Fast and correct measurements are difficult

Fredrik Arrhén, a senior researcher at RISE, have worked with dynamic measurements since the beginning of the 2000s, first focusing on pressure and over time expanding his area of interest to force and torque.

"Basically, we are studying how well sensors and measuring instruments keep up with rapid changes and dynamic events. We can measure these events and take note of the value presented by the measuring instrument, but when the pressure or force changes very quickly, we do not know how well that value reflects reality. If we use the measured value to control a production process, it is very important that the value is correct", says Fredrik Arrhén.

Big challenge producing standards

To know how well a measuring instrument reflects reality, the first step is to be sure what change has actually taken place, what reality looks like. Exactly how high was the pressure at any given point of time during the change or event?

"The big challenge of dynamic measurements in all areas of metrology is to produce standards with very low measurement uncertainty that we can use to calibrate the measuring instruments. We must know the properties of the rapid change very well before we can determine how well the instrument measures it", says Fredrik Arrhén. 

The idea of standards or references is simple. To measure how long an object is you need something to compare it with, a reference, such as a ruler. To be certain that the ruler is as long as it claims to be the ruler in turn needs to be compared with something even more accurate than the ruler, and so on. As the National Metrology Institute of Sweden, one of the main tasks for RISE is to maintain traceability for several physical quantities. Traceability means that measurement results must be traceable back to the definition of the unit, through chains of calibrations (comparisons) against standards or references with specified measurement uncertainties. A measured value without traceability is not reliable.

"Traceability is the foundation for quality-assured measurements which is lacking in dynamic measurements today. As it stands, virtually all calibrations take place under static conditions. A certain pressure is set, we wait until it stabilizes and then compare the value from the measuring instrument with the standard", says Fredrik Arrhén.

"What we do know however is that measuring instruments often do not behave as stated in data sheets and manuals during rapid changes and events. The instruments take longer than promised to react to the changes and the measured values often fluctuates during the event. This can have major impact for the industries using the measuring instruments to provide the basis for decisions.", says Fredrik Arrhén.

Develops traceability

To establish traceability in dynamic measurements and to increase knowledge of the properties of measuring instruments during rapid changes, RISE runs several projects in areas such as pressure, force, torque and temperature.

"One example is the development of a shock tube, which we hope will be internationally accepted as a standard for dynamic pressure. Using the shock tube, it is possible to generate a pressure wave which properties we know with very low uncertainty. We also invest heavily in developing our collaboration with trade and industry to build knowledge and to direct our efforts based on their actual needs", said Fredrik Arrhén.