The Performance of the Ultrasonic Flowmeter in Complex Velocity Profiles

The principle of operation of the ultrasonic flowmeter, detecting phase shifts or pulse transit times, is examined. It is shown both theoretically and experimentally that the sensitivity of the flowmeter depends on the fluid velocities averaged along the path of the ultrasound between the transducers. The instrument therefore overestimates the net fluid velocity by 33 percent in case of steady laminar flow and by approximately 7 percent in case of steady turbulent flow, provided the ultrasound traverses the entire lumen of the (cylindrical) vessel. The signal obtained in case of oscillatory flow generally is larger in amplitude than and lagging in phase behind the true flow. The errors are between 0 and 33 percent and between 0 and 7°, respectively. The effect of this on natural blood flow signals is discussed in detail. It is concluded that the flowmeter detects the waveform of aortic flows with a fidelity which should be satisfactory in most cases.