A proposal for three categories of units within the SI

Abstract

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In this letter we examine the organization of the traditional base units and defining constants established in the 2019 redefinition of the International System of Units (SI). For the next redefinition of the SI, which will accompany the anticipated redefinition of the candela and second, we propose an organizational change to improve clarity while maintaining practicality. We propose three distinct categories of units: The first category contains only the four base-measurement units comprised of the second, meter, kilogram, and ampere. The second category contains physiologically-relevant derived units. The third category contains the remaining units derived from the base units.