= Primary reference measurement procedure
A <primary reference measurement procedure> determines a result without calibration against a <measurement standard> for a quantity of the same kind. It can still depend on calibrated measurements of other quantities. The VIM example determines delivered liquid <volume> from a mass difference and liquid <mass density>, rather than comparing it with another calibrated delivered volume.<International vocabulary of metrology by JCGM>[VIM3 entry2.8, definition and example]{ref}
For an original ideal calculation, let the receiving vessel's <tare mass> be $m_t=12.000\,\mathrm{g}$ and its <gross mass> after delivery be $m_g=20.000\,\mathrm{g}$. Suppose the liquid's <mass density> at the measurement conditions is $\rho=0.8000\,\mathrm{g\,mL^{-1}}$. Then
$$
V=\frac{m_g-m_t}{\rho}=10.000\,\mathrm{mL}.
$$
With independent <standard uncertainties> $u(m_t)=u(m_g)=0.001\,\mathrm{g}$ and $u(\rho)=0.0004\,\mathrm{g\,mL^{-1}}$,
$$
u^2(V)\simeq\frac{u^2(m_g)+u^2(m_t)}{\rho^2}+\frac{(m_g-m_t)^2}{\rho^4}u^2(\rho),
$$
so $u(V)\simeq0.0053\,\mathrm{mL}$. These are stipulated values, not a recommended liquid or an actual calibration result.<Guidelines for Evaluating and Expressing the Uncertainty of NIST Measurement Results by Barry N. Taylor and Chris E. Kuyatt>[AppendixA, equationA-3]{ref}
The physical procedure would weigh the receiver before and after delivery, establish the liquid temperature and density, and include significant corrections in its <measurement model>. Evaporation, buoyancy and correlated mass readings can invalidate the simplified expression above. Primary describes the reference relation, not freedom from <measurement uncertainty> or the absence of any instrument calibration. Its primary status is therefore a claim about the complete procedure, not about this equation alone.
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