sensors or probes are great products for powder and bulk solids
applications where the material being sensed has a very low
dielectric constant that might be lower than an RF Capacitance
probe could detect. They are also good alternatives for rotary
paddle bin indicators in application with extremely low bulk
density values. Expanded poly-styrene beads have a bulk density
that is typically less than 2 lbs/ft3 (.032g/cm3). Paddle type
sensors and especially high level paddle type sensors would
very likely cavitate when covered with material this light
and give no indication that material is indeed present.
What if we have an application for a bin whose material frequently
changes and we were using a rotary paddle bin indicator? If
the bulk density decreases, at some point the paddle being
used will start to cavitate and a paddle with a higher surface
area (increasing the effective sensitivity) would need to be
installed to allow the paddle type sensor to regain functionality.
There will be a lower limit of bulk density vs. area of the
paddle that once reached will ultimately render the paddle
type sensor useless for the application in question. If an
RF Capacitance probe is being used in silos, hoppers or bins
and the material is changed to something with a radically different
dielectric constant, probe re-calibration might be required.
This is where a vibratory probe is a great choice because there
is no calibration per se. Many vibratory probes on the market
do have a general sensitivity setting that can be changed but
that is an adjustment the rarely needs to be set differently
than the factory default.
The main downside of a vibratory probe is that it is generally
more expensive than a paddle or RF Capacitance type of sensor.
Another area of concern for a vibratory probe, and main point
of this article, is that models that are high in sensitivity
and able to detect materials with very low bulk densities are
typically not as physically strong in comparison to other point
level technologies. So when we use a vibratory probe, we must
strictly adhere to the manufacturer's installation guidelines.
According to a conversion website I frequent:
150 Newtons = 33.7 pound-force.
500 Newtons = 112.4 pound-force.
1000 Newtons = 224.8 pound-force.
"So what?” you exclaim.
One of the most common reasons for vibratory probe failure
is from the probe having been subjected to forces beyond their
design limitations. Such a condition can actually bend the
probe rendering it useless. Other modes of failure include
broken components within the probe housing or within the probe
element if it is a rod type or diamond shaped design.
Below are examples of parameters for vibratory
PZP PROBE LIMITS
OK, here come the Newtons. Our model PZP has two separate
force limitations. The probe must be installed such that the
narrow edge (knife edge if you will) of the probe is always
pointing up. The PZP can withstand 1000 Newtons max in this
plane but will only withstand a maximum fore of 150 Newtons
along and perpendicular to the axis on the wide side of the
probe. Our VibraRod probe is constructed using a hollow tube
that is cylindrical shape and is rated at 500 Newtons max in
any plane along and perpendicular to the axis.
Generally speaking, either probe should be shielded from direct
impact from material falling directly on the probe during the
filling process depending upon material characteristics. There
are suggestions for doing so in our Installation & Operation
Bulletins for these products.
So, when you need or want to use a vibratory probe, be mindful
of all forces the probe will be exposed to during fill and
also when material is being drawn out. How and where the probe
is mounted to the vessel will have a direct influence on the
amount of force the probe is subjected to when the material
contained in the vessel is in a dynamic state. Mass flow silos
and hoppers will produce more force against the probe than
its funnel flow counterpart would exhibit. Properly placed
baffles will correct this problem. Also determine if there
will be any significant side or lateral horizontal loads that
either require silo modification or require the selection of
a different style of probe so ensure that maximum force specifications
are never exceeded. If you find yourself needing or desiring
a vibratory probe and you have determined a need to install
protective baffles or shields but you are unable to install
them, you should consider using a top mounted cable extension
model or a rigid pipe extension model. A cable extended probe
will allow the actual sensing portion of the probe to hang
freely at the desired point of material detection without exceeding
specified lateral force limitations. See examples below.
|VibraRod with cable extension
||PZP with cable extension
Now if this seemed all too gloomy for you, relax. We have
many successful applications in materials with bulk densities
as high as 100 lbs/ft3. You just need to think about the application
and install the probe in such a manner that its limitations
are not exceeded. When in doubt, give our Technical Support
department a call at 800-766-6486 for advice and guidance.
We are here to assist you by providing practical solutions
on and at every level we can.
For more information on VibraRod, please visit
For more information on the PZP, please visit
Or call us in the USA at 800-766-6486 or from anywhere at
630-365-9403. Also, check out our Level
Measurement blog at http://monitortech.typepad.com.