Primary measurement standard

A primary measurement standard is a measurement standard established by a primary reference measurement procedure, or an artifact selected by convention. A primary reference measurement procedure obtains its result without calibration against a standard of the same kind of quantity; it may still require measurements of other kinds of quantity.^[ref]

For an original ideal pressure realization, independently established force $F=10.000\mathrm{N}$ acts on an effective piston area $A=1.0000\times 10^{-4}{\mathrm{m}}^2$. The measurement model givesIf the inputs have independent standard uncertainties $u(F)=0.010\mathrm{N}$ and $u(A)=2.0\times 10^{-7}{\mathrm{m}}^2$, first-order propagation of uncertainty givesso $u(p)\simeq 224\mathrm{Pa}$. This is an illustrative model, not a complete piston-gauge uncertainty budget: a physical realization must also account for effects such as piston deformation and friction.^[ref]

The essential distinction is how the pressure value is established. Merely comparing a gauge with another pressure gauge does not make it a primary measurement standard. Primary status also does not mean zero measurement uncertainty.