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The K value obtained by water testing might not be valid for non-Newtonian fluids (with velocity-dependent viscosity) or magnetic slurries (those containing magnetic particles). Magmeters can measure flow in both directions, as reversing direction will change the polarity but not the magnitude of the signal. For this reason, flowtubes are usually calibrated at only one velocity. The K value thus obtained is valid for any other conductive liquid and is linear over the entire flowmeter range. Manufacturers determine each magmeter's K factor by water calibration of each flowtube. Compensation is also provided by shaping the magnetic coils such that the magnetic flux will be greatest where the signal weighing factor is lowest, and vice versa. The velocity differences at different points of the flow profile are compensated for by a signal-weighing factor. Because the magnetic field density and the pipe diameter are fixed values, they can be combined into a calibration factor (K) and the equation reduces to: If a conductive fluid flows through a pipe of diameter (D) through a magnetic field density (B) generated by the coils, the amount of voltage (E) developed across the electrodes-as predicted by Faraday's law-will be proportional to the velocity (V) of the liquid. A pair of magnetic coils is situated as shown in Figure 4-1, and a pair of electrodes penetrates the pipe and its lining.

The magnetic flowmeter consists of a non-magnetic pipe lined with an insulating material. The newer designs have reduced that requirement a hundredfold to between 0.05 and 0.1. Early magmeter designs required a minimum fluidic conductivity of 1-5 microsiemens per centimeter for their operation. Magmeters can detect the flow of conductive fluids only. The operation of magnetic flowmeters is based on Faraday's law of electromagnetic induction. And today's ultrasonic flowmeters owe their successful application to sophisticated digital signal processing. Vortex meters depend on piezoelectric sensors to detect vortices shed from a stationary shedder bar. , for example, are the most directly electrical in nature, deriving their first principles of operation from Faraday's law.

All have no moving parts (well, maybe vibrating), are relatively non-intrusive, and are made possible by today's sophisticated electronics technology. While the flow measurement technologies discussed in this chapter-magnetic, vortex, and ultrasonic-are neither exclusively nor exhaustively electronic in nature, they do represent a logical grouping of flow measurement technologies.

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