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News from the Institute Branches
| West of England Branch
Measurement or NDT method?
Tony Dunhill reports
On 11 March 2023, the West of England Branch held an informal discussion with Professor Pete Loftus on non-destructive testing (NDT) and measurement with eight members. This topic had been raised by a number of Branch members who, as site Level 3s, are often asked to decide if an inspection is a measurement or an NDT method, so having an understanding of the differences and similarities was thought to be helpful. This is especially true for thickness gauges with just a digital readout.
We started with a definition of metrology, which is ‘the science of measurement and its application’. It is an inter-governmental activity with regular ministerial meetings and national scientific advisory groups. In 1875, the ‘Metre Convention’ system was signed and there is now a website at www.bipm.org/en containing all of the relevant documents. Control is via ISO 10012 (Control of measurement systems), which currently requires a company board member to be identified as responsible for ensuring the measurement systems used by the organisation are adequate. These individuals rely on their metrologists to ensure they understand all the processes used. (The ISO is in revision and the new one is better suited to today’s matrix organisations, but the principle of accountability for metrological control is still at its core.)
The next topic was accuracy; however, Pete persuaded us not to use the term. The true size of any measured quantity is unknowable and the term used to describe the margin of error is ‘uncertainty’. When deciding on any measurement technique, it is important to assess the sources of uncertainty and try and put in place controls to minimise their effect. Repeatability addresses the precision element of uncertainty but other sources exist, such as ‘bias’, depending on how a method is used. Statistical exercises, similar to a probability of detection (POD) in NDT, can be used to establish an overall uncertainty, once procedures and training are in place. There are procedures to allow new methods to be evaluated and introduced and indeed these have been applied to some of the International System of Units (SI) standards themselves such as the kg, which has now been relegated to a museum as it is replaced by a definition involving three fundamental constants that may be realised anywhere in the universe (currently using a device known as the ‘Kibble balance’ to realise the kg from electrical units).
Unlike NDT, there is not much personal qualification in the metrology field; however, there are now trailblazer apprenticeships available and a skills framework produced by the National Metrology Skills Alliance and the National Physics Laboratory has Level 1 and 2 training grades (see the links at the end of the article). The sector is moving towards a system closer to the NDT model, which has provided inspiration for the work.
We then discussed cases where measurement systems were used by NDT scanners to allow for profile following. This is a similar case to some machine tools and their rule of thumb is that the accuracy – sorry, I mean uncertainty! – for the defining system should be ten times better than the following system needs.
A significant measurement challenge for NDT is in critical sizing of an indication. Often in ultrasonic testing, the obscure value of ‘–X dB of a 1.2 mm-diameter flat-bottomed hole’ is used to overcome the vagaries of ultrasonic signal heights caused by the unknown acoustic impedance mismatch of the flaw and parent material. Full matrix capture images provide a possible way forward. It is also rare to use a +/– uncertainty value when quoting through-wall thickness values of cracks, but exercises could be carried out to establish these at a cost.
Where is machine learning taking metrology? This is a hot topic within the international community although, as with non-destructive testing, the large datasets needed to train the algorithms are rare.
The topic of decision rules and ‘guard-banding’ was briefly discussed. Having arrived at a measured result with defined uncertainty, it is necessary to determine how it will be used to inform a decision: is the wall thick enough?; is the crack small enough?; etc. These decisions should take account of the uncertainty, ie you do not know that the wall is as thick as the measurement result says it is. You can only prove that it is as thick as the measurement result minus the uncertainty.
The standard reference for all questions about uncertainty assessment is: JCGM 100:2008(E), ‘Evaluation of measurement data – Guide to the expression of uncertainty in measurement (2008)’, www.bipm.org/documents/20126/2071204/JCGM_100_2008_E.pdf
For applying uncertainty to form decision rules, etc, see: JCGM 106:2012, ‘The role of measurement uncertainty in conformity assessment (2012)’, www.bipm.org/documents/20126/2071204/JCGM_106_2012_E.pdf
Skills links:
| Early Careers Branch
Charity event
The Early Careers Branch is participating in a charity event, the Tough Mudder Classic, scheduled to take place on 6 July 2024 in the Midlands at Belvoir Castle, Grantham.
The Tough Mudder Classic is an endurance event that aims to make its participants knackered, soaking wet, uncomfortable, covered in mud and, at times, rather shocked (figuratively and literally). It consists of 20 rather creative obstacles designed to push participants past their comfort zone, scattered across a ten-mile stretch, all to be completed either individually or as a team.
The Branch is hoping to raise awareness and support for Primary Engineer, a non-profit organisation that aims to encourage more young students to consider careers in science, technology, engineering and mathematics (STEM), and the British Institute of Non-Destructive Testing’s Benevolent Fund.
As a group, the Branch believes it is imperative to inspire future engineers; what better time than in the early stages of their education and development? It goes without saying that this is especially important in light of the rapid technological advances we are bound to experience in the coming years. Furthermore, we feel strongly about supporting the immediate network of professionals in the industry we all find ourselves working in.
To join the team and participate in the event, or to donate to the fundraising effort, visit: www.justgiving.com/crowdfunding/ndtcharityevent
Measurement or NDT method?
