Magnetic Flow Meter Principle

The purpose of a flowmeter system is to measure the movement, or flow rate, of a given volume of fluid and to express it through an unambiguous electrical signal. A standard flowmeter consists of a series of linked components that transmits signals indicating the volume, rate of flow, or volume of fluid moving through a specific channel, and it ideally functions with minimal interference from environmental conditions. A magnetic flowmeter is a relatively noninvasive measuring device that is well-suited for flow rate analysis due to its straightforward range of functions.

A magnetic or electromagnetic flowmeter can be installed in a comparatively simple fashion insofar as an existing pipe network can be converted into a measurement system by applying external electrodes and magnets. These flowmeters can track forward and reverse flow and are minimally affected by flow disturbances related to viscosity or density. They are linear devices that can be calibrated to measure a range of different variables while also reacting to changes in fluid movement. Progress in flowmeter technology has focused on producing devices that are smaller, less expensive, and capable of making more refined measurements.

Faraday’s Law

Like many other electrical devices, magnetic flowmeters function under the principles of Faraday’s law of electromagnetic induction. According to this law, a conductor that passes through a magnetic field produces voltage proportional to the relative velocities between the magnetic field and the conductor. The law can be applied to flowmeter systems because many fluids are conductive to a certain degree. The amount of voltage they generate as they move through a passage can be transmitted as a signal measuring quantity or flow characteristics.

The functional range for a flowmeter system is based on the movement of a conductor perpendicular to a magnetic field. For example, as a conductor of a certain length moves through a magnetic field with a specific flux density, it remains perpendicular to the field along the X, Y, and Z axes, producing a voltage across both ends of the conductor. This voltage will equal the conductor length times the field flux density and velocity. Faraday’s law extends to flow measurement because the conductor length in a fluid will equal the inside diameter of the flowmeter itself, and the basic formulas of electromagnetic induction can thus be applied to liquid flow rates.

For more information on Faraday’s law, please visit HyperPhysics.

Velocity and Voltage

When a flowmeter is installed and activated, its operations begin with a pair of charged magnetic coils. As energy passes through the coils, they produce a magnetic field that remains perpendicular to both the conductive fluid being measured and the axis of the electrodes taking measurements. The fluid moves along the longitudinal axis of the flowmeter, making any generated induced voltage perpendicular to the field and the fluid velocity. An increase in the flow rate of the conductive fluid will create a proportionate increase the voltage level.

Flow Profiles

Fluid movement within a flowmeter system can be characterized as square, with a turbulent fluid velocity; distorted, with weak upstream flow; or parabolic, with a laminar velocity. But regardless of the profile, a magnetic flowmeter will provide the average voltage from a metering cross-section, so that the signal transmitted to operators tends to closely reflect the average velocity of the flowing liquid. Given a fixed pipe diameter and a constant magnetic field, induced voltage will only correlate to fluid velocity. If the fluid has sensors attached to a circuit, the voltage will create a current that can be translated as an accurate flow rate measurement.

Although flowmeters are designed to provide as close of a linear connection between voltage and flow as possible, there are numerous factors which may disrupt this relationship. Possible sources of interference include:

Unintended extra voltage in the processing liquid. Electromechanical voltage accidentally induced in the electrodes or the fluid. Capacitive coupling between the signal circuit and the power source. Inductive coupling between the magnetic components in the system. Capacitive coupling between connective leads.

These and similar sources of external voltage or noise can disrupt normal flow measurement, so it may be worthwhile to set up a flowmeter under conditions as carefully controlled as possible.