is assumed to be low compared with that of the fluid. Variations of electrical 

 conductivity can generally be neglected if the impedance of the measuring instru- 

 ment is high compared with the internal impedance of the meter. 



B 



ELECTRODES 



Figure 8. Transverse magnetic field configuration, electromagnetic flowmeter. 



DESIGN PARAMETERS 



The most important design pai'ameters include choice of ac magnetic field 

 frequency; length and diameter of pipe; choice of materials; effects of meter length 

 on magnetic field uniformity; methods of reducing and compensating unwanted ac 

 pickup, and use of electrodes or coils for measuring induced voltage. One meter 

 used in oceanography has a faired housing around the tube. 



ADVANTAGES 



The induction flowmeter used no moving or intricate parts. The method is 

 inherently capable of measuring direction as well as magnitude of flow. Linearity 

 is good; errors of less than one percent appear possible with careful design and 

 proper use. The output is independent of the conductivity of the fluid if the mea- 

 suring instrument impedance is high compai'ed with the internal impedance of the 

 meter. The output is therefore practically independent of changes in temperature, 

 pressure, and density of the liquid. Without moving parts, upper frequency limits 

 are high and the device can measure turbulent flow. In circular transverse field 

 meters, concentric deposits of solid matter on the pipe walls, if they have the 

 same conductivity as the fluid, do not change the calibration.^^ The simple 

 mechanical design allows for easy cleaning. 



16 



