Transit Time FlowTransit Time transducers typically operate in the 1-2 MHz frequencies. Higher frequency designs are normally used in smaller pipes and lower frequencies for large pipes up to several meters in diameter. So operators must select transducer pairs/frequencies according to the application. Doppler transducers usually operate at 640 kHz to 1 MHz frequencies and work on a wide range of pipe diameters.Transit Time flowmeters measure the time it takes for an ultrasonic signal transmitted from one sensor, to cross a pipe and be received by a second sensor. Upstream and downstream time measurements are compared. With no flow, the transit time would be equal in both directions. With flow, sound will travel faster in the direction of flow and slower against the flow. Because the ultrasonic signal must cross the pipe to a receiving transducer, the fluid must not contain a significant concentration of bubbles or solids. Otherwise the high frequency sound will be attenuated and too weak to traverse the pipe.
Doppler FlowDoppler flow meters measure flow from outside a pipe with a clamp-on sensor. Doppler meters continuously transmit high frequency sound that travels through the pipe wall and into the flowing liquid. Sound is reflected back to the sensor from solids or bubbles in the fluid. If the fluid is in motion, the echoes return at an altered frequency proportionate to flow velocity. Doppler flow meters continuously measure this frequency shift to calculate flow. The Doppler technique only works on liquids which contain solids or gas bubbles to reflect its signal. These are “difficult” liquids that may damage regular flow meters: slurries, sludge, wastewater, abrasives, viscous and corrosive chemicals. Because the sensor mounts on the outside of the pipe, there is no pressure drop and no obstruction to flow.
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