The amount of level-sensing devices that might 'pop up' in any given process plant depends on a number of factors. Level-sensing devices are used to facilitate batch processing, augment process control, track inventory, provide overfill protection, and prevent damage to rotating machinery, such as pumps and motors. The need for them is obvious.

According to Bill Sholette, product manager at Ametek Drexelbrook (Horsham, Pa.), 'The percentage of level measurement in a given process facility is dependent on what the facility is making. If pressed for a hard answer, however, 15 to 20% is probably a good one. The U.S. level market is currently at about $430 million for 2002 with a total number of units at about 999,000. This is for all level, both electronic and mechanical, point and continuous.'

Level measurement says nothing about the chemical or physical state of any process variable. And although things like weight or volume can be inferred by a level measurement, it takes knowledge of vessel dimensions or a material's pressure or temperature or to hone the accuracy of these measurements. Even when level values help fine tune process control and batching operations, it is a still 'place holder' that lets the control system know when media is where it should be.

Although any number of level technologies can be applied to process optimization or batching operations, several exhibit the accuracy and repeatability needed for these operations. Designs include ultrasonic-, radar-, and floatation-based systems. Properly applied, all of these systems are viable. With float systems, however, maintenance and cleaning are key to accuracy and proper operation. Depending on the process media, this can be very difficult.

Maintaining accuracy in small vessels with aggressive fluids is especially taxing. A Germany-based Solvay GmbH plant uses an Endress+Hauser (E+H, Greenwood, Ind.) Micropilot M device to measure level in a buffer vessel with their process. The vessel, which contains hydrogen fluoride, is only 6-in. dia. and 8-in. tall. The Micropilot M device, which is available in two frequencies (6 or 26 GHz), was chosen by Solvay because higher accuracy 26 GHz option could use a very small 1.5-in. horn antenna. And because the Micropilot M's radar technology requires no blocking distance, level can be continuously measured to the top of the small tank.

The high accuracy and repeatability of radar-based devices also make them ideal for production monitoring of high-quality, high-cost products. Expensive reagent materials and other feedstocks, pharmaceuticals, and fine chemicals often come under the scrutiny of radar-based devices. A device like Ohmart/Vega's (Cincinnati, O.) Puls 42 has the level of accuracy and repeatability suited to this task.

Manufacturers of radar level sensing devices have improved the safety and, hence, the versatility of these devices. The power required to operate many of the currently commercially available devices has decreased substantially. In fact, the power requirement is so low that many radar devices can now be loop powered. Using low power means that these types of devices can be used without FCC restrictions and are approved for use in nonmetallic and open topped vessels. They pose no health or accident threat to personnel or do not interfere with other inplant signals.

Because they are noncontact, radar and ultrasonic-based devices can be adapted to measure sticky or aggressive media. Unlike 'bottom-of-the-tank' pressure sensors that are prone to product build-up and resulting inaccuracy, these gages stay cleaner and usually require much less routine maintenance. With penetration above the vessel's media level, there is no threat of product leakage due to device or seal failure.

According to Shawn Rigby, product manager for STI Automation Product Group (Logan, Ut.), 'Top-of-the-tank level sensors with specialty-material transducers, like the stainless steel US06, have been adapted to difficult environments. Replacing bottom-mounted sensors for a candle manufacturer eliminated more than maintenance issues. The ultrasonic US06 provided required accuracy while dealing with an assortment of 60+ colors and varying translucencies of hot wax. It can also withstand the 300 °F washdown temperatures required to clean the vessels and remove hardened wax buildup.'

Tank configurations are not always conventional. In some cases, electronics and antenna cannot be mounted inline. Krohne Inc. (Peabody, Mass.) offers its loop-powered BM702 frequently modulated continuous wave radar level device in modular units. For unusual, hazardous, or hostile environments, microwave extensions are available to link the electronics with the horn antenna. These extensions accommodate lateral or angled installations, as well as provide isolation for electronics in high-temperature applications. For extreme applications, the BM702 can be equipped with antenna cooling or an antenna flushing system.

While radar and ultrasonic-based level continuous measurement devices require only a single top-mount penetration, they are not the only technologies that offer this desirable attribute. RF admittance, capacitance, and various types of mechanical devices also can be mounted this way. Even when the level device (electronics plus transducer) may be heavy, cumbersome, or difficult to install and wire in certain applications, mounting above the liquid line is still preferable.

However, if the particular process media is under pressure or gives off vapor into the headspace, other precautions are necessary. Sealing a top-mounted device against leaks and fugitive emissions of vapors and osmotic liquids requires special apparatus. Ametek Drexelbrook has developed Seal-Tyte, a sealing method used with its RF transmitters, which incorporates a flange-mounted sensing element with a special facing that is hermetically sealed to the probe insulation. The facing acts as a flange gasket to completely seal the vessel. The hermetic seal of the facing to the rod insulation prevents material from leaking. A secondary seal is incorporated into the sensor mounting, providing additional security in case of sensor damage. The primary seal is tested to 3,000 psi.

Dust presents a health and safety hazard in many process industries. Companies that grind, mix, or process dust-producing materials, such as grains or mineral aggregates, often deal with the problem by enclosing dust-producing operations when they can. Use of explosion-proof or intrinsically safe controls/control elements becomes a requirement and use of complex and sophisticated dust collection and disposal systems become an art form.

