In measurement, traceability is critical because to have any real value, measurement systems must provide results that have traceable accuracy. What does that mean?
Measuring up
Let’s consider a question: why do we measure things? The correct answer is that we do so because measurements allow us to compare one thing to another: taller or shorter, heavier, or lighter, a fit between two conjoined objects or not, and so on. This being the case, for measurements to have value all, or any two measurement systems must be consistent with each other and yield the same results. For instance, my scales should tell me I’m the same weight as I’d be if I were to use your scales to weigh myself. If this wasn’t the case, you can see the potential for problems.
Here’s an example that illustrates this. I have two tape measures at home. One is an expensive model from a well-known Swedish brand and the other is a no-brand cheap model. Objectively the difference between the two seems only to be a 20 Euro price differential and an almost 5-millimetre length differential (yes, the no- brand tape measure has markings showing almost 5 millimeters less per meter.) Which one are you going to trust for making your measurements?
The expensive Swedish seems the obvious answer, but why? It’s not simply because it’s more expensive and feels like its higher quality but because it has a reference to a European Standard class rating, confirming its accuracy. The cheaper model has no such rating, just the name of its country of manufacture. The former is traceable; the latter isn’t.
In our world, references are essential – and the gold standard in the metrology industry is the “International vocabulary of metrology” (3rd edition – 4 is coming soon!), administered by the BIPM, and to which we will refer repeatedly in this series.
Traceability in practice
This is a big deal. Another example illustrates why. If you order two components that to operate properly must fit together, your order will be based on dimension requirements. If the supplier of one of the components used the no-brand tape measure to arrive at its dimensions while the supplier of the second component used the Swedish tape measure when they manufacture their part, when you receive your order the two parts will not fit together.
When you complain to both suppliers, each of them will tell you that the other one’s wrong; their part was sized and measured correctly. But who is right?
Without the existence of an agreed-upon definition, the concept of measuring has no meaning. For something measured with a tape measure, things are easy to resolve. An international standard exists for lengths and units and one meter is well definedas “the length of the path travelled by light in a vacuum in 1/299 792 458 of a second.” (Yes, that’s the definition, maybe not so practical in everyday life but nevertheless specific.)
Most countries have an Institute of Standards to handle matters of measurement and definitions. They will usually keep a reference, or equipment to verify a reference to the standard definition.
So, when the Swedish manufacturer wants to make a tape measure, they go to their institute of standards and get a reference measurement which they bring back to their factory and calibrate their manufacturing process so that the markings on their tape measures come within the required accuracy.
Examples of organizations that provide traceable references are the Bureau of International Weights and Measures (BIPM)[1] and Physicalisch-Technische Bundesanstalt (PTB)[2].
This is the process known as traceability and it’s why the Swedish tape measure can be relied on. It’s traceable.
Handling traceability: the Conoptica and the Calibration Certificate.
Traceability can be confirmed by a Calibration Certificate. For instance, Conoptica provides documentation to its customers that their systems provide traceable measurements. This is because when we at Conoptica assemble a system, we calibrate it using traceable reference objects, as you can see in the figure below.
Traceable reference objects
In fact, Conoptica uses several different objects for the calibration process. Since we measure the diameters and ovality of round holes (among other things), one of our reference objects is a coated glass plate. The coating has circular holes with specific sizes and one glass plate acts as a company reference. This plate has been measured by the PTB – or National Metrology Institute of Germany. Its method is described in a long and highly technical document and contains a reference back to the meter definition. All Conoptica’s calibration objects are referenced to this Company Reference Plate.
Using this approach, we create sets of customer references. These are the objects that can be used to validate calibration and ensure that the traceability of the necessary measurements. For the user, that means that when a Conoptica system measures an object, you can expect the value obtained to be as close to the true value as specified accuracy claims.
Conclusion
Traceability is the key to all measurements. The concept of tracing all physical parameters to a set of common references (one for length, one for weight and so on) is the foundation of measurement technology.
Conoptica has been providing high tech camera-based measurement solutions for the metal working industry since the 1993. We make sure that the metal working industry has access to key quantitative data about their products and tools. Conoptica is the market leader for measurement equipment in the wire & cable industry.
Further reading:
https://en.wikipedia.org/wiki/Metre
https://eur-lex.europa.eu/legal-content/EN/TXT/PDF/?uri=CELEX:32004L0022&from=EN
http://www.npl.co.uk/educate-explore/factsheets/traceability-and-uncertainty/
[2] www.ptb.de