Written by Dr Lewis Newton
The science of measurement
In many scientific disciplines, not least your daily life, there is often a need for measurement. In many ways, it is the cornerstone of the scientific method – although this may be bias coming from a Metrologist!
For us metrologists, we are concerned with everything to do with measurement. From the 7 SI units (mass, length, time, etc.) to the instruments that measure them!
Think of it as confidence in our results – or more positively, certainty!
While it all sounds negative, you’ll often see this in the form of simple “±” with your result, and it’s just a metrologists way of admitting that the world isn’t perfect.
Uncertainty and it’s calculation is all about picking apart the things that have contributed to the value you get for your measurement. These errors can be:
- Forgetting to start from zero – happens more often than you think!
- Differences between your repeats – repeatability
- Temperature changes – materials can shrink/grow a couple of nanometres!
Being highly uncertain isn’t bad! I’m sure you can forgive your scales at home for not being as certain as CERN!
It’s just always better to know your uncertainty in a measurement result.
Système international (d’unités)
Ooo la la: This is what SI stands for! The International System of Units
They are realised by the various national laboratories in different countries (NPL in the UK) – from whom we all check our lengths, masses, etc. are the same as theirs!
The metre is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second.
To realise it: NPL fires a laser and measures how fast it travels – in a super controlled way!
In short, even your ruler is connected to the speed of light…
If you think of metrology as a pyramid, the thing you want to measure is at the bottom; you don’t need it to be super certain – like 0.10000 ± 0.00001 m, but you do want the unit you are using to be the same one as everyone else! – you want to be traceable to the definition of the metre at the top of the pyramid!
So you have connected measurements that link them all together – this is calibration.
This is a word that needs some clarity! Especially in metrology
While it has other uses in wider day to day life, in metrology we aren’t adjusting ‘the scales’ when we calibrate! This is more of a correction.
Calibration involves comparing the measurement result from your system with a known-value (from a ‘better’ measurement system, which might have uncertainties associated with it!). BUT you can use this knowledge (maybe there’s some systemetic errors?!) to apply corrections!
General sites to learn more:
- National Physical Laboratory – Engineering. Available at: https://www.npl.co.uk/engineering
- BIPM – International bureau of weights and measures. Available at: https://www.bipm.org/en/worldwide-metrology/
Books for reference reading:
- GUM: Guide to the Expression of Uncertainty in Measurement. Available at: https://www.bipm.org/utils/common/documents/jcgm/JCGM_100_2008_E.pdf
- VIM: International Vocabulary of Metrology. Available at: https://www.bipm.org/utils/common/documents/jcgm/JCGM_200_2012.pdf
- The Beginner’s Guide to Uncertainty of Measurement. Available at: https://www.npl.co.uk/special-pages/guides/gpg11_uncertainty.pdf
About the author:
Lewis is a Research Fellow at the University of Nottingham within the Manufacturing Metrology Team. Currently, his research is in the development of optical measurement systems for the measurement of aesthetic surfaces relating to human perception and further investigations into feature-based characterisation pipelines for novel applications.
This is part of our engineering series, find more here.