The Health and Safety Executive (HSE) guidance on ultrasonic sizing errors and their implication for defect assessment is now available at: www.hse.gov.uk/comah/assets/docs/ndt4.pdf
  
  
• A report, published by the National Physical Laboratory (NPL) on ‘Uncertainty of Measurement for Non-Destructive Testing (NDT)’, came out a couple of months after my previous article in 2022 and is available at: https://eprintspublications.npl.co.uk/9509/1/MAT105.pdf

Hopefully, on the back of Tony Dunhill’s report, this column will open the floodgates to a vigorous discussion on how to address uncertainty in NDT. Don’t all rush at once!

Please note that the views expressed in this column are the author’s own personal ramblings for the purpose of encouraging discussion within NDT News. They do not represent the views of Jacobs or BINDT.

Letters can be mailed to The Editor, NDT News, Midsummer House, Riverside Way, Bedford Road, Northampton NN1 5NX, UK. Email: ndtnews@bindt.org or email Bernard McGrath direct at bernard.mcgrath1@jacobs.com