Magnetic flowmeters use Faraday’s Law of Electromagnetic Induction. According to this principle, a voltage is generated in a conductive medium when it passes through a magnetic field. This voltage is directly proportional to the density of the magnetic field, the length of the conductor, and the velocity of the conductive medium. In Faraday’s Law, these three values are multiplied together, along with a constant, to yield the magnitude of the voltage.
Magnetic flowmeters use wire coils mounted onto or outside of a pipe. A voltage is then applied to these coils, generating a magnetic field inside the pipe. As the conductive liquid passes through the pipe, a voltage is generated and detected by electrodes, which are mounted on either side of the pipe. The flowmeter uses this value to compute the flowrate.
Magnetic flowmeters are used to measure the flow of conductive liquids and slurries, including paper industry pulp slurries and black liquor (produced during wood to paper pulp process). Their main limitation is that they cannot measure hydrocarbons (which are nonconductive), and hence are not widely used in the petroleum industry. “Magmeters,” as they are often called, are highly accurate and do not create pressure drop. Their initial purchase cost is in the medium range, and most magmeters are priced lower than equivalent Coriolis meters.
AC vs. DC
When magnetic flowmeters were first introduced, many had coils powered by continuous alternating current (AC). These devices had the disadvantage that they were subject to noise that interfered with the proper reading of the meter. As a result, they needed to be calibrated regularly against an onsite hydraulic zero to maintain their accuracy.
Direct current (DC) magmeters were developed to solve the problems associated with the noise associated with AC meters. The DC meters were based on pulsed direct current. When the current is turned on, a voltage is generated in the magnetic flowmeter, showing the velocity of a flowing liquid. When the current is turned off, any remaining voltage is assumed to be due to noise. The meter computes flow velocity by subtracting this extra remaining voltage.
While DC pulsed technology was first introduced in 1974, it became popular in the 1980s, and its popularity has grown since then. In 2019, 93 percent of magnetic flowmeter revenues were from meters using some type of DC technology. Pulsed DC technology does have the drawback, however, that many of them have lower signal strength than AC meters. This gives AC meters an advantage for measurement of some dirty liquids and slurries.
To compensate for low signal strength, some DC meter suppliers developed “high strength” DC meters. These high strength meters still use the pulsed on-off technology of DC meters, but they have a higher coil current. This makes them better able to handle high noise applications such as slurries and dirty liquids than standard DC meters. As a result, they can successfully handle some applications that formerly could only be handled by AC meters.
Liners — the ‘secret sauce’
Suppliers have regularly brought out magnetic flowmeters with new liner types, and the increasing availability of many types of liners is another growth factor for magnetic flowmeters. This is an area where magnetic flowmeters distinguish themselves. Liners are the “secret sauce” of magnetic flowmeters that enable them to measure both very dirty and very clean liquids. With the appropriate liner option, they can measure the dirty liquids and slurries common to the pulp & paper and wastewater industries, as well as the hygienic and sanitary liquids common to the food & beverage and pharmaceutical industries.
There are nine main types of liners for magnetic flowmeters. The two most popular ones are PFA (perfluoroalkoxy) and PTFE (polytetrafluoroethylene). Teflon® is the familiar trade name for PTFE made by DuPont. Hard rubber is widely used for water and wastewater applications.
While, PFA, PTFE, and hard rubber are the dominant liners, a number of other types of liners meet special needs. For example, special liners exist for sanitary applications. And ceramic measurement tubes have excellent abrasion resistance (up to ten times greater than that of polyurethane liners) and feature superior heat and pressure resistance. On the other hand, ceramic liners are not good candidates for highly concentrated alkaline solutions.
This choice that users have in liners increases the durability and reliability of magnetic flowmeters and make it possible to use them with almost any type of liquid (with the exception of nonconductive liquids). No other flowmeter that measures liquids has such versatility when it comes to the wetted material in the flowmeter that makes contact with the liquid.