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An ultrasonic flow meter uses Doppler meters to measure the shift of a frequency signal that is sent into the liquid flowing through a pipe. From the diagram below it can be noted that the transmitting element sends a signal with the given frequency into the fluid flow. Bubbles in the fluid or any suspeneded solids in the fluid reflect back to a receiver. The receiving element, also called a Doppler meter, is placed a short distance downstream and it detects the the frequency as the fluid flows past it. A special circuit called a time-of-travel meter measures the time delay or shift in frequency that is caused by the fluid flow. The faster the fluid flows, the more the frequency is shifted. This type of flow meter is useful because it can measure the flow without creating any obstruction. it's important to understand that some amount of suspended solid or bubbles must be present in the flow to get the best reflection of the signal.
This type of meter is also widely used as a portable flow meter. The portable flow meter is used to detect flow when troubleshooting complex hydraulic systems. When a hydraulic system starts to fail, it generally leaks fluid past an inoperable valve back to the reservoir. Since the hydraulic piping is made of metal or hoses, it can be difficult to tell how much fluid is being bypassed, so an ultrasonic flow meter can be temporarily placed around each segment of the hydraulic piping and the amount of flow in the main lines and in the return lines to the reservoir can be determined.
The Ultrasonic flowmeter belongs in a category of flowmeters called Velocity Flow Meters. Velocity flow meters use the change in velocity (speed) that occurs when fluid flow changes to measure the amount of flow. Velocity meters can be used in systems that are not adversely affected by changes in viscosity. Examples of velocity flow meters include the paddlewheel flowmeter, the turbine flowmeter, the vortex flowmeter, the electromagnetic flowmeter, and the ultrasonic (Doppler) flow meter.