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PQS SI system units classes
The SI categorisation of units into into coherent and incoherent units 1 accepts some non-coherent units with conversion factors consisting of powers of 10 for use with the SI.( e.g kilometers, megawatts), while excluding other non-coherent units ( usually legacy units, generally with conversion factors that are not a simple power of 10). PQS clarifies the distinction between allowed and non allowed units by classifying allowed SI units as SI normative units and non allowed units as unit conversions. 2
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[1] The SI system identifies 2 classes of unit, coherent units and non-coherent units . The SI also acknowledges other units outside the SI which are associated with SI units by means of conversion factors, which are also non coherent units .
Coherent units consist entirely of some product of SI base_units. There is no use of conversion factors (including integer powers of 10).
Incoherent units are related to coherent units by means of a conversion factor which can convert their numeric value to some value expressed in coherent units.
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[2] The PQS SI implementation also defines the standard SI base units, which are a subset of normative_units.