Measuring devise, usually located at a point where ownership is transferred, for example at the entry to a customer’s premises, city gate, national border or gas processing plant. Note that to determine the energy content of the gas it will be necessary to monitor the composition & hence calorific value of the gas. This is done at high pressure meter stations but not normally downstream of a city gate. Depending on the size of flow, meters may be: Flow meters, measuring gas throughput using a rotor which is made to revolve by the gas flowing through them; Ultrasound meters, more sophisticated, based on the principle that sound waves travel faster with than against the flow. By measuring the difference in travelling time between the two sets of measurements the gas flow rate can be measured to an accuracy of more than 0.5%; Orifice plate meters, older, and less accurate than other meters. Calculate flow rates by measuring the drop in pressure over a pierced obstructing plate placed in the pipeline. See also Telemetry.