16
Level Measurement
RAdio Detection and Ranging … Huh?
Joe Lewis
Managing Director,
BlueLevel Technologies
Radar technology (yes it is an
acronym: RAdio Detection and
Ranging) is used for continuously
measuring the level of material within a vessel.
Other technologies that can also be used for
the level measurement and indication include
glass gauges, magnetic liquid level gauges, a
variety of electromechanical devices, acoustic,
capacitance, magnetostrictive, laser, radiometric
and differential pressure (hydrostatic). Did I forget anything? The list is long. However, when
we speak specifically of measuring the level of
powder and other bulk solid material the list
shortens a little, still including radar, electromechanical (weight and cable), acoustic, capacitance, laser and radiometric technologies.
On the price scale, radar is somewhere
between the mid-high end of the spectrum;
and on the time scale, it is more recently developed than most others. In fact radar is one of
the technologies where there continues to be
much development, promotion and continued
improvement in the technology by the primary
manufacturers. However, when talking about
radar technology it is important to clarify a
couple of points.
Radar continuous level measurement technology can be classified into two categories:
that which is generally in contact with the
material being measured and that which is
non-contact. The contact technology is known
by names like guided wave radar, TDR (time
domain reflectometry) or even MIR (micro-power impulse radar) and has been commercially
available as a level measurement sensor at least
since the early-mid 1990s. Non-contact radar
or through-air radar is available in two primary
versions that differentiate their basic method
of measuring distance through air. These are
FMCW (frequency modulated continuous wave)
and pulsed radar. Non-contact radar technologies have been in industrial level measurement
use for tank gauging from the late 1980s and
in general widespread use since the late 1990s
with much of its advanced development occurring in the last decade.
Why all the fuss about radar units? They
handle more applications than almost any other
technology, except radiometric or nuclear
level measurement sensors; and their price,
while moderate-high, has come down over the
past 20 years and is now arguably considered
“reasonable” for mass use in general industrial
applications. They have also shown success in
very challenging applications such as where you
must measure during filling of a dusty powder
or granular material.
Guided Wave Radar
The contact radar devices known as guided
wave radar generally use TDR, or time domain
reflectometry, to measure the distance of
empty space between the level sensor and
material surface. Radar energy is transmitted
in pulses and guided to the material surface
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along the waveguide or probe that is continuously suspended within the vessel. The radar
pulses are reflected from the material surface
and the time from transmission of the pulses to
receipt of the reflections is directly related to
the empty space distance. However, not all the
radar energy is reflected. In fact, the percentage of reflected energy is very small. A rule
of thumb used by some manufacturers is that
the energy reflected is related to the dielectric
constant of the material. The lower the dielectric, the lower the amount of energy that is
reflected. The higher the dielectric, the greater
the amount of energy that is reflected. With
a dielectric constant of about 5.0, whole corn
will reflect about 5 percent of the radar energy.
However, polyethylene pellet with a dielectric constant of 1.6 reflects far less (estimated
about 1.6 percent reflected) of the radar energy and absorbs nearly all of it. The impact of
this is on the effective measuring range of the
level sensor, pointing out a limitation of radar
level measuring technology in general, i.e. the
lower the dielectric constant, the shorter the
effective measuring range.
Non-contact / Through-air Radar
Two different technology approaches exist
regarding non-contact radar for continuous
level measurement. The first is pulsed radar
that measure the empty space distance by
time-of-flight or transit time methods like guided wave or TDR radar previously discussed.
FMCW, or Frequency Modulated Continuous
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18
Level Measurement
Wave, is the alternate. FMCW radar continuous level sensors work by
directly relating the empty space distance to the frequency shift of the
radar energy resulting from contact and reflection off the material surface.
Proponents of each exist and claims abound from manufacturers, especially regarding performance in powder and bulk solid applications and
effectiveness in dusty environments during vessel filling. Success can be
found for both technology implementations. But they are both non-contact radar implementations and this is a big attraction in many liquid and
bulk solid applications over guided wave radar and other technologies.
Like guided wave radar, through-air radar also suffers from distance
and performance limitations as the material dielectric goes down.
Through-air arguably suffers from this drawback more than guided wave
or TDR radar. While guided wave radar units are available for use with
materials having extremely low dielectric constants (as low as 1.2), generally they are used with materials having dielectric of 1.5 and up. Throughair radar units are generally used with materials having a dielectric constant of 2.0 and above.
Another difference between the guided wave and through-air types
of radar level sensors is the precision of
their energy propagation. Because the
radar pulses from a guided wave radar
unit are “guided” to the material surface
by the waveguide (rod or cable probe) the
point on the material surface being measured can be relatively precise. However,
through-air units use an antennae and the
energy propagates in a beam to the material surface. Beam angles for through-air
radar units is much smaller than for acoustic devices generally, but still 2-3 degrees
and the larger the measuring distance the
wider the measured area. Measuring a
single point on the surface of the material
isn’t a negative with liquids, but for solids
where the material surface can be sloping and irregular this can be a problem
when converting level to volume or mass.
But that is the nature of virtually all level
sensor technologies, including both radar
level sensor technology types. But that is
another story for another time.
Conclusion
Radar level sensor technology has
come a long way in the last two decades,
both in its capability and reasonableness
of price point. These devices, contact
or non-contact, can be used across the
board in many liquid and solid material
applications, including some difficult to
measure situations like dusty powders,
interface measurement and even vessels
with some agitation. Check the performance of the specific device you are considering by reviewing references of similar
applications.
For more information about level
measurement and detection technology,
application and use, contact the author at
[email protected] and
visit www.blueleveltechnologies.com.
You can also follow BlueLevel and the
author on Twitter @BlueLevelTech and
check out their Facebook page at
www.facebook.com/pages/
BlueLevelTechnologies/97182916337.
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