Water Well Journal
Issue link: https://read.dmtmag.com/i/655200
of which must be calculated and machined for each applica- tion. The differential pressure flowmeter follows the principle of the annubar that was outlined in last month's column. Electromagnetic flowmeters (Figure 2) can handle most liquids and slurries, provided the material being metered can conduct electricity. Major components are the flow tube (primary element), which mounts directly in the pipe. The pressure drop across the meter is the same as it is through an equivalent length of pipe because there are no moving parts or obstructions to the flow. Electromagnetic flowmeters operate on Faraday's Law of Electromagnetic Induction: a voltage will be induced when a conductor moves through a magnetic field. In this case, the liquid serves as the conductor and the magnetic field is cre- ated by energized coils outside of the flow tube. The amount of voltage produced is directly propor- tional to the rate of flow. Two electrodes mounted in the pipe wall detect the volt- age, which is measured by the second- ary element. Electromagnetic flowmeters have major advantages in many potable and non-potable water applications. They can measure difficult and corrosive liq- uids and slurries and can measure for- ward as well as reverse flow with equal accuracy. This makes them a meter of choice for many aquifer storage and re- covery (ASR) systems. They also have no moving parts to wear. Electromagnetic flowmeters are also available as insertion meters for retrofit applications. Magnetic insertion meters use a probe inserted into the pipe. They have to be installed carefully as the di- ameter of the pipe affects the depth the probe has to extend into the pipe. Inser- tion meters work well when expensive major modifications would be required to install a full pipe meter and are in common use today in numerous water and waste- water applications. Ultrasonic flowmeters (Figure 3) in- clude Doppler meters and time-of-travel (or transit) meters. Doppler meters measure the frequency shifts caused by liquid flow. Two transducers are mounted in a case attached to one side of the pipe. A signal of known fre- quency is sent into the liquid to be measured. Solids, bubbles, or any other discontinuity in the liquid cause the pulse to be reflected to the receiver ele- ment. Because the liquid causing the re- flection is moving, the frequency of the returned pulse is shifted. The frequency shift is proportional to the liquid's ve- locity. A portable Doppler meter capable of being operated on AC power or from a rechargeable power pack is available and commonly used for field testing of pumps and pipelines. The sensing heads are clamped to the outside of the pipe and the instrument is ready to be used, as shown in the figure. A set of 4 to 20 mA output terminals permits the unit to be connected to a conventional strip chart recorder or other remote device. Time-of-travel meters have transducers mounted on each side of the pipe. The configuration is set so the sound waves traveling between the devices are at a 45° angle to the direc- tion of liquid flow. The speed of the signal traveling between the transducers increases or decreases with the direction of transmission and the velocity of the liquid being measured. A WWJ April 2016 39 Twitter @WaterWellJournl ENGINEERING continues on page 40 Make your mission easier. Call us for special pricing on missionary packages. lonestardrills.com • 800.227.7515 Y o ur M i ssi on , Our Jo ur n e y The road to clean water is full of challenges. Lone Star Drills are portable, reliable and rugged. Travel over rough terrain with our durable, lightweight trailers, and drill as deep as 300 feet through the clay, sand and hard rock formations. You can rely on us for everything you need from tools to training, because getting you through your mission is our journey every day.