Orifices have been used to measure flowrate for thousands of years, but economic pressures recently have dictated a careful evaluation of the procedure. The National Bureau of Standards currently has two major research programs (funded by API and GRI) underway with the goals of increasing the accuracy and range of orifice measurements. The equations presented in this work represent a significant departure from those normally used. The new expressions are equally rigorous and possess many advantages compared with the conventional relationships. The new expression for mass flow rate measured by differential pressure contains no discharge coefficient, only fluid properties and observables.
