Contributions of precision engineering to the revision of the SI

Abstract

All measurements performed in science and industry are based on the International System of Units, the SI. It has been proposed to revise the SI following an approach which was implemented for the redefinition of the unit of length, the metre, namely to define the SI units by fixing the numerical values of so-called defining constants, including c, h, e, k and NA. We will discuss the reasoning behind the revision, which will likely be put into force in 2018. Precision engineering was crucial to achieve the required small measurement uncertainties and agreement of measurement results for the defining constants

Dimensional artefacts to achieve metrological traceability in advanced manufacturing

“…For example, in ancient Egypt the royal cubit (the length of the Pharaoh's forearm) served as the length reference and wooden copies of it (see Fig. 4-a) were manufactured and maintained by civil servants and distributed for practical length measurements in daily life [38]. These copies or dimensional artefacts were directly traceable to the reference, the royal cubit, and thus guaranteed the comparability and reliability of length measurements performed in ancient Egypt.…”

“…In France, the metric system was established in 1799 based on the Mètre des Archives, an end standard made from platinum whose length was defined to be the 1⁄10 000 000 part of the quarter meridian of the earth, which was assumed as a stable and universal dimension [38]. In 1875, the metre convention was signed by 17 member states which laid ground for the international acceptance of the metric system of units and at the first General Conference of Weights and Measures (CGPM) in 1889, the length of the international metre prototype made from Pt-Ir alloy, or the spacing of its line engravings at ice water temperature, was defined as the length unit traceable to the former Mètre des Archives.…”

“…Following this resolution, the numerical values of the so-called defining constants of the SI are fixed. The contributions from precision manufacturing and precision dimensional metrology for experiments to determine the numerical values of the above natural constants with the smallest possible uncertainties in the current SI were described in a previous CIRP keynote [38].…”

An insight on optical metrology in manufacturing

“…In the current SI system of units, some physical quantities are associated with each other, as shown in figure 18 [113]. Optical sensors for the measurement of length and displacement therefore contribute to the measurement of other physical quantities, such as mass and temperature [114]. Figure 19 shows an example of optical displacement sensors used in a system for the measurement of mass, which is referred to as the Kibble balance [115,116].…”

“…Also, another optical system composed of a corner cube, a laser source and a position-sensitive detector (PSD) is employed to measure the motion of the air-bearing shaft in the horizontal direction. Optical form interferometry, which is described in the following section of this paper, is also contributing to the establishment of a mass standard for the SI base units [114,117].…”