Electromagnetic flow meters operate on Faraday's law of electromagnetic induction, which states that a voltage is induced when a conductor moves through a magnetic field. The liquid serves as the conductor; the magnetic field is created by energized coils outside the flow tube. When selecting electromagnetic flow meters, parameters include pipe diameter, mounting type, end fittings, performance specifications, output options, and special features. The ability to meter liquids with suspended solids (slurries) and monitor media temperature are also important considerations.
Electromagnetic flow meters differ in terms of pipe diameter and mounting type. Pipe diameter is the diameter of the process pipe to be monitored. Choices for mounting type include in-line, insertion type, and non-invasive. In-line flow meters are installed directly in the process line and typically require a straight run of pipe. By contrast, insertion-type devices are inserted perpendicular to the flow path. Usually, they require a threaded hole in the process pipe. Non-invasive electromagnetic flow meters do not require direct-mounting in the process flow and can be used in closed piping systems.
There are many types of end fittings for electromagnetic flow meters. Choices include clamp, compression, flanged, plain end, socket weld or union, threaded, and tube end or hose nipple. External clamp-on flow meters are non-invasive. Compression fittings tighten down a sleeve or ferrule over a joint to prevent leakage. Flanges are often circular or square and connected via bolting or welding. Plain-end devices can be inserted into the bell end of the connecting pipe. Threaded designs often use National pipe thread (NPT). Tube-end or hose-nipple fittings for electromagnetic flow meters are suitable for short pieces of pipe, usually with male threaded studs.
There are two types of output options for electromagnetic flow meters: electrical output and interface. Choices for electrical output include analog current, analog voltage, frequency and switch output. Interface options include serial or digital, and network or fieldbus. Analog current levels (transmitters) such as 4 – 20 mA are suitable for sending signals over long distances. Analog voltage outputs are simple, usually linear functions of the measurement. Frequency or modulated frequency outputs for electromagnetic flow meters include amplitude modulation (AM), frequency modulation (FM), sine waves, and pulse trains.
Electromagnetic flow meters differ in terms of features. Some devices can measure density, level, or temperature. Others feature audible or visual alarms, averaging or multi-insertion functions, controller functions, programmability, and recorder or totalizer functions. Electromagnetic flow meters that are designed for use in sanitary environments, such as in medical or food processing applications, are also available.