In a developed flow of a homogenous material, the velocity will depend only on radius. Placing a mag meter too soon after an elbow or other piping change will cause less dramatic but still significant errors. The layers shown have different cross-sections, so the actual net flow is distinctly non-zero. Since the voltage measured is a path integral, the contributions from each layer cancel out and the mag meter reads no flow. In this hypothetical example, most of the liquid is stationary but there are two equally thick layers moving in equal and opposite directions (shaded). The flow distribution illustrated below would be difficult-to-impossible to create in real life, but it provides a clear illustration of the meter’s sensitivity. Last time we looked at the measurement principle of mag meters, and here we’ll explore the effects of flow distribution. In the next post, we start to explore where problems might arise. Conductivity only affects equilibration time, not the final voltage. Note that the conductivity of the fluid does not appear in the equation. With a known magnetic field and pipe diameter, the flow is found. The cross product is perpendicular to its arguments in-line with the voltage measurement.Ī path integral of the electric field between electrodes gives the voltage. Equilibrium means no net force, so we get an electric field that is equal and opposite to the cross product of the flow velocity and the magnetic field. Assuming flow is uniform, and charges can move in response to the force, i.e., the material is conductive, they will find equilibrium within microseconds. ![]() Every charge going through the pipe experiences the Lorentz force. The mag meter creates a magnetic field across the pipe flow and measures the voltage perpendicular to both the pipe and the magnetic field. Still, they do not work in all scenarios and understanding the physics of the measurement is key for anticipating and avoiding errors and failures. This post was originally published in four parts which have been combined below.Įlectromagnetic flow meters, or mag meters, are the most robust and generally applicable flow meters for slurries.
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