Maintenance and plant management use point level sensors as an important component in controlling the production process. Used as high, low, or mid-level sensors, they can assure a continuous supply of material into a production process and prevent downtime. When used in conjunction with an alarm—such as a horn or light—or wire to a PLC, they make the workplace safer by eliminating the need to climb vessels. Point level sensors are easy to install through an existing mounting connection, and many offer varied power supplies, making them convenient to wire to the existing electrical system.
Affordably priced with most at a few hundred dollars, point level sensors may fall into a company’s MRO budget instead of requiring the approval of a capital expense. An MRO purchase can often be done on a credit card, unlike a complete inventory management system that likely needs corporate authorization. Compact and light, point level sensors can be installed incrementally in a few vessels at a time, without a huge investment in either time or money.
An often-overlooked advantage of point level sensors is that they play an important role in redundancy in an inventory management system. Used as a high-level indicator, point level sensors can alert to potential overfills. Working in tandem with continuous contact or non-contact sensors, these highly reliable devices may detect a potential issue that could cause material waste or damage to the structure, expensive equipment, or other sensors in the vessel.
Meet Your Expert
Scott Bonine (pronounced Bo-neen) is a 23-year veteran of the level measurement industry, working with customers all over the world. Scott joined BinMaster in 2010 and is currently sales manager for the Midwest region. His expertise in applications spans many industries including agriculture, aggregates, cement, plastics, and mining, among others. Scott is a familiar face at many trade shows and industry events including GEAPS Exchange, the Powder Show, NPE, and ConExpo.
Process or Inventory Control
Bonine says the best place to start is with an understanding of the difference between point level detection and continuous inventory control. “Point level does exactly as the name infers—the sensor alerts you when material in the bin reaches a certain point,” he explains. “Most often they are used for process control to stop filling a vessel when it is almost full. Conversely, a low-level alert can signal to add material before running out.”
Continuous inventory control uses other types of sensors such as non-contact or guided wave radars, cable-based sensors, ultrasonic, or 3DLevelScanners that continuously measure as levels change.
“Continuous level sensors serve a different purpose in ongoing inventory management,” says Bonine. “However, it’s always a good idea to use a point level sensor as a backup. They are relatively inexpensive and using them as a high level shut off prevents overfilling or damaging a more costly continuous level sensor. It’s a small price to pay for peace of mind.”
Rotary Level Indicators
Rotaries are a familiar and common device used for high or low-level point level indication in bins, tanks, and silos. Rotaries are versatile enough to use in nearly any material from powders and granules with a minimum bulk density of 2 lb./ft.3 to coarse, lump materials with bulk densities up to 150 lb./ft.3 The principle of operation for rotaries is quite simple. When the rotary is used to alert that material has reached a high level while the tank is filling, the paddle rotates continually until material reaches the paddle. When the paddle meets resistance due to the presence of material, it stops rotating and sends an alert to a control room or via a horn, light, or an alarm panel. Conversely, as a low-level indicator, the paddle will begin turning when material drops below the level of the paddle and will send an alert or can be wired to shut off a process system. When tied to process control, it is important to use a rotary with a fail-safefeature to ensure you are alerted in the event of a rotary failure. Rotaries are increasingly applied in new and innovative ways. For high level detection at the interior of a vessel, a vertical extension on a rotary can allow it to be extended as far as 12 feet down into the bin, tank, or silo. This configuration is recommended for a center-fill vessel when the operation requires a specific amount of headroom. Mounted on the top of the vessel, a vertically-extended rotary can alert when material is higher toward the center of the container versus simply detecting the level of material near the sidewall which could be at a lower level when filling (cone up) and at a higher level when emptying(cone down). For thick tank walls, such as those in cement silos, a horizontal extension allows a rotary to be used to detect material levels through the sidewall. When a horizontal extension is combined with a collapsible paddle, the rotary can be installed through a 1-1/4˝ or 1-1/2˝ NPT opening without entering the vessel.
Plants Run on Rotaries
Rotaries are by far the most common point level device. “They are known to work, are a familiar fixture on many vessels, and are used in every industry,” said Bonine. “There is nothing to calibrate and, outfitted with the right paddle, rotaries will work in about any dry solid material.” “When used for process control, a rotary with a fail-safe option is recommended. This feature keeps you apprised of the status of the unit, and most importantly, let’s you know if it’s not working,” he explains further. “For example, a failsafe rotary can alert to the loss of power, failure of the motor, or failure of the electronics.” Another consideration is customizing a rotary. “You might not realize how versatile a rotary can be and how many options there are. There are 19 different paddles for very light to heavy materials, vertical, horizontal, and flexible extensions, heat tubes for high temperatures, and stainless-steel connection options for sanitary or corrosive applications. Mini versions of rotaries are ideal for small vessels and tight spaces.”
Capacitance sensors are designed for a wide array of applications and can easily be customized with different types of probes, lengths, or extensions. These sensors may be used for high, mid, and lowlevel detection in bins, silos, tanks, hoppers, chutes, and other types of vessels where materials are stored, processed, or flowing. Capacitance sensors operate by detecting the presence or absence of material in contact with the probe, sensing minute changes (as low as 0.5 picofarad) in capacitance caused by the difference in the dielectric constant of the material versus the air. Capacitance probes are available with a wide range of options. If your facility has an explosion proof requirement, you will need a capacitance sensor designed and certified for hazardous location applications. If the application is in a high temperature environment or in an area where there is excessive vibration, it is appropriate to install a capacitance probe that houses the electronics and probe in separate enclosures. An extended, flexible cable extension can be attached to a capacitance probe in instances when the sensor is mounted on top of the tank and will be used for high, mid, or low-level detection. Or, a flush mounted probe can be used in narrow or space-constrained areas or in applications where material flow or bridging may damage a standard probe. When the vessel is small or has internal obstructions, a bendable probe can be used to avoid obstructions while still allowing adequate probe surface area to detect the presence or absence of material.
