A primary standard is derived from the 7 SI base units. The 7 SI base units (International System of Units) are length; time, mass, temperature, electric current, amount of a substance (moles) and luminous intensity. There are only 7 basic parameters used by national laboratories with international consensus as to their values.
Other units, called SI derived units, are obtained from a system of equations involving the 7 SI base units. These SI derived units include volume and pressure (a combination of length, mass, and time).
With no SI base unit for gas flow, it is necessarily a derived measurement. Volumetric flow consists of volume divided by time. To obtain volumetric flow, most national laboratories use provers similar to Mesa’s primary piston provers, measuring the time it takes to displace a known volume of gas. These instruments are classified as primary because their readings are directly derived from the SI units.
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