Magnetic Flowmeters Two-wire vs. Four-wire Chemical Engineering©

To optimize process control and overall plant performance, magnetic flowmeters (mag meters) are the most widely used devices in many industrial flowmeasurement applications. Mag meters are used to measure the flow of water and other conductive liquids by applying a magnetic field to the flowing liquid. Following Faraday’s law of electromagnetic induction, conductive liquid flowing through the magnetic field induces a voltage signal that can be detected by electrodes on the pipe walls. Fluid flowrate is proportional to voltage: faster-moving fluids generate higher voltage signals. Mag meter technologies produce signals that are linear with flowrate.

Mag meter advantages
Mag meters have several key advantages over alternatives, including exceptionally high accuracy, (within the range of 0.5%).

Chemical feed applications require highly accurate and stable flow measurements, making them a good fit for mag meters.

Mag meters are volumetric flowmeters, free of moving parts. This means they are much less susceptible to wear-and-tear than contact meters, particularly those with moving parts, such as turbine flowmeters. Also, the lack of moving parts minimizes the loss of accuracy over time that many other meter types can experience.

Mag meters can be used with pipes of almost any size and are ideal for applications where low pressure drop and minimal maintenance are required.

While mag meters can measure a wide range of aqueous solutions and slurries, they are not suitable for low-conductivity solutions, such as distilled water, hydrocarbon-containing solutions and non-aqueous solutions and gases.

Two- versus four-wire mag meters
Mag meters are offered in both two-wire, loop-powered format and four-wire, separately powered configurations. The basic difference is that two-wire mag meters do not require separate wiring to a power source, as they receive power from the 4–20-mA loop power supply. Aside from the wiring, another main difference between the two- and four-wire flowmeters is the strength of the magnetic field. With their greater power input, four-wire mag meters can generate a more powerful magnetic field, which can be an advantage for some applications.

Four-wire mag meters are most commonly used for more standard applications because that type was developed first. Because of that, they are widely supported by thousands of compatible devices, including wireless transducers. They work with many different wire conductors and connectors, even with poor-quality connections. Modern variations of 4–20 mA, such as those using the HART communication protocol, continue
to drive support for the four-wire meter as the standard.

Selection considerations
Both meter types can provide good value in the applications for which they are intended.

Both have advantages and disadvantages, so engineers need to decide which type is better-suited to the application. The following are a number of considerations that are important to the selection of a twowire or four-wire mag meter.

Slurries or clean fluids. The consistency of the liquid being measured is the most important determining factor when choosing which type of mag meter to use. Four-wire mag meters are better for heavy-slurry applications. Two-wire mag meters are used for clean fluids, but can also be used for light slurries.

Signal noise. Four-wire mag meters typically are more tolerant of process-generated noise in the fluid to be measured than the two-wire type. Two-wire magnetic flowmeters are good for low-noise applications with cleaner fluids.

Cost and size. Four-wire mag meters are slightly less expensive at initial purchase, but two-wire mag meters offer cost savings in installation because they require less wiring overall and do not require an independent power source.

Energy requirements. Four-wire mag meters generally use more power. However, since a two-wire mag meter is powered by the controller’s analog input module, it is important not to overload the module. Engineers should check the nameplate on the meter for a Watt- or burden-rating. For example, a 4–20-mA two-wire mag meter on a 24-V d.c. loop supply will draw nearly 0.5 W at full span (24 V x 0.02 A = 0.48 W).

The latest two-wire meters offer exceptional energy efficiency, consuming as little as 1–4% of the power needed for a four-wire unit. This 96% reduction in power consumption also reduces annual CO2 emissions from 46 kg to 1.2 kg. Lower energy consumption also contributes to a lower total cost of ownership for a two-wire mag meter.

For example, the estimated lifetime energy savings from a two-wire mag meter, could exceed $10,000:
$10/y x 100 m = $1,000/y
Lifetime (10 y) savings = $10,000

Advanced designs. There are certain instances in which advanced two-wire mag meters can be used in applications that traditionally would have called for the use of a four-wire mag meter. A two-wire mag meter is now available that uses a dual-frequency excitation method. The dualfrequency excitation provides the two-wire instrument with the same level of processgenerated noise immunity as a four-wire flowmeter under a wide range of fluid conditions, including light slurries.

The new type of two-wire mag meters are also capable of suppressing noise without sacrificing fast response time. This is due to an advanced electromagnetic design that generates a relatively strong magnetic field with only the limited power available from the two wires in a 4–20-mA loop power installation.

Handling corrosive liquids. Magnetic flowmeters can be used to measure corrosive liquids and abrasive slurries. However, it is important to choose a flowmeter fabricated with materials that are compatible with the types of fluids used in the application.

Editor’s note: Content for this edition of “Facts at Your Fingertips” was contributed by Sam Hassan, product specialist at Yokogawa Corp. of America (Newnan, Ga.; www.yokogawa.com/us).