Instrumentation & Measurement Magazine 23-6 - 3

The Impact of the New SI in the
Measurement Sector
Alan Steele and Barry Wood

R

ecent changes to the world's measurement system
[1], [2] have been carefully designed to appear to
the general public, and even to most of the scientific community, as if nothing substantial has changed. The
same units exist, no new ones have been created, and the values of the properties of physical artifacts remain the same for
all but the most extreme accuracies. In fact, only the definitions
of the seven base units have been altered while keeping their
sizes virtually unchanged. And so it might appear that these
changes to our measurement system are simply a bit of tidying up and of no practical consequence to any except those
few metrologists responsible for realizing and maintaining
the measurement system at its highest levels of accuracy and
precision.
But this is just appearance, and in fact a wide range of measurement options have changed or become available. The
purpose of this review is to explain the changes to our measurement system that occurred in 2019 and to illustrate some
of the measurements that have become possible or more practical as a result. So while the 2019 changes to the SI certainly
impact metrologists, it is these new measurements that will
more broadly impact measurement science.

Our Measurement System
Decades of study and quiet debate have finally culminated in
official changes to the world's measurement system, the International System of Units, or the SI [1], [2]. The SI is a creation of
the Convention of the Metre [3]-an intergovernmental treaty
first signed in 1875 and used by essentially every country in
the world. The purpose of this treaty is to develop and promulgate a worldwide measurement system that is practical and
suitable for use for all measurements, from those in daily life to
even the most accurate in support of fundamental science. The
term SI was only adopted in 1960 but this measurement system began much earlier, at the beginning of the signing of the
Convention of the Metre with the construction of physical artifacts for the units of length and mass. These units were in turn
based on the even earlier metre kilogram second (MKS) system in use since Napoleonic times. The MKS system not only
September 2020	

specified the three base units for length, mass, and time, but
also their sizes. The meter was 1/1000000 of the quadrant of
the earth. The kilogram was the mass of a cubic decimeter of
water. The second was defined as 1/(24×60×60) of the rotation
time of the earth about its axis. Significant efforts were made to
quantify these unit sizes and to embody them into more practical artifacts [4].
Soon after the signing of the Convention of the Metre, the
then-new SI units of length and mass were established as the
meter and kilogram, and physical artifacts were commissioned to embody their size. The International Prototype of
the Kilogram (IPK) and the International Prototype of the Metre were constructed of a platinum iridium alloy (PtIr), and
while a number of artifacts were constructed, only one of each
was chosen as the defining international prototype. The mass
of the new kilogram and the length of the new meter were not
directly compared to the earlier definitions (quadrant of the
earth and cubic decimeter of water) but instead to the previous
Napoleonic artifacts: the older definitions serve to conceptually describe the size of the unit, but traceability in the SI was
to the new international prototypes. For mass and length a major difference between the MKS and SI systems was the apex
of traceability, but continuity of the size of these units was ensured during the selection of the new artifacts. The SI was
based on the belief and assumption that the properties of the
physical international prototypes were sufficiently stable for
all activities (Fig. 1).
Since 1875, the SI and its base units have undergone many
changes. Four additional base units have been incorporated:
the kelvin for thermodynamic temperature, the ampere for
electrical current, the candela for luminous intensity, and the
mole for amount of substance [2]. As well, the SI definitions of
the sizes of these base units have been successively refined to
achieve ever smaller uncertainties in their practical realization.
As examples, the second has been based on the rotation of the
earth about it axis and then on the rotation of the earth about
the sun. Similarly, the triple point of water, used to anchor the
thermodynamic temperature scale, has had its chemical and
isotopic compositions specified.

IEEE Instrumentation & Measurement Magazine	3
1094-6969/20/$25.00©2020IEEE



Instrumentation & Measurement Magazine 23-6

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