Flow measurement is perhaps of greater importance to the oil and gas industry than it is to any other industry and a good understanding of the metering process remains central to the efficient production of hydrocarbons.  For example, inter-well allocation, custody transfer, gas flaring and produced water discharge all require metering.  The reasons for flow measurement vary from quality control, revenue calculation to environmental regulation.  The required accuracy of flow measurement varies with application from better than 1% for fiscal metering, up to 10% for environmental regulation.