Doppler Shift Ultrasonic Flowmeter

Measuring with Doppler shift ultrasonic flowmeter

A fundamental requirement of Doppler shift ultrasonic flowmeter is the presence of scattering elements within the flowing fluid which deflect the ultrasonic energy output from the transmitter such that it enters the receiver. These elements can be provided by solid particles, gas bubbles or eddies in the flowing fluid. The scattering elements cause a frequency shift between the transmitted and reflected ultrasonic energy, and measurement of this shift enables the fluid velocity to be inferred.

The instrument consists essentially of an ultrasonic transmitter—receiver pair clamped on to the outside wall of a fluid-carrying vessel. Ultrasonic energy consists of a train of short bursts of sinusoidal waveforms at a frequency between 0.5 MHz and 20 MHz. This frequency range is described as ultrasonic because it is outside the range of human hearing.

The electronics involved in Doppler shift flowmeters is relatively simple and therefore cheap. Ultrasonic transmitters and receivers are also relatively inexpensive, being based on piezoelectric oscillator technology. As all of its components are cheap, the Doppler shift flowmeter itself is inexpensive. The measurement accuracy obtained depends on many factors such as the flow profile, the constancy of pipe-wall thickness, the number, size and spatial distribution of scatterers, and the accuracy with which the speed of sound in the fluid is known. Consequently, accurate measurement can only be achieved by the tedious procedure of carefully calibrating the instrument in each particular flow measurement application. Otherwise measurement errors can approach ±10% of the reading, and for this reason Doppler shift flowmeters are often used merely as flow indicators, rather than for accurate quantification of the volume flow rate.

Versions are now available which avoid the problem of variable pipe thickness by being fitted inside the flow pipe, flush with its inner surface. An accuracy of ±0.5% is claimed for such devices. Other recent developments are the use of multiple-path ultrasonic flowmeters which use an array of ultrasonic elements to obtain an average velocity measurement which substantially reduces the error due to non-uniform flow profiles. There is a substantial cost penalty involved in this, however.

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