The FMG prefers to present flow meter calibration results in a dimensionless format that takes
into account the physical model for the flow meter type. The dimensionless approach helps
facilitate accurate flow measurements by the flow meter user even when the conditions of usage
(i.e., gas type, temperature, pressure) differ from the conditions during calibration. For example,
for a CFV calibration, the calibration report will present Reynolds number and discharge
coefficient, and for a laminar flow meter, a report presents the viscosity coefficient and the flow
coefficient [8]. However, we point out that there may be additional uncertainties introduced
when the dimensionless approach is used to extrapolate a NIST calibration to conditions or gases
that were not specifically tested. Unless special provisions are made between NIST and the
customer, it is the customer’s responsibility to determine any additional uncertainty when using
the dimensionless approach to extend a NIST calibration beyond the measured range.