Transit-time flowmeters are known, and are the flowmeter of choice for most fluids. See for example, U.S. Pat. Nos. 4,232,548 and 3,869,915. The disclosures of these and all other patents and prior art mentioned herein are expressly incorporated by reference. However, up to the present, transit-time flowmeters have had disadvantages due to their sensitivity to operating conditions as well as the need for close acoustic matching between their transducers and the pipe in which flow is measured
These flowmeters employ transducers for passing ultrasonic pulses through the wall of a conduit and then receiving the transmitted pulses at a point along the conduit spaced from the transmitting transducer, either on the opposite side of the conduit, or on the same side of the conduit. In the former case, the acoustic energy passes directly through the conduit walls and the flowing liquid, whereas in the latter case, the acoustic energy is reflected by the opposite internal wall of the conduit. Circuitry processes the energy received at the receiving transducer and produces a display of flow rate.
The transducers are clamped to the exterior surface of a pipe and spaced along the pipe axis. They transmit and receive ultrasonic energy through the liquid in either the upstream or the downstream direction. Two measurements are made, namely the difference between the upstream and downstream travel time (Δt) and the travel time in the liquid (tL). These parameters are measured by circuitry and used to compute the liquid flow rate.