Per ISO 5167-2:2003 with Reader-Harris/Gallagher
Understand AGA Report 3 equations, discharge coefficient, beta ratio, and flow measurement
An orifice meter measures gas flow by creating a pressure drop across a thin plate with a precise bore hole. The flow rate is calculated from the differential pressure, pipe and orifice diameters, gas properties, and a discharge coefficient per AGA Report No. 3 / ISO 5167.
The Reader-Harris/Gallagher (RHG) equation calculates the orifice discharge coefficient as a function of Reynolds number, beta ratio, and tap configuration. It is the standard method in ISO 5167-2:2003 and provides improved accuracy over older empirical correlations.
The calculator supports flange taps (1 inch from plate faces), corner taps (at the plate surface), and D-D/2 taps (one pipe diameter upstream, half diameter downstream). Flange taps are most common in North American natural gas measurement.
ISO 5167 specifies a beta ratio (orifice bore / pipe ID) between 0.10 and 0.75 for flange taps. The optimal range for accuracy is 0.20 to 0.70, with lower beta ratios providing better measurement uncertainty at the cost of higher permanent pressure loss.