| 摘要 |
A system for ultrasonic flow measurement operates by transmitting a signal along one or more paths (1, 2) through a fluid to determine average flow along the paths (1, 2) from the transit time of the signal. These measurements are then combined to define an invariant, from which a meter factor may be set to yield average flow across the conduit (C). Flow in the conduit (C) is described by one or more first variables related to a coordinate system, and by at least one second flow-related variable such as friction factor or Reynolds number unrelated to a coordinate system, wherein the first variables are separable or substantially separable from the second variables. The method includes determining average velocities of the fluid on two paths (1, 2), or taking a first path measurement and at least one point measurement, and combining the first path (1) measurement and the second measurement (2) to form an expression independent of the second variable. In one embodiment, the first and second path (1, 2) measurements are combined in accordance with the velocity relationships given by the Pao equation to produce a flow measurement which is independent of the friction factor. A single calibration then allows the meter to be used on diverse conduits with different wall roughness or friction factor.
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