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Electromechanical Plumb Bob System: An Oldie But Definitely a Goodie

December 3, 2013
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For as long as there has been a need to store material there has been a need to know how much is being stored at any given time. Whether it is for inventory counts, loading out or in, or batching, there is always someone somewhere in a plant needing to know what is in a silo or other holding bin. Because there hasn’t always been the fancy high-tech methods of measuring level in a bin we know today, or even electricity for that matter, yet still a need to know, this need has led to innovative methods of measuring how much material one has in a bin.
    From hitting the side of a bin with a hammer (or even throwing rocks up the side of a silo) while listening to the type of “bong” the hit creates, to waking up extra early to see where the morning dew on the side of vessel begins or ends, there is no shortage of ways people have come up with to measure level. Probably the most tried and true method, and maybe even the most common still used today, is the plumb bob style or “weight and cable” method.
  There is evidence of the plumb bob being used for vertical alignment as early as ancient Egyptian times, and somewhere along the way someone came up with the idea to use this concept for measuring the vertical depth of empty space in a bin, and using that measurement to calculate how much material is in the vessel from the bottom up or percent full. Assuming human error is controlled, this method has proved to be reliable and repeatable, inexpensive, and easy to execute. This manual method of bin level measurement is still by far the most used today in modern plants by operators who have tied a rock to a tape measuring device and lower the apparatus into a bin while looking at how much tape has paid out when the rock hits something in the bin. The only downside is that someone has to climb to the top of a bin or silo to conduct this measurement, which requires valuable resources and introduces a possible safety hazard.
  It is this issue that drove the development of the electromechanical plumb bob level system several decades ago. Early systems utilized several pulleys and large motors that resulted in bulky, complex machines that, without proper preventative maintenance applied, tended to fail due to a number of reasons. Failures included cables breaking or tangling, fouling of the electronics due to material being drawn up into the housing, motors burning up, and weights being caught under material as it fed into the bin. Measurements were conducted using a counting wheel that generated an electronic pulse with every turn of the wheel relating to a predetermined distance of cable fed out of the unit during the plumbing. These pulses were typically sent to a counter that interpreted each pulse as a distance and displayed the resulting value to the operator.
  Even with all of these initial challenges, the electromechanical plumb bob device was perceived to be an invaluable tool in the field of inventory management, and as technology improved in electronics and materials, so did the plumb bob. We are now seeing fourth and fifth generation designs that employ microprocessor-based electronics, simpler pulley designs, high-efficiency motors, and stronger cables (or tape in the case of a few suppliers) that address many of the issues the technology saw with its ancestors. Engineering controls that are built-in to the programming of these units monitor cable and motor life, and notify users if there is cable, pulley, and motor failure due to any number of reasons. Some manufacturers offer direct analog 4…20mA outputs, relays, and digital MODBUS or Profibus communications that eliminates the need for an intermediate remote start unit (RSU) thereby eliminating significant cost.
  Today electromechanical plumb bob devices are the most prolific and widely used inventory measurement systems, but they still hold a reputation for being less reliable and more maintenance intensive than the newer technologies such as microwave radar, ultrasonic, and laser. The following are five common arguments (I’ll be bold and refer to them as myths) against electromechanical plumb bob level devices and the industry response to those arguments.

Myth #1: Having moving parts makes the electromechanical system maintenance intensive.
  Yes, electromechanical devices by definition have both electrical and a mechanical components and yes, some of those parts have to move in order to function as designed. However, with advances in motor technology and with simpler pulley designs in use, the reliability and resulting life cycle of these systems has increased to match that expected for devices with no moving parts, such as microwave and ultrasonic. It is not unreasonable to expect 3500 running hours with standard motors or unlimited lifetimes for systems that use high efficiency brushless motors. Users may expect at least 100,000 measurement cycles of a cable-based system or more than 500,000 measurement cycles for a tape-based plumb bob device. For an operator making 24 measurements per day, that equates to an expected lifetime of 10 years for a cable or more than 50 years for a tape-based system. In addition, any supplier worth considering will utilize on-board diagnostics to notify the user as to any potential failure so that it can be addressed before becoming a resource-consuming problem. A quality electromechanical unit will monitor and notify a user of motor/electronics failures, sensor weights being jammed or buried, broken cables/tapes, adequate supply current, and other functions for which the failure of each would cause downtime.

