Vibratory point-level 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 probes.

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
http://www.monitortech.com/product_p_vrod.shtml

For more information on the PZP, please visit
http://www.monitortech.com/product_p_vpzp.shtml

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.

Product Manager