Redefinition of the SI unit kilogram: METTLER TOLEDO explains all.
15 Nov 2018
METTLER TOLEDO has published a White Paper to help customers find out more about the reasons, methods involved and benefits of redefinition of 1 kilogram.
Revised definitions of four of the base units of the SI will come into force in May 2019. The redefinition of the ‘Kilogram’ ensures a stable definition for the unit kilogram, but many users will be unaware of the backgrounds and benefits of the revision.
The first General Conference of Weights and Measures (GCPM) defined the unit kilogram based upon the mass of the International Prototype Kilogram (IPK), known as the ‘Paris kilogram’ in 1889. The new definition was formally adopted November 16th, and it will be practically implemented from May 2019.
The kilogram had been the last measurement unit of the International System of Units (SI) to be defined by a man-made physical body, namely an artefact. However, the problem with this definition is that this physical body, although intended to have a stable mass, actually changes its mass over time. Since this body itself is the reference for all mass measurements, this change cannot be determined by any measurement.
From World Metrology Day 2019 (May 20) onwards, the new definition will be in force and the kilogram will be defined in terms of a combination of fundamental constants: the Planck constant, h, which is stable and universally accessible. Until that point in time, h was measured directly linked to the IPK. However, from this date, the definition simply changes its direction. This also implies that the change of definition will not introduce any (step) change in the moment of switching.
This major reexamination of the SI since its foundation in 1960 may be perceived as ground-shaking to the weighing world. However, the goal of this redefinition is to ensure long-term stability and traceability of the unit for mass by making it independent of the material artefact, without affecting calibration or measurements.