have emerged as an extremely popular method of flow measurement for a
number of reasons, but most importantly because they excel in measuring
most kinds of liquids. They are also relatively cost-efficient and
straightforward to install. Further, with no moving parts, magmeters,
as they’re called, are easy to maintain and offer good accuracy for
In essence, magnetic flowmeters measure flow via magnetic coils
powered by either alternating current (AC) or direct current (DC). When
the current powers the coils, a magnetic field is created in the area
of the flowing liquid. When conductive liquid flows through a magnetic
field, a voltage is generated that is directly proportional to the
velocity of the fluid. The magmeter detects this voltage using
electrodes that are typically positioned on either side of the pipe and
computes flow velocity based on the amount of voltage present.
From Continuous AC to Pulsed DC
|Figure 1. EMCO Flow Systems’ Unimag magnetic flowmeter is an example of a pulsed AC measurement system.
were first introduced for commercial use in Holland in 1952 by The
Tobinmeter Company. Foxboro (www.foxboro.com
) released magmeter
technology in the United States in 1954. Today there are more than 50
suppliers of magnetic flowmeters.
First-generation magnetic flowmeters were powered by continuous
alternating current, which is subject to noise that interferes with the
proper reading of the meter. As a result, continuous AC meters need to
be regularly calibrated against an on-site hydraulic zero to compensate
for the presence of noise.
DC magmeters were invented mainly as a solution to the
zero-calibration issues associated with continuous AC magmeters. These
magmeters operate based on a pulsed direct current. When the current is
turned on, a voltage is generated, showing the velocity of the flowing
liquid, plus any noise. When the current is turned off, any remaining
voltage is assumed to be noise. By subtracting this voltage
measurement, the meter provides a flow measurement that accounts for
the effects of noise.
In addition, pulsed DC magnetic flowmeters use less power and
have a lower coil excitation frequency than continuous AC magmeters.
They also typically cost less than continuous AC magnetic flowmeters,
because their design does not have to minimize the effects of eddy
currents. Pulsed DC magmeters are also easier to install and cost less
to operate than continuous AC magnetic flowmeters.
|What Causes Media Noise?
• Electro-chemicals (e.g. Hydrogen ions + ve, hydroxyl ions –ve)
• Entrained magnetically charged particles
• Solids/particulate impact against electrodes (piezoelectric)
• Frictional effects against electrodes — fibers, etc. (triboelectric)
• Low and variable media conductivity
• Coated electrodes
Pulsed DC magmeters became popular in the 1980s. Today, over 85
percent of magnetic flowmeters sold worldwide use pulsed DC technology,
and about 90 percent of magmeter suppliers offer DC technology. Even
so, close to one-third of magnetic flowmeter suppliers still offer some
type of AC magnetic flowmeter, and some suppliers have both continuous
AC and pulsed DC magmeters in their product line.
To compensate for the lower signal strength of many DC magnetic
flowmeters, suppliers have developed high-strength DC magmeters. These
high-strength meters employ the on-off current technology of other
pulsed DC meters, but they produce a stronger signal. This stronger
signal results from a higher coil current. As a result, these
high-strength meters can handle high-noise applications, such as
slurries and dirty liquids, better than other pulsed DC magmeters.
Close to 25 percent of magnetic flowmeter suppliers today offer some
type of high-strength pulsed DC magmeter.
Error Sources for Pulsed DC Meters
Even though pulsed
DC magmeters dominate the market today, they still have their
limitations, especially for slurries and other liquids containing
solids. Two potential sources of error for pulsed DC magmeters are
media noise and liner and electrode seal microporosity. Both of these
sources of error can interfere with the integrity of the flowmeter
signal and its ability to provide fast response with good accuracy.
There are multiple sources for media noise (see sidebar). These
include entrained magnetically charged particles, solids impacting
against the electrodes, low- and variable-media conductivity, and
coated electrodes. Liner and electrode seal microporosity refers to the
possibility that media molecules can migrate past the electrode seals
or through the liner itself. Liner microporosity normally results from
wear or large temperature differentials across the liner. Electrode
microporosity emanates from minute liner and seal movements due to
shock or variation in temperature or static pressure.
Pulsed AC Magmeters
While pulsed DC
technology has a number of advantages, AC magmeters maintain advantages
for certain applications. The strength of the current exciting the
coils is typically less in DC magmeters than in AC magmeters.
Consequently, AC magmeters have very good signal strength and
relatively high exciter frequency, which makes them suitable for
slurries, pulps, and other noisy media. The combination of relatively
high exciter frequency, typically greater than 30 Hz, and exciter coil
current, typically in the amps regime, significantly improves signal
quality by producing a strong, noise-free signal.
|Figure 2. The Unimag’s solid-state sensor is designed to eliminate
liner and electrode seal microporosity and substantially reduce the
effects of media noise.
To counteract the disadvantages of continuous AC magmeters, while
retaining some of the advantages of pulsed DC meters, some suppliers
have introduced pulsed AC magnetic flowmeters. One such company is
Siemens Energy & Automation (www.siemens.com
), which sells the
Transmag 2. Another company is EMCO Flow Systems (www.emcoflow.com
which manufactures the Unimag (Figure 1). EMCO refers to its technology
as “pulsed hybrid.”
This meter has an auto-zero feature and compensates for eddy
currents, even at relatively high exciter frequencies up to 40 Hz. It
has a strong exciter current in the amps regime, together with a
typical exciter frequency of 40 Hz, making it a good choice for
slurries, pulp, dirty, and clean media.
Another EMCO design features a magmeter without a liner.
Electrodes, exciter, and reference coils are contained in a removable
solid-state insulated sensor, positioned on either side of the
flowtube. The large ratio of sensor diameter to flowtube diameter, as
well as a plurality of sensors and a magnetizing current in the amps
regime, gives this design the same hydraulic accuracy as a
conventionally designed continuous AC or a pulsed DC meter. For media
with very thick coatings, optional extended electrodes are made long
enough to protrude through the media coating. This solid-state sensor
design eliminates liner and electrode seal microporosity and
substantially reduces the effects of media noise (Figure 2).
Balancing the Benefits of AC vs. DC
technology has come a long way since 1952. Like other flowmeter types,
magmeters have gone through a technological evolution, with many new
innovations and improvements designed to compensate for previous
disadvantages. Pulsed DC magmeters were introduced to counter the
calibration and power issues of early-generation AC magmeters and have
become the dominant technology. Now pulsed AC magmeters retain some of
the benefits of both continuous AC and pulsed DC designs. However, no
technology is perfect for all applications.
Despite all the advances made by magnetic flowmeter suppliers, no
one yet has figured out how to enable magmeters to effectively measure
the flow of hydrocarbons, whose conductivity is usually very low.
Magnetic flowmeters also cannot measure the flow of gas or steam. But
suppliers have made advances in enabling magmeters to measure liquids
with lower conductivity levels, and they have also significantly
increased accuracy. Look for suppliers to continue to bring out new
features and products as they deal with the unique challenges posed by
magnetic flow measurement.
Jesse Yoder, Ph.D.,
is a regular contributor to Flow Control magazine. He has been a
leading analyst in the process control industry since 1986. He
specializes in flowmeters and other field devices, including pressure,
level, and temperature technologies. Dr. Yoder is currently president
of Flow Research Inc., an analyst firm specializing in flow technology.
He can be reached at email@example.com
or 781 245-3200.