Introduction to Ultrasonic Doppler Flow Meters

What is a Doppler Ultrasonic Flow Meter?

An ultrasonic flowmeter (non-intrusive Doppler flow meters) is a volumetric flow meter which requires particulates or bubbles in the flow. Ultrasonic flowmeters are ideal for wastewater applications or any dirty liquid which is conductive or water based. Ultrasonics flowmeters will generally not work with distilled water or drinking water. Aerations would be required in the clean liquid applications. Ultrasonic flowmeters are also ideal for applications where low pressure drop, chemical compatibility, and low maintenance are required.


PRINCIPLE OF OPERATION

The basic principle of operation employs the frequency shift (Doppler Effect) of an ultrasonic signal when it is reflected by suspended particles or gas bubbles (discontinuities) in motion. This metering technique utilizes the physical phenomenon of a sound wave that changes frequency when it is reflected by moving discontinuities in a flowing liquid. Ultrasonic sound is transmitted into a pipe with flowing liquids, and the discontinuities reflect the ultrasonic wave with a slightly different frequency that is directly proportional to the rate of flow of the liquid (Figure 1). Current technology requires that the liquid contain at least 100 parts per million (PPM) of 100 micron or larger suspended particles or bubbles.


ULTRASONIC DOPPLER FLOW METER SELECTION

The key questions which need to be answered before selecting an ultrasonic flow meter or doppler flowmeter are:

Does the liquid have particulates of 100 ppm of 100 microns in size?

Do you require a handheld or continuous process monitor?

Do you require an analog output?

What is the minimum and maximum flow rate for the flow meter?

What is the minimum and maximum process temperature?

What is the minimum and maximum process pressure?

What is the size of the pipe?

Is the pipe always full?

DESIGN VARIATIONS

Clamp-on ultrasonic flow meters come in either single or dual-sensor versions. In the single-sensor version, the transmit and receive crystals are potted into the same sensor body, which is clamped onto a single point of the pipe surface (Figure 4-8). A coupling compound is used ultrasonically connect the sensor to the pipe. In the dual sensor version, the transmit crystal is in one sensor body, while the receive crystal is in another. Clamp-on Doppler flowmeters are subject to interference from the pipe wall itself, as well as from any air space between the sensor and the wall. If the pipe wall is made of stainless steel, it might conduct the transmit signal far enough so that the returning echo will be shifted enough to interfere with the reading. There are also built-in acoustic discontinuities in copper, concrete-lined, plastic-lined, and fiberglass-reinforced pipes. These are significant enough to either completely scatter the transmitted signal or attenuate the return signal. This dramatically decreases flowmeter accuracy (to within only ±20%), and, in most cases, clamp-on meters will not work at all if the pipe is lined.

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