LeHigh Portland Cement in Leeds, Ala., needed to keep track of level in a 70-ft long by 20-ft wide bin that received hot, dusty product at 375 °F. LeHigh engineers evaluated a number of continuous sensing technologies before settling on the SonaMaster sonic level detector developed by Thermo Ramsey (Minneapolis, Minn.).

The sonic device-its operating frequency is below 20 kHz-uses lower frequencies better suited for long distances, and penetrating environmental factors such as dust, vapors, and foam. The SonaMaster's software allows the device to 'map' vessels to account for ladders, supports, mixers, etc. that interfere with return echo readings and verification of fill times. The SonaMaster also has the ability to 'power up' if it loses an echo instead of going into default. This feature allows the device to remember where the signal was located and keep 'beefing up' the sound amplification until it re-finds the level. 'Cement companies can now actually control the process instead of just monitoring the inventory when they're done filling,' according to Bruce Erickson, manager of Thermo Ramsey's Proline Div.

'Low sound frequency has a better penetration with less interference. That is why ship foghorns are such low tones,' says Russ Carlson, level product manager at Sor Inc. (Lenexa, Kan.). 'The use of low frequency (down to 5 kHz) and a high-power signal allows Sor Inc.'s ultrasonic units to be adapted to a wide variety of applications where either sound absorbency or other barriers to penetration, like foam, vapor, dust, or protective grating above an application, are present.'

Hot dusty environments can also pose problems for point-level sensors. High temperatures can 'fry' electronics and dust can cake on mechanical transducers rendering them useless. Fargo Controls (Eatontown, N.J.) has developed a high temperature capacitive proximity sensor that can be used to detect a wide variety of liquids and solids between -382 to 482°F. The device, which is IP68 rated, uses a high frequency oscillator to create a field near its sensing surface. When the presence of an object in this field changes the oscillation amplitude, the device's remotely mounted electronics changes its output state. Prox sensors of this type-well known in discrete manufacturing applications-can handle liquid and solid point-level applications.

Extremely finely divided dust can prove to be a match for many systems. In the case, of the Luzenac America plant in Three Forks, Mont., talc powder proved to be the undoing of several level-sensing systems. Talc, which has a bulk density of approximately 50 lb/ft³ and is used as an additive in ceramics, rubber, and paper, produced so much dust that it consistently caused default readings in the ultrasonic system and fouled and eventually broke the cables in the weight and cable system electromechanical system that replaced it.

To track the product in its five 12-ft dia. by 50-ft high closed silos, engineering management at the Three Forks plant replaced the functional but high maintenance cable and weight systems with SiloPatrol units developed by Monitor Technologies (Elburn, Ill.). 'These units, also cable-based devices, provided Luzenac America sufficient accuracy for their operation. They did not need high accuracy, reliability was their main concern,' says Jim Stovall, application engineer at Monitor who worked with the engineering staff at Luzenac America.

The SiloPatrol units are top-of-the-tank devices. The housing, which contains the integral motor/worm drive, optical encoder, cable drum, and related control electronics, is air purged to maintain a positive pressure of 3-5 psi to keep dust from entering through the cable entry. The entry point is also mechanically sealed and incorporates a wiper to remove talc from the entering cable. All controls are now solid state. Outputs available are 4-20 mA, RS-485, and a 'retropulse' consistent with older Monitor weigh and cable equipment that SiloPatrol may be called on to replace.

Hill & Griffith Co. (Cincinnati, O.) serves the metal casting industry as a supplier of metal casting and industrial consumable products. Bulk solids they must process and then inventory include a variety of additive minerals for cast metal production, sand, and for their line of strainer and metallurgical cores.

To maintain proper inventory and ensure timely customer order fulfillment of these specialized materials, Hill & Griffith uses BinMaster's (Lincoln, Neb.) new SBR II remote sensor as an integral part of its nationwide inventory system. An electromechanical cable and weight (the bob) system, SmartBob II, is fitted in Hill & Griffith's storage vessels at each location. The units communicate through a digital network using BinMaster's IMS software to Hill & Griffith's corporate headquarters where inventory is closely monitored.

Using a weight system to accurately measure solids can be tricky. Using an encoder on the cable drum, these systems first measure the distance a weighted bob descends to the surface of the material. Once the bob reaches the surface, it is immediately retracted and the encoder readings compared to ensure both full retraction and measurement accuracy. Solids, depending on properties, can behave significantly different than liquids. A bob sliding down the angle of material repose or into a discharge cone can yield erroneous results. In the case of BinMaster's bob system, its mechanical braking system prevents this and minimizes measurement errors that may result in these cases.

One of the most difficult tasks that continuous levels sensors are called on to do is accurately track material interface(s) in a production vessel. Ronan Engineering Co. (Florence, Ky.) has introduced a motorized density level system that is said to provide this. Designed to work in liquids or slurries, this motorized gamma density gage (both source and detector) travels protected by dry wells set 12-18 in. apart over the measurement range of a customer's vessel. Devices are custom applications and the dry wells are matched to the customer's vessel by pressure/temperature rating and metallurgical specifications.

Level sensing may be one of the least represented instruments on the plant floor but that does not diminish its importance to the process. Even in its mundane role of overflow protection, the presence of a humble point-level switch serves to remind the control engineer that it's better to have a process under control and not over the top of the tank.