OTHER FLOW METERSFlow DetectorsTo measure this flow, the motion of the shaft generates a cone with the point, or apex,down. The top of the shaft operates a revolution counter, through a crank and set ofgears, which is calibrated to indicate total system flow. A variety of accessories, suchas automatic count resetting devices, can be added to the fundamental mechanism, whichperform functions in addition to measuring the flow.Hot-WireAnemometerThe hot-wire anemometer, principally used in gas flow measurement, consists of an electricallyheated, fine platinum wire which is immersed into the flow. As the fluid velocity increases, therate of heat flow from the heated wire to the flow stream increases. Thus, a cooling effect onthe wire electrode occurs, causing its electrical resistance to change. In a constant-currentanemometer, the fluid velocity is determined from a measurement of the resulting change in wireresistance. In a constant-resistance anemometer, fluid velocity is determined from the currentneeded to maintain a constant wire temperature and, thus, the resistance constant.ElectromagneticFlowmeterThe electromagnetic flowmeter is similar in principle to the generator. The rotor of the generatoris replaced by a pipe placed between the poles of a magnet so that the flow of the fluid in thepipe is normal to the magnetic field. As the fluid flows through this magnetic field, anelectromotive force is induced in it that will be mutually normal (perpendicular) to both themagnetic field and the motion of the fluid. This electromotive force may be measured with theaid of electrodes attached to the pipe and connected to a galvanometer or an equivalent. For agiven magnetic field, the induced voltage will be proportional to the average velocity of the fluid.However, the fluid should have some degree of electrical conductivity.UltrasonicFlowEquipmentDevices such as ultrasonic flow equipment use the Doppler frequency shift of ultrasonic signalsreflected from discontinuities in the fluid stream to obtain flow measurements. Thesediscontinuities can be suspended solids, bubbles, or interfaces generated by turbulent eddies inthe flow stream. The sensor is mounted on the outside of the pipe, and an ultrasonic beam froma piezoelectric crystal is transmitted through the pipe wall into the fluid at an angle to the flowstream. Signals reflected off flow disturbances are detected by a second piezoelectric crystallocated in the same sensor. Transmitted and reflected signals are compared in an electricalcircuit, and the corresponding frequency shift is proportional to the flow velocity.IC-04 Page 10 Rev. 0
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