Orifice meters are built to rigorous standards that can achieve 0.5percent uncertainty of the discharge coefficient (Cd) without the need for any flow calibration. This is achieved by relying on a vast array of empirical data collected in several test facilities over many years. The standards specify the orifice plate, the fitting, the pressure taps and the meter tube, to ensure that geometric and dynamic similarity is maintained with the empirical database. Daniel had been supplying orifice meters for 70 years when they brought the first gas ultrasonic meter to market. Users who were familiar with specifying meter tubes for orifice plates just continued applying the same specification to ultrasonic meters. However, ultrasonic meters differ from orifice plates in many respects: (1) No detailed construction standards exist (2) Performance is not based on similarity with empirical data (3) They are less sensitive to velocity profile changes (4) There are no local pressure tap effects (5) Calibration is common to achieve < 0.5percent uncertainty This paper discusses these differences and shows why meter tubes for multi-path ultrasonic meters do not need to meet the same strict specifications as orifice plates.
展开▼
