Senior forskareContact Jan
A central concept in metrology is measurement uncertainty, which is not about being unsure, but about being very sure of what you do and do not know.
Many factors affect measurements: the human factor, temperature, humidity, properties of the object being measured, the measuring instrument we use, etc. But no matter how good the measuring instruments we use are or how ideal the conditions are, it is not possible to know that the measured value is the true one.
“There are many concepts that describe quality in measurements, but one of the most central for us metrologists is measurement uncertainty. Note that it is uncertainty, not accuracy or precision. All measurements have systematic or random errors, and the word uncertainty confirms this fact”, says Jan Johansson, responsible for the National Laboratories at RISE.
All measurements have systematic or random errors, and the word uncertainty confirms this fact.
The measurement uncertainty indicates how much the measured value of the measuring instrument can differ from the true value with a certain probability (usually 95 percent). The measured value together with the measurement uncertainty gives a measurement result. The measurement result 5.00 m +- 0.02 m means that the true value is, with great probability, between 4.98 and 5.02 meters.
“If you are going to build a porch, you need to think about which plank lengths, tools, measuring methods and measuring instruments are available. With the right measurement methods and calibrated measuring instruments, it possible to reduce the measurement uncertainty, but this comes at a cost. In the case of the porch, the measurement uncertainty may be at a reasonable level with an ordinary tape-measure ruler, the cost of buying a new measuring instrument and calibrating it may be greater than the benefit”, says Jan Johansson.
The National Laboratories at RISE offer calibration services with the lowest possible measurement uncertainties to business, academia, and society. A lot of the research conducted here aims to reduce measurement uncertainty in different fields. But the level of measurement uncertainty that the national laboratories can offer, at a level of parts per million or parts per billion, is not necessary in most cases.
“No, being aware of the measurement uncertainty and using that knowledge to make measurements with the right level of measurement uncertainty is what’s most important. What level of measurement uncertainty does the production process require in your company? It is a balancing act between the cost of reducing measurement uncertainty and the cost of having to discard products when they are outside the tolerance requirements”, says Jan Johansson.
As the National Metrology Institute of Sweden RISE maintains metrological traceability through the National Laboratories for different physical quantities. Chains of calibrations, national standards and international comparisons makes certain that a kilogram is a kilogram and a meter a meter everywhere.