Tony Dunhill reports
On 11 March 2023, the West of England Branch held an informal discussion with Professor Pete Loftus on non-destructive testing (NDT) and measurement with eight members. This topic had been raised by a number of Branch members who, as site Level 3s, are often asked to decide if an inspection is a measurement or an NDT method, so having an understanding of the differences and similarities was thought to be helpful. This is especially true for thickness gauges with just a digital readout.
We started with a definition of metrology, which is ‘the science of measurement and its application’. It is an inter-governmental activity with regular ministerial meetings and national scientific advisory groups. In 1875, the ‘Metre Convention’ system was signed and there is now a website at www.bipm.org/en containing all of the relevant documents. Control is via ISO 10012 (Control of measurement systems), which currently requires a company board member to be identified as responsible for ensuring the measurement systems used by the organisation are adequate. These individuals rely on their metrologists to ensure they understand all the processes used. (The ISO is in revision and the new one is better suited to today’s matrix organisations, but the principle of accountability for metrological control is still at its core.)
The next topic was accuracy; however, Pete persuaded us not to use the term. The true size of any measured quantity is unknowable and the term used to describe the margin of error is ‘uncertainty’. When deciding on any measurement technique, it is important to assess the sources of uncertainty and try and put in place controls to minimise their effect. Repeatability addresses the precision element of uncertainty but other sources exist, such as ‘bias’, depending on how a method is used. Statistical exercises, similar to a probability of detection (POD) in NDT, can be used to establish an overall uncertainty, once procedures and training are in place. There are procedures to allow new methods to be evaluated and introduced and indeed these have been applied to some of the International System of Units (SI) standards themselves such as the kg, which has now been relegated to a museum as it is replaced by a definition involving three fundamental constants that may be realised anywhere in the universe (currently using a device known as the ‘Kibble balance’ to realise the kg from electrical units).
Unlike NDT, there is not much personal qualification in the metrology field; however, there are now trailblazer apprenticeships available and a skills framework produced by the National Metrology Skills Alliance and the National Physics Laboratory has Level 1 and 2 training grades (see the links at the end of the article). The sector is moving towards a system closer to the NDT model, which has provided inspiration for the work.
We then discussed cases where measurement systems were used by NDT scanners to allow for profile following. This is a similar case to some machine tools and their rule of thumb is that the accuracy – sorry, I mean uncertainty! – for the defining system should be ten times better than the following system needs.
A significant measurement challenge for NDT is in critical sizing of an indication. Often in ultrasonic testing, the obscure value of ‘–X dB of a 1.2 mm-diameter flat-bottomed hole’ is used to overcome the vagaries of ultrasonic signal heights caused by the unknown acoustic impedance mismatch of the flaw and parent material. Full matrix capture images provide a possible way forward. It is also rare to use a +/– uncertainty value when quoting through-wall thickness values of cracks, but exercises could be carried out to establish these at a cost.
Where is machine learning taking metrology? This is a hot topic within the international community although, as with non-destructive testing, the large datasets needed to train the algorithms are rare.
The topic of decision rules and ‘guard-banding’ was briefly discussed. Having arrived at a measured result with defined uncertainty, it is necessary to determine how it will be used to inform a decision: is the wall thick enough?; is the crack small enough?; etc. These decisions should take account of the uncertainty, ie you do not know that the wall is as thick as the measurement result says it is. You can only prove that it is as thick as the measurement result minus the uncertainty.
The standard reference for all questions about uncertainty assessment is: JCGM 100:2008(E), ‘Evaluation of measurement data – Guide to the expression of uncertainty in measurement (2008)’, www.bipm.org/documents/20126/2071204/JCGM_100_2008_E.pdf
For applying uncertainty to form decision rules, etc, see: JCGM 106:2012, ‘The role of measurement uncertainty in conformity assessment (2012)’, www.bipm.org/documents/20126/2071204/JCGM_106_2012_E.pdf
Skills links:
- Metrology Technician (www.instituteforapprenticeships.org/apprenticeship-standards/metrology-technician)
- Senior Metrology Technician (www.instituteforapprenticeships.org/apprenticeship-standards/senior-metrology-technician-v1-0)
- National Metrology Skills Alliance – The skills framework (www.instmc.org/sigs/national_metrology_skills_alliance)
- National Physical Laboratory (NPL) training solutions (https://training.npl.co.uk/sector/training-courses)
| Early Careers Branch
Charity event
The Early Careers Branch is participating in a charity event, the Tough Mudder Classic, scheduled to take place on 6 July 2024 in the Midlands at Belvoir Castle, Grantham.
The Tough Mudder Classic is an endurance event that aims to make its participants knackered, soaking wet, uncomfortable, covered in mud and, at times, rather shocked (figuratively and literally). It consists of 20 rather creative obstacles designed to push participants past their comfort zone, scattered across a ten-mile stretch, all to be completed either individually or as a team.
The Branch is hoping to raise awareness and support for Primary Engineer, a non-profit organisation that aims to encourage more young students to consider careers in science, technology, engineering and mathematics (STEM), and the British Institute of Non-Destructive Testing’s Benevolent Fund.
As a group, the Branch believes it is imperative to inspire future engineers; what better time than in the early stages of their education and development? It goes without saying that this is especially important in light of the rapid technological advances we are bound to experience in the coming years. Furthermore, we feel strongly about supporting the immediate network of professionals in the industry we all find ourselves working in.
To join the team and participate in the event, or to donate to the fundraising effort, visit: www.justgiving.com/crowdfunding/ndtcharityevent
