Back to the Basics: Magnetic Flowmeters

They don’t read out in mass flow units, but when combined with an ancillary density measurement device (often, for larger diameter pipes a gamma nuclear densitometer), they can produce a high-precision mass flow measurement. This combination of devices is used regularly in any water-based fluid flow situation where the pipe size is larger than 12 in. (nominally 300 mm). This application is commonly found in the mining industry and in dredging applications in harbors and rivers around the world.

Most importantly, they will not work on non-conductive fluids or on gases at all. The minimum conductivity of a fluid is usually considered to be 5 μS (microSiemens) before a magnetic flowmeter will measure its velocity. In practice, it’s not wise to use a magmeter on a fluid whose average conductivity is this low.

Finally, magmeters (except again for specially-designed units) have trouble working on fluids with extremely high or highly variable conductivity. Saline brine and seawater, are examples of this kind of fluid.

Using Magmeters

There are some simple rules for using magmeters, which, if you follow them, will produce a satisfactory application.

Straight Run

Magnetic flowmeters need less straight run than most flowmeters, often as little as three diameters upstream of the electrode plane (the centerline of the meter body, usually), and no diameters of straight run downstream. However, there are circumstances in which a better choice is to go with as much straight run as you can get. For example, spiraling flow (swirl in the pipe) can propagate for hundreds of diameters after a three-dimensional turn in piping. Spiraling flow causes severe inaccuracy in a magmeter, sometimes as much as 40% of measured value.

“How a Magmeter Works Figure 2. Some basic rules of thumb for using magmeters.

Vertical Mounting

One of the ways to make sure you have a fully developed flow profile moving through the meter is to mount your magmeter so that the flow is through the meter in the vertical direction. This helps in cases of spiraling flow and also helps reduce air entrainment.

Right Sizing

Although a magmeter will operate over the entire range from 0.3 fps to 33 fps (0.09 to 10 meters per second) velocity, it isn’t wise to install a magmeter that’s going to operate permanently at the lower end of that range. In applications where there are solids, this can cause buildup of solids inside the flow tube and sometimes on the electrodes themselves. If buildup occurs inside the flow tube, the calculated volume is now in error, and if buildup occurs on the electrodes, the insulating properties of the buildup can either reduce the voltage or break the circuit entirely. Either will cause inaccurate readings. It’s better to size the flowmeter for a normal flow that is about 60% of maximum for that pipe size, and if necessary, install a properly designed meter run. Fortunately, for a magmeter, that meter run doesn’t need to be as long as it does with some technologies for measuring flow.

Proper Grounding

Remember that the pipe section of the magmeter needs to be non-conductive for the circuit to work. The electronics that process the induced voltage, however, are susceptible to interference if they’re floating above ground. Magmeter vendors all have grounding procedures, which you ignore at your peril.

Temperature and Pressure

Magnetic flowmeters are designed to work at moderate temperatures and pressures and should not be stressed above or below those specifications. Magnetic flowmeters should not be operated where a vacuum can be pulled inside the flow tube unless specifically designed for that service. This is so especially when there is a pressed-in polyurethane or Teflon lining, because the vacuum can pull the lining right out of the meter, causing potential hazard, as well as inaccuracy in reading. Both Teflon and polyurethane, which are the most common magnetic flowmeter liners, are de-rated for pressure at the upper end of their temperature range and will deform if overheated.

Magnetic flowmeters have become one of the most widely used flow technologies in the 50 years since their first introduction. They’re simple, easy to maintain, and because they have no moving parts, able to operate for years without maintenance.

Walt Boyes is Control’s editor in chief.