The effect of motion to a sound source, relative to the frequency shift of the sound, was first observed and described by Christian Johann Doppler. A practical application for flow measurements is made by sending bursts of sound waves through a fluid filled pipe. The measurement of flow is based on the principle that sound waves travelling in the direction of the fluid flow travel faster than sound waves travelling against the fluid flow. At zero velocity, the transit time differential or delta T is zero. Since ultrasonic signals can penetrate solid materials, the transducers can be mounted onto the outside of the pipe. Ultrasonic flow meters use one of these two principles; The Doppler Effect, or Time of Flight measurement.
The Doppler Effect Flow meter uses reflected ultrasonic sound waves to measure the velocity of particles in the fluid. By measuring the frequency shift between the ultrasonic frequency source, the fluid carrier, and the receiver, the relative fluid velocity is measured. This method requires that there are reflecting particles in the fluid. This method is not suitable for clear liquids. Since the Doppler flow meters performance is so highly dependent on the physical properties of the fluid, such as the sonic conductivity, particle density, and flow profile, this method is only suitable for applications that do not require a high accuracy.
With the Time of Flight Ultrasonic Flow meter, the time for the sound to travel between a transmitter and a receiver is measured. This method is not dependent on the particles in the fluid. Two transmitters/receivers (transceivers) are located on each side of the pipe. The transmitters send pulsating ultrasonic waves in a predefined frequency from one side to the other. The difference in frequency is proportional to the average fluid velocity.
Ultrasonic flowmeters are chosen due to their; clear flow path, low pressure drop, corrosion resistance, and relatively low power consumption (compared to Magnetic flow meters). Typical accuracy about 1% of flow rate, although multi-beam ultrasonic meters can offer higher levels of accuracy. Clamp-on (or bolt-on) ultrasonic flow meters are available at very economical prices. Inline styles typically range from one thousand to five thousand dollars.
Reliable operation of ultrasonic flowmeters requires high frequency sound transmitted across the pipe. Liquid slurries with excess solids or with entrained gases may block the ultrasonic signals. Ultrasonic flow meters are not recommended for primary sludge, mixed liquor, aerobically digested sludge, dissolved air flotation thickened sludge and its liquid phase, septic sludge and activated carbon sludge.
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