Words of Wisdom about Probes
Bonine’s basic advice is, “A capacitance probe can be a great next step if you are using a rotary and want to move up to the next level.” “When side mounting a probe, you will want to keep the probe length short to avoid material bending the probe. A 6-1/2” probe generally works well. Better yet, use a heavy-duty probe, like the one commonly used in power plants or mining operations,” he suggests. “It’s best to talk with someone at the manufacturer before you spec in a capacitance probe. You might not know you should avoid using probes in low dielectric materials, as they may not sense them.” Bonine adds, “If you are changing materials frequently in a silo, you may need to recalibrate a probe. A rotary may be a better solution.” When it comes to probe selection, there are Delrin, Teflon, 3A, bare, shielded, unshielded, hanging, and flush options. “A Delrin probe is a great all-purpose choice. If you are using a probe in high temperatures or chemicals, a Teflon probe may be a better alternative.”
The vibrating level sensor or vibrating rod is a piezoelectric driven vibration type level switch that can be used for level detection in bins, silos, and hoppers filled with dry bulk solid materials. A vibrating level sensor can detect extremely light, fluffy materials as light as 1.25 lb./ft.3 such as powders and flakes or can be used for heavy materials such as granules or pellets. Vibrating rod level sensors vibrate when there is no material covering the active rod. When the rod is covered with material, the vibration is dampened, and an electronic circuit causes a relay to switch and sends an alert. When the rod becomes uncovered, the vibration restarts, and the relay will switch back.
With advancements in product design, most vibrating rods do not require calibration and easily adjust to the desired sensitivity level. For process-critical applications, be sure to look for features such as a fail-safe alert that provides notification when power is interrupted to the unit to avoid overfills and empty tank situations that could shut down operations. Increasingly versatile, available features may include models for high temperatures or with remote electronics. Some vibrating rods can be extended to a custom length, allowing the vibrating sensor to be used in a top-mounted application for high level detection. Hazardous locations can be equipped with vibrating rods or forks manufactured with FM or CSA approvals.
The Lowdown on Vibrating Rods
“Vibrating rods are often the best choice for extremely lightweight or low dielectric materials that can’t be sensed by a capacitance probe,” says Bonine. “They are also good for light, fluffy materials like Styrofoam that won’t cause enough resistance to activate an alert when using a rotary. Sawdust bins and hoppers containing plastic pellets are places where vibrating rods are often the best choice.” Bonine cautions, “When using a vibrating rod be sure the target material is not sticky, viscous, or clingy as this can dampen the vibration and cause false signals.” “Vibrating rods can offer a lot of versatility in processing operations. There are models that can be used in super high temperatures up to 482 degrees Fahrenheit. They can also be extended up to 13 feet into a vessel when rigid or flexible extensions are added. You might not know they can be used to detect sediment settled at the bottom of a tank.”
A diaphragm—or pressure—switch was the earliest type of point level sensor and is still among the most prolific today. These switches are the most affordable and are highly reliable when used in free-flowing dry and granular materials. Models are available for ordinary as well as hazardous environments where there is a risk of combustible dust. The diaphragm switch works by activating a sensitive microswitch when material reaches the level of the switch in the bin. It sends a signal that can be used to start or stop a process or alert to a high, medium, or low-level in the vessel. Diaphragm switches are offered for either internal or external mounting and with a variety of diaphragm materials. They can be wired to a light, horn, alarm, or into a company’s PLC to indicate an alert status.
A tilt switch is an affordable, reliable high-level indicator that is easy to install and requires no routine maintenance. A hanging tilt switch is installed by suspending it from a flexible cable over a control point. Its principle of operation is quite simple— as material rises below the switch, it will tilt and activate a microswitch when the tilt reaches 15 degrees. Tilt switches are routinely used in bins or silos or over a conveyor belt or open pit. A hanging tilt switch can also be used for plugged chute detection. Alternatively, a fixed-mount tilt switch mounts from the outside on the top of a vessel through a process connection. It operates by utilizing an angular motion transferred into linear motion to activate an electrical microswitch that can be used for a direct input to a control system or to activate an external alarm. A fixed mount tilt switch can be custom-made in lengths from one to eight feet, depending on the distance from the top of the bin an alert should be activated. Newer, patented models are available in a mercury-free design for applications that prohibit the presence of the substance in their operations.
Myriad Mounting Options
Flexibility is a key attribute of point level sensors. While more commonly used for high level indication, they can trigger an alarm anywhere along the vessel wall, in the cone, or a hopper— alerting to low levels for timely refills and adjusting for variances in seasonal high-level inventory fluctuations.
The Easiest Way to Avoid Mistakes When Selecting a Point Level Sensor
Point level sensors are inexpensive, easy to use, and are very durable. The key to success is selecting the right one for an application. “Within each type of point level sensor there are so many options it can be overwhelming. You don’t want to use a baseball bat to hit a hockey puck,” joked Bonine. “Talk is cheap. Make a phone call or send an email to the manufacturer to get free advice on the best solution for your material, application, and budget.”