Hot-Wire Anemometer

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OTHER FLOW METERS Flow Detectors To 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  of gears, which is calibrated to indicate total system flow.   A variety of accessories, such as automatic count resetting devices, can be added to the fundamental mechanism, which perform functions in addition to measuring the flow. Hot-Wire Anemometer The hot-wire anemometer, principally used in gas flow measurement, consists of an electrically heated, fine platinum wire which is immersed into the flow.  As the fluid velocity increases, the rate of heat flow from the heated wire to the flow stream increases.   Thus, a cooling effect on the  wire  electrode  occurs,  causing  its  electrical  resistance  to  change.    In  a  constant-current anemometer, the fluid velocity is determined from a measurement of the resulting change in wire resistance.   In a constant-resistance anemometer,  fluid velocity is  determined from the current needed to maintain a constant wire temperature and, thus, the resistance constant. Electromagnetic Flowmeter The electromagnetic flowmeter is similar in principle to the generator.  The rotor of the generator is replaced by a pipe placed between the poles of a magnet so that the flow of the fluid in the pipe  is  normal  to  the  magnetic  field.    As  the  fluid  flows  through  this  magnetic  field,  an electromotive  force  is  induced  in  it  that  will  be  mutually  normal  (perpendicular)  to  both  the magnetic field and the motion of the fluid.   This electromotive force may be measured with the aid of electrodes attached to the pipe and connected to a galvanometer or an equivalent.   For a given 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. Ultrasonic Flow Equipment Devices such as ultrasonic flow equipment use the Doppler frequency shift of ultrasonic signals reflected   from   discontinuities   in   the   fluid   stream   to   obtain   flow   measurements. These discontinuities can be suspended solids, bubbles, or interfaces generated by turbulent eddies in the flow stream.  The sensor is mounted on the outside of the pipe, and an ultrasonic beam from a piezoelectric crystal is transmitted through the pipe wall into the fluid at an angle to the flow stream.   Signals  reflected  off  flow  disturbances  are  detected  by  a  second  piezoelectric  crystal located  in  the  same  sensor.    Transmitted  and  reflected  signals  are  compared  in  an  electrical circuit, and the corresponding frequency shift is proportional to the flow velocity. IC-04 Page 10 Rev. 0


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