During 2010 and 2011, NIST piloted a 12-laboratory comparison of hydrocarbon liquid flow calibration standards spanning the range 3.8 L/min to 38 L/min. The laboratories were in mutual agreement within the expected 0.3 % uncertainty, which is approximately half as large as the differences measured in a similar 1988 comparison. The transfer standard (a pair of turbine flow meters in series) introduced an uncertainty of 0.17 % into the comparison. The comparison protocol used methods that were developed during international comparisons including: using uncertainty weighting to generate a best fit a comparison reference curve, using statistical criteria to remove discrepant results from the fit, assessing and including in the data analysis the uncertainty contributed by the transfer standard, and reporting a standardized degree of equivalence between the participants. Several laboratories used mixtures of propylene glycol and water (PG + W) instead of Stoddard solvent (the commonly used surrogate for jet fuel) because the PG + W mixtures are safer and cheaper to manage environmentally. This comparison and other studies show that there is no significant difference in the calibration results between Stoddard solvent and a PG + W mixture with the same kinematic viscosity. Therefore, NIST is changing its calibration fluid to PG + W and encourages other laboratories to do the same.
Cal Lab: International Journal of Metrology
comparison, flow, propylene glycol, Stoddard solvent, transfer standard, turbine