An independent set of units of measurement that forms the basis of a basis of units. In the SI unit system (Système International d’Unités), there are seven fundamental units: kilogram, metre, second, kelvin, mole, ampere, and candela. Derived units are also acceptable in the SI system and include the units for pressure as newtons per square metre (Nm−2) with base units kgm−1s−1. The pascal (Pa) is also acceptable. In British Imperial units, the units are foot, pound, second. Many process instruments are still to be found with these units. See f.p.s. engineering units.