Traceable to the SI

The SI is an abbreviation for Système International d’Unités (International System of Units) is defined by the CGPM (General Conference of Weights and Measure) and includes the units used internationally today. The integrity of the calibration is established by the traceability and level of confidence of the measurement results. The traceability refers to the measurement’s relation to the SI achieved through an unbroken sequence of calibration steps or comparisons all of which have stated uncertainties along with their level of confidence.

Level of confidence is the term used by metrologists to indicate the reliability of the measurement uncertainty. The term, “level of confidence”, is derived from the term “confidence level”, that is defined in the mathematical field of statistics. Traceability is so important in modern metrology that there have been specific elements assigned to the concept that must be satisfied to be able to successfully claim that the measurement result is traceable to the SI. These properties are as follows:

  1. The unbroken chain of comparisons
  2. The measurement uncertainty must be calculated according to defined methods (true for every step in the chain)
  3. Each step in the chain must be performed according to documented and generally acknowledged procedures; the results must be equally documented
  4. Laboratories in the chain must supply evidence for their technical competence (e.g. they are accredited)./li>
  5. The appropriate standards must be primary standards for the realization of the SI units. Calibrations must be repeated at appropriate intervals (traceability may be lost by the passage of time)

During the calibration of a DUT (device under test) our job at miller instruments is to establish a link between the measurement (or generation) of the DUT and the SI, to document the calibration, calculate the combined measurement uncertainty of all influential items in the chain of comparisons going back to the SI and the level of confidence of this measurement uncertainty. The results of this process are documented in a calibration report which the customer receives along with the instrument when it is returned.