Measurement is the basis of commerce between producers, royalty owners, transporters, process plants, marketers, state and federal governmental authorities, and the general public. In fact, accurate measurement of hydrocarbon fluids has a significant impact on the Gross National Product of exporting and importing countries, the financial performance and asset base of global companies, and the perceived efficiency of operating facilities. The need for accurate fiscal measurement is obvious. Given the present or future levels of the cost of these critical resource materials one can quickly quantify the material and economic value unaccounted for that is associated with each ± 0·01 per cent systematic uncertainty that might unknowingly exist in the measurement systems for these materials. For reasons such as these, it is essential that fluid quantity and flowrate measurements are precise and accurate with minimal bias errors. Furthermore, it is incumbent upon those involved in fluid custody transfer to establish and maintain the traceability chains that link their measurements to appropriate domestic and international standards. In this manner, fiscal transfer of fluids can be done equitably with the confidence of both seller and buyer alike. Accurate flow measurement is defined as measurement with a low uncertainty. Stated another way, accurate flow measurement requires maximum absolute accuracy and high precision. An important objective is to minimize the bias error associated with the measurements. These errors are best minimized through the use of primary or secondary calibration systems