18
Level Measurement
Choosing an Inventory Monitor for Bulk Solids
Joe Lewis
Managing Director,
BlueLevel Technologies
Strategies
Before you begin to look at
technologies available for level
sensing, pick a strategy. There are three possible strategies: use level sensors; use a weight
measurement system; or use manual measurement. Inventory applications typically do not
need frequent measurement. Accuracy and
repeatability of measurement is more important.
Each company that processes, handles and
stores bulk solid materials will have their own
perspective. Most bulk solids are produced or
purchased in measures of weight, by pounds
or tons, which means there is usually a desire
to have the inventory measurement results in
terms of weight. Making empty space or
material level measurements manually or by
using automated level sensors will require
calculations to convert from the empty space
distance or level measurement to volume
displaced, and then finally to weight. Using a
weight measuring system does not.
Weighing
Directly measuring the material weight by
use of load stand systems or bolt-on strain
sensing devices can be done for small bins and
hoppers, as well as for large silos. However,
the weighing strategy seems to be the most
practical and economical in smaller vessels, and
justifiable where the best degree of accuracy in
terms of material weight is required.
One challenge with weighing systems is
existing vessels. These applications present
more costly and difficult installation issues for
load stand systems, especially for large vessels
such as industrial silos and large bins. Bolt-on
strain sensors are easier to install in retrofit
situations, which is their main selling point and
attraction for facility personnel. However, they
can still present a costly solution requiring
calibration and setup which may be an issue
depending on the vessel, material weight and
the physical environment. The expense and
challenge of retrofitting existing silos and bins,
with load stand systems or bolt-on systems,
Level
There are a number of parodies that have
popped up in trade advertisements regarding
the manual material level sensing strategy. I
recall one ad that featured an illustration of a
direct labor worker throwing rocks at a bin while
the aged and slightly bald supervisor stood by
watching. Both would be listening to the sound
of the rocks as they bounced off the bin… “tink,
tink, tink, TONK” and viola they have pinpointed their material level in the bin, well, relatively
speaking. Another more recent advertisement
TECHNOLOGY
NONCONTACT
NONINVASIVE
SENSITIVE
TO DUST
Weight &
Cable
Periodic
No
No
SENSITIVE
DIELECTRIC PRICE
TO ANGLE SENSITIVITY
OF REPOSE
No
No
Low
Acoustic
Yes
No
Yes
Yes
No
Moderate
RF Capacitance/ No
Admittance
No
Guided Wave
Radar
Yes
Through-Air
Radar
Yes
Laser
No
Yes
No
Yes
Moderate
No
No
No
Yes
Moderate
No
Some
Some
Yes
High
Yes
Yes
No
No
(but color)
High
Reproduced from the forthcoming book, Solids Level Measurement and Detection Handbook, by Joe Lewis, (c)
2012 Momentum Press, LLC. All rights reserved.
Write In 220
June 2011 • www.ProcessingMagazine.com
is why many companies resort to a manual or
automated level measurement as an alternative.
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19
features a western character using his sixshooter to search the bin for material presence
and absence to determine the level of the
material. Neither of these manual methods are
very effective for inventory measurement.
Another tried-and-true manual method, and
more realistic than the previous examples, is the
use of a tape measure, hand-held laser distance
finder, or even a rope with knots every
foot, to determine the empty space distance manually by personnel that would
climb to the top of the bin, open a hatch
and make the measurement using the
tool of choice. There are a few problems
with this, including the safety of having
personnel climb bins and silos (I have
done it many times myself). Other practical problems include how to determine
when the end of your tape measure or
rope has touched the material surface
without slack (more difficult with large
empty space distances); how accurate is
the measurement tool; how accurate and
precise is the facility personnel reading
the measurement tool; and where is the
measurement point (to the roof of the
bin, some point inside, etc.).
Using continuous level sensors for
inventory measurement is commonplace
and there are several technologies to
choose from. No matter which technology may be chosen the conversion
calculation from distance/level to volume
and weight may still be required. This
conversion assumes a flat surface and
volume is calculated based upon dimensions provided by the facility personnel.
Often the level sensor can make the
calculation conversion or it is made by
the information receiving device. Angle
of repose, material packing, dimensional
inaccuracies and bulk density assumptions all impact the accuracy of the volume and weight value.
Accuracy of the distance/level varies
by technology, typically from 0.25 to 1
percent of reading. The accuracy of the
level sensing technology often does not
have as much impact on the accuracy of
the conversion to volume and weight as
does the other factors previously mentioned. Therefore, cost of purchase and
installation as well as reliability becomes
an important factor in selecting level
sensor technology. Refer to the table on
page 18 for a summary of six of the primary level sensing technologies.
Conclusion
When choosing a level sensor technology for inventory monitoring it is best
to select the least expensive technology
that will meet all or most of your needs.
If your needs require high accuracy in
weight measurement, consider load
stands for new installations or bolt-on systems
for retrofitting. Selecting the best fitting level
sensor technology can be a mine field at times,
and vendors often show bias based upon their
product offerings. The previously mentioned
table offers a guideline. If you would like to
discuss a specific application please contact me
for consultation and available technical
documents. It won’t cost you anything but your
time, regardless of what you decide to buy or
from whom.
Mr. Lewis can be reached at 888-61LEVEL,
815-625-2224, or e-mail [email protected].
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