Myth #2: Electromechanical devices are not as reliable or repeatable as newer technologies such as ultrasonic or microwave radar.
  Aside from devices that provide weight measurements of a bin or silo, electromechanical plumb bob technology is the only inventory management system that is not adversely affected by most conditions one might see in a silo containing a solid material. The performance of ultrasonic, microwave radar, and laser all depends on how amenable the conditions are within the vessel. With each of these technologies, an instrument is projecting a wave and looking for a return signal strong enough to detect and analyze. Dust, steam, foam, high temperatures, low dielectric constant, low bulk density, and angle of repose can all seriously affect the quality and reliability of a non-contacting time-of-flight instrument. Because a plumb bob utilizes a mechanical measurement, all of the conditions just listed do not affect the action by which this instrument generates a measurement thereby improving the chances of a repeatable and reliable measurement. Because a plumb bob is a straight line measurement, each time it is activated it takes a reading from the same spot within the bin every time. There is no worrying about installation angles in order to catch the optimal surface area as would be the case with many of the non-contacting technologies. Accuracies of measurement differ depending on which manufacturer one is considering but it is reasonable to expect a measurement accuracy of 0.25% to 0.5% of measured range.

Myth #3: Electromechanical devices are not directly compatible with current control systems.
  Before the introduction of modern electronics, this statement held more credence, but today most brands of electromechanical plumb bob devices will communicate directly with PLC control systems via an analog 4…20ma signal or for some suppliers even a MODBUS RTU or Profibus digital protocol. If a supplier is still specifying an intermediate remote monitoring unit as standard in order to generate an analog output, find another supplier.

Myth #4: The use of pulleys and cables is inefficient and makes the device prone to failure.
  One of the more common questions about this technology regards the perceived high rate of failure due to cables getting tangled in pulley systems. Whether it was a result of constant winding and unwinding or material getting drawn up onto the pulley system and causing interference with the cable setting correctly on the cable, this used to be a legitimate concern. Many plumb bob suppliers have recognized that less is more with pulley systems and have introduced designs that utilize two or less pulleys. One manufacturer has even introduced a single pulley design. Many designs also employ the use of a brush-cleaning system or a spring-tensioned tape scraping apparatus that removes material from the cable or tape as it is retracted into the housing. Manufacturers of plumb bob units that use a tape instead of a cable claim to eliminate tangling completely because of the tape’s tendency to lie only on top of itself as it winds onto a pulley. Add to this the qualities of improved materials as previously stated and these improved features have resulted in a reduction of pulley/cable failures and have enhanced the reliability of plumb bob devices.

Myth #5: A weight and cable device is intrusive. The weight can get caught in the material and can break off into the silo causing damage to components downstream.
  Writing that this is a myth is a bit of a stretch because this has been known to occur. A weight and cable is intrusive in that it does have to enter the bin in which it is measuring material. However, it must be pointed out that most technologies have to intrude in some manner in order to “see” the material it is tasked with measuring. One should remember that the sensor weight and cable is not exposed to the material in the bin except when it is in the process of taking a measurement, and then it is only touching material for an instant before it is retracted back into the housing of the instrument. A well-designed plumb bob device will seal itself during downtime so as to prevent material from blowing up into the housing. With regard to sensor weights breaking off and causing damage, this may be minimized with the advance of stronger cables and engineering controls designed to prevent conditions which could lead to this issue. Many systems now include minimum travel distances for weights in the case that the silo is empty to prevent weights from getting caught in the bottom of the bin. Many systems also have controls built in to prevent motors from continuously pulling on cables in the case that a sensor weight is buried, which itself is a rare occurrence if care is taken during the installation stage. For those prospects who still worry about sensor weights breaking off, most suppliers now offer breakable “digestible” bags that may be filled with the material that is in the bin so that if the weight does break off, it will simply pass through the system.
    Is the electromechanical plumb bob system perfect for every application? No. Because it is only taking a measurement when asked to do so, it is not a true continuous monitoring device. If one absolutely needs a 24/7 measurement for process control, another technology may be a better choice. Also, as with any device installed at the top of a bin, plumb bob units are measuring linear level and not true volume. Volume or weight of material in a bin must be calculated using bin dimensions and bulk density which may lead to slight inaccuracies. Finally, because power is needed to drive a motor capable of moving a sensor weight and cable, plumb bob devices are not available with two-wire 24VDC loop power, as is the case with many of the electronic level systems.
  Inventory management is an important function in any plant environment. Choosing the best solution depends on application considerations, desired output, and, of course, cost. When considering which level measurement solution best suits your needs, seek advice from a tank-monitoring professional, and remember that sometimes an oldie is still a goodie.
      Matthew Hikade is director of sales & marketing for UWT Level Controls USA, the North American division of UWT GmbH, a leading global supplier of level measurement technologies for bulk solids. Hikade can be reached at 877-898-5385 or mhikade@uwtlevel.com. Visit www.uwtlevel.com or www.levelmeasurement.com for more information.

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