6
IndustryInsight
December 2013
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Material Density Effect
on Level Measurement
Technology
Joe Lewis
[email protected]
T
The density of the material being measured can
have either a direct or indirect impact on the choices
of level measurement technology for use on a given
application, and also may impact the performance and
reliability of that technology.
When speaking about the density of a powder or
granular material the definition depends upon what
density we are speaking of. There are several common references to “density”, including skeletal density,
particle density, and bulk density. We are referring to
bulk density in regards to level measurement and detection.
Joe Lewis
In a vessel the bulk density of the material is the
ratio of the mass of the material to the volume it displaces. The bulk density includes the material particles, as well as voids between particles and voids within
the particles themselves. Because the voids are included, the bulk density can
be when the material is in a loose or packed condition.
The bulk density of a material can be determined by filling a container that
has a known volume (a cubic foot), with the material and then weighing it. The
loose bulk density is determined by weighing the full container without having tamped the container or packed the material during and after filling. The
packed bulk density is calculated the same way, but while filling you tamp the
container to pack the material in order to fit as much of the material as possible
in the container, minimizing voids between the particles. When evaluating the
use of a level measurement or detection device, it is important to understand
what bulk density is used, or if you are just given the range of bulk density between loose and packed.
Bulk density can impact the choice of sensor technology. In regards to point
level sensing (presence/absence detection), some sensing methodologies directly respond to material density. Rotary paddle and vibrating element technology are two examples. The size of the paddle used in a rotary paddle bin
level indicator is dependent upon the bulk density of the material. The larger
the paddle, the greater the sensitivity of the unit and the lower the minimum
bulk density is that the unit can detect. The smaller the paddle, the higher the
minimum density of the material will be. This is because the rotation of the
paddle is more easily stopped by material with a higher bulk density. Therefore,
the bulk density, loose (such as at the top of the material pile; high level) and
packed (such as at the bottom of the material pile; low level), is an important
material characteristic to know in order to properly apply rotary paddle bin
level indicators, specifically in selecting the most appropriate paddle.
With a vibrating rod or fork point level sensor the mechanical structure is
placed in vibration at its natural resonant frequency with known amplitude.
The frequency changes when material present at the probe dampens the vibration. Based on a given amplitude and frequency the minimum material density
is determined. Most have a minimum bulk density in the 2-3 lb/cu ft range.
Knowing the material bulk density is a key to the successful use of vibrating
rods and forks.
An example of indirect impact of material density is in regards to the dielectric properties of the material. The dielectric constant or permittivity of a
material is an expression of how the material is affected by an electric field.
Air has the lowest dielectric constant, 1.0. The lower the dielectric constant of
a material the more challenging is its detection by an RF admittance or capacitance sensor, and the shorter the effective measuring range will be for radar
based continuous level transmitters. Bulk density is related to dielectric because
lighter weight materials typically have more air present and a lower dielectric
constant as a result. Therefore, the lower the bulk density of the material is, the
more challenging for RF probes to detect and the lower the measuring range
might be for radar based units.
Whether you have a point level sensor or continuous level sensor application
the bulk density of the target material should always be considered.
Joe Lewis is co-founder and managing director of BlueLevel Technologies Inc.
Lewis authored “Solids Level Measurement and Detection Handbook.” He has a BS
in electrical engineering from Roger Williams University, and a Masters in Business
from Bryant University.
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