Reference Instrument
Metrological references that possess the highest accuracy and do not rely on other standards for their values define the base units of measurement. These instruments, known as primary measurement standards, are maintained by national metrology institutes and are used to calibrate secondary standards. They realize their units directly from physical constants, such as the speed of light or the Planck constant, ensuring absolute accuracy and long term stability.
This direct realization eliminates dependency on any other physical artifact.
Traceability Path
Calibration of industrial sensors depends on an unbroken chain that leads back to these primary measurement standards. Working standards in industrial labs are calibrated against secondary standards, which are in turn calibrated against primary ones. Each step up the chain reduces the measurement uncertainty.
Environmental Control
Operating these high level standards requires an extremely stable environment with strict controls on temperature, humidity, and vibration. Even minor environmental changes can alter the physical behavior of the primary reference system.
System Verification
Inter-laboratory comparisons verify the agreement between primary references maintained by different nations. Metrologists transport highly stable transfer standards between laboratories to perform these checks. This process ensures that a measurement made in one country is equivalent to a measurement made in another.
