Building traceabilityWhat is traceability?
Property of the result of a measurement or of the value of a standard such that it can be related to stated references, usually national or international standards, through an unbroken chain of comparisons all having stated uncertainties (International Electrotechnical Vocabulary, term 311-01-15)
An unbroken chain of comparisons, all with stated uncertainties, is called a traceability chain and forms a basis for quality controlled measurements.The basic units are defined by international agreement in the SI-system as a choice of seven well-defined units which by convention are regarded as dimensionally independent: the metre, the kilogram, the second, the ampere, the kelvin, the mole, and the candela. http://www.bipm.org/en/measurement-units/ From these, all other units are derived by combining the base units according to the algebraic relations linking the corresponding quantities.
Realisations of basic units of measurement
The practical realisation of units for electrical quantities is based on two quantum-mechanical effects, the Josephson Effect for voltage and the Quantised Hall Effect for resistance. These permit interlaboratory measurements accurate to less than 10-9 in uncertainty. http://www.bipm.org/en/publications/mises-en-pratique/electrical-units.html The practice in our primary measurement laboratory is then to scale from the value given by the realisation to other values by using bridge techniques, which can be classical analogue bridges or state-of-art digital instruments.
Link from dc to ac quantities
To link AC measurement we heat a pure resistor with the AC quantity, measuring the temperature, connecting DC and adjusting the value until the same temperature is reached. This is a realisation of true rms value.
Link from low to high voltage
Scaling from voltages below 1 kV to the levels needed in high voltage is accomplished by a combination of methods. For AC voltage we have a very precise gas capacitor with a negligible voltage dependence (<10-6 change up to 350 kV), which permits extremely accurate scaling of ac quantities. For DC we have two DC dividers for 1000 kV of slightly different designs. Using a combination of low voltage measurements and high voltage comparisons we have established an uncertainty of 0.002%.
The CIPM Mutual Recognition Arrangement (CIPM MRA) is the framework through which National Metrology Institutes demonstrate the international equivalence of their measurement standards and the calibration and measurement certificates they issue. The outcomes of the Arrangement are the internationally recognized (peer-reviewed and approved) Calibration and Measurement Capabilities (CMCs) of the participating institutes. Approved CMCs and supporting technical data are publicly available from the CIPM MRA database (the KCDB).