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Ultrasonic Flow Meter Home Page | |||
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What is an 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 FLOW METER SELECTION The key questions which need to be answered before selecting an ultrasonic flow meter or doppler flowmeter are: 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. Clean Liquids As noted, the basic ultrasonic Doppler flowmeter requires that the liquid to be measured contain a minimum of at least 100 PPM of suspended solids or bubbles at least 100 microns or larger in size. The transducer frequency for these requirements is 1 megahertz. Lower frequencies require more PPM and a larger micron size. Until recently, 25 PPM of suspended solids or bubbles of at least 30 microns or larger, constituted the cleanest measurable application. The FD-7000 ultrasonic flowmeter has bridged the gap towards the use of cleaner liquid applications. Requiring no solids or entrained gas, this type of flowmeter can work on applications the typical ultrasonic Doppler meter cannot measure. The principles of operation are similar, with the exception that the Doppler signal is reflected by turbulent swirls in the liquid rather than by solid particles or bubbles. Because of this, the FD-7000 can only be used where there are no suspended solids or bubbles in the fluid. The unit is specially calibrated must be installed one to three pipe diameters downstream of a 90° elbow. The FD-7000 is one of the very few flowmeters not requiring a straight upstream pipe run. This product is not compatible with DI water applications. |
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