2-wire magmeter creates freedom from 120 V

A two-wire electromagnetic flowmeter—long sought for ease of installation and freedom from a power supply—become reality with an addition to the Bailey-Fischer & Porter (Warminster, Pa.) "COPA" line of magmeters.With global flow sensor sales running at $3.2 billion, and magmeters at 8% or $256 million of that, the company expects its 2-wire revolution to quickly capture th...

04/01/1998


A two-wire electromagnetic flowmeter—long sought for ease of installation and freedom from a power supply—become reality with an addition to the Bailey-Fischer & Porter (Warminster, Pa.) "COPA" line of magmeters.

With global flow sensor sales running at $3.2 billion, and magmeters at 8% or $256 million of that, the company expects its 2-wire revolution to quickly capture the magmeter majority and eat into other flowmeter market shares, particularly the largest: differential pressure flowmeters (36%).

"This is a once-in-a-lifetime introduction," says Blake Doney, a senior product manager with Bailey-Fischer & Porter. Trying hard not to understate the importance of the technology, he adds, "People have been trying to make a commercially viable 2-wire magmeter for 35 years."

The COPA-XT (copa is a German abbreviation for compact) combines a microprocessor, low-power electronics, more efficient magnetic coils, and a capacitor to store power from and send signals through the 24 V, 2-wire supply (14-55 V dc). The patent-pending design avoids the need to run separate conduit with 120 V power supply (or run the wires in the same conduit with concern about signal corruption).

The 2-wire magmeter avoids the $10/ft installed cost of conduit, eliminates the 120 V potential for signal disruption, and reduces chances of electrical malfunction (and related environmental hazards), Mr. Doney says. The 2-wire magmeters will first be available with HART (highway addressable remote transducer) communications and will take advantage of digital industrial networks by year-end, such as Fieldbus Foundation's 31.25 kbit/sec protocol, and Profibus-DP.

Smarter, better excitation

Besides more powerful microprocessors, the unit uses more refined coils, creating less signal interference in creating the pulsed-dc field to measure fluid conductivity. Because the pulses arrive at the target range faster than with other magmeter coils, the pulses can be shorter, requiring less power, just 0.5 W. (See "Power-saving" line chart.)

The low power allows direct changeout with other 2-wire flow devices using differential pressure, turbine, vortex, and other technologies. A capacitor accumulates necessary power to excite the coils—like a charge builds in a camera's flash—without interfering with analog output, explains Mr. Doney. "Our patented algorithm maximizes the power to the coil drive circuit, while assuring that the current draw does not overshadow the 4 to 20 mA output." The excitation frequency increases as the analog output approaches 20 mA. Refresh is fast enough to handle the maximum flow in each model's range.

The beta product ships in March; commercial products will be available in the third quarter, he adds. With this development, the installed cost of magmeters, about $2,500-$3,500, approaches the total installed cost of differential pressure flowmeters, about $2,000, with accuracy of 0.5% of rate (velocity 1.67 ft/sec). Repeatability is comparable to 4-wire Bailey-Fischer & Porter magmeters:

The units provide migration paths for existing B-F&P customers: wafers or flanges have the same place-to-place dimensions for easy replacement. They are year 2000-compliant and use an EEPROM to store data, avoiding any need for the customer to rekey configuration data, should the converter need replacing. The flow rate is totalized internally in any of 15 engineering units for each flow direction. Displayed values can be any of 42 engineering units. A 2316 character liquid crystal display shows up to four flow information values:

  • Flow rate in percentage, engineering units, or as a bargraph;

  • Totalizer values in engineering units for forward or reverse flow;

  • User-configurable tag number; and

  • Current output value.

Nonpetroleum fluids

Magmeters only work on conductive fluids (nonpetroleum-based), making them big in the chemical industry, which accounts for about 45% of magmeter use, Mr. Doney says. As long as the liquids, slurry, or sludge has conductivity exceeding 50the unit. (See "How a magmeter works.")

COPA-XT comes in 10 sizes from 3/8 to 4 in., for flow rates from a minimum of 0 to 2.25 l/min. to a maximum flow range of 0 to 240 m3/h. In installation, the flowmeter's electrode axis should be horizontal, if possible, and never vertical. The meter tube needs to remain full of fluid.



How a magmeter works

Faraday's Laws of Induction say a voltage is induced in a conductor as it moves through a magnetic field. In an electronic flowmeter, a magnetic field is generated perpendicular to the fluid's flow direction. (See schematic.) Two diametrically opposed electrodes measure the voltage induced in the fluid. Signal voltage U E is proportional to the magnetic flux density B, the electrode spacing D, and the average flow velocity v.

Because magnetic flux density and the electrode spacing are constant values, a proportionality exists between the signal voltage at the electrodes and the average flow velocity. From the volume flow rate equation, it follows that the signal voltage is also linearly proportional to the volume flow rate.

The induced flow signal is converted into scaled, analog, and digital signals in the converter.



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