unraveling the complexities of level detection · turck inc. 3000 campus drive minneapolis, mn...

TURCK Inc.3000 Campus DriveMinneapolis, MN 55441Phone: (763) 553-7300Fax: (763) 553-0708Application Support:1-800-544-7769www.turck.com

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Unraveling the Complexities of Level Detection

TURCK Inc. 3000 Campus Drive Minneapolis, MN 55441-2656Phone: 763-553-7300 • Application Support: 1-800-544-7769 • Fax: 763-553-0708 • www.turck.com

When it comes to using sensors for level detection, the parameters know no bounds. There is really no clear-cut sensor solution that can be implemented across all fi elds and/or applications. This is largely due to the extensive amount of applications that require level detection. Basically, if you can imagine it, it is being sensed; from food and beverage products like cookie dough, pancake batter and seasoning mix, to rocks and gravel, hot wax and glue. Nearly all products produced during an industrial process application need to be monitored to determine level.

For many applications, accurately gauging level in a container has more implications and poses more risks than merely reporting a container’s empty/full conditions. Medias that “run dry” can cause damage to other parts and processes in the line, and messy over-fi lls are not just a pain-staking process to clean, but may pose risks involved with the media being sensed and environment where located (hazardous materials and locations).

Many factors must be considered when determining an application’s sensing needs. The material used for the container, the media being sensed, and the environmental conditions the application consists of, merely touch on the variety of issues a manufacturer/designer/engineer must take into consideration. Only after careful examination can one determine the best sensing option. And there are many options: from probe to capacitive to ultrasonic, sensors have been consistently fi ne-tuned and redesigned to encompass more application requirements. This variety of sensing options enables users to choose the sensor most appropriate per application. However, making this choice is not often easy, and can become confusing. This paper will help you unravel the complexities and differences between sensor types and applications they are used in.

Unraveling the Complexities of Level Detection


SENSORS FOR LEVEL DETECTION

Generally, there are two overarching sensor categories for level detection: single point level detection and continuous level detection. Single point level detectors determine one point of the level of a media, whereas continuous level detection provides constant evaluation of the contained media.

CAPACITIVECapacitive sensors can “see through” lower dielectric materials such as plastic or glass to detect higher dielectric materials, such as liquids. View industrial products and their dielectric constants. This allows capacitive sensors to detect the level of many materials directly through the wall of a plastic container, or by utilizing a sight glass or tank well for metal tanks.

Capacitive sensors are used to detect a wide variety of granular or powdered materials, for instance plastic pellets in a hopper for injection molding processes. Further, intrinsically safe NAMUR capacitive sensors are used in explosive areas such as grain elevators, to detect materials ranging from rice and barley malt, to corn and soybeans.

Capacitance is a function of the surface area of two electrodes, the distance between the electrodes, and the dielectric constant of the material between

the electrodes. Capacitive sensors work when two metallic electrodes (A and B) are placed in a feedback loop of a high frequency oscillator. When no target is present, the sensor’s capacitance is low and the oscillation amplitude is small. When a target approaches the face of the sensor, it increases the capacitance and the amplitude of oscillation. The amplitude of oscillation is measured by an evaluating circuit that generates a signal to turn the output ON or OFF. Generally, the larger the dielectric constant of a material, the greater the achievable operating distance of the sensor.


ULTRASONICUltrasonic sensors emit an ultrasonic pulse that refl ects back from any object entering the sonic cone.

Because sound has a constant velocity at a given temperature and humidity, the time taken for this echo to return to the sensor is directly proportional to the distance of the object. The sensor’s output status is dependent on the comparison of this time with the setting of the detection zone.

Ultrasonic sensors are typically mounted above a container and emit the pulse downward to detect the level of the contained media. Solids, fl uids, granular and powdery targets can be detected by ultrasonic sensors. Both air temperature and humidity infl uence the pulse, and consequently change

the sensor’s rated operating distance. Note that hot surfaces refl ect sonic waves less than cold ones, and ultrasonic sensors may not be suitable for use above turbulent areas.

Some ultrasonic sensors have been developed to sense through metal container walls. The sensors are non-invasive and mount to the outside of a container without compromising the structural integrity. The container’s empty or full conditions are programmed into the sensor. The sensor is able to “see-through” a container wall by generating a high-frequency ultrasonic pulse that is transferred into the container wall through a coupling gel. As the pulse enters the container wall it is infl uenced by the container contents and refl ected back to the sensor where the information is analyzed and compared to the pre-set conditions of the sensor.

See-through ultrasonic sensors are designed for use where non-invasive level detection is desired, but the container is metal and unable to be sensed through using capacitive sensors. The sensors are used in high-pressure, hazardous and/or sterile applications, like those found in food, beverage and pharmaceutical industries.

How does an ultrasonic see-through sensor operate?

All manufacturers create devices differently. TURCK ultrasonic see-through sensors operate via two modes:

REVERB MODE: Evaluates the ultrasonic pulse as it reverberates within the container wall. This pulse travels through the container wall until it reaches the inner wall. The reverb mode is best suited for liquid with low viscosity, and when stirring devices are used inside the container.

ECHO MODE: Evaluates the ultrasonic pulse as it travels through the liquid and echoes offof the opposite container wall. The echo mode is best suited for liquid with high viscosity, as the mode can see-through media that may have coated the container’s inner wall.


PROBESMagnetostricitive linear displacement transducers (LDT) provide continuous, absolute position detection. LDTs detect the position of an external magnet along the active stroke of a sensor without causing any wear on the sensor parts, providing longer and (potentially) better performance.

LDTs can be used in conjunction with probes for continuous liquid level detection by monitoring the position of a magnetic fl oat along the active stroke of the sensing tube. Multiple LDT probe sensor variations increase level detection possibilities, including models with sanitary and intrinsically safe ratings for food, beverage and pharmaceutical applications.

Another method for providing continuous point level detection is to use a linear variable displacement transducer (LVDT) that incorporates a fl oat coupled to a nonmagnetic rod attached to the core of the LVDT. When the level changes, the fl oat moves up or down bringing the LVDT core with it. LVDTs are good for low-pressure and low-viscosity liquids.

Conductive probe sensors detect empty and full conditions by extending two probes into a container and sensing the continuity between them to determine the level of media in the container. In order for this single point detection to work, the media in the tank must be conductive. Conductive probes can also be used in applications to control the fi lling and draining of a container, or as high and low level alarms.

PRESSUREPressure sensors detect the level of media as it exerts pressure on the sensor. Generally used for liquid level detection, the sensor is able to continuously output this pressure reading that is easily converted to a liquid level. Pressure sensors are externally mounted and therefore do not interfere with the internal contents of the container.

LEVEL SENSING CONSIDERATIONS

ContainerThis paper uses container as a generic term for anything used to hold a medium, whether it is a tank, cylinder or anything else used for this purpose.


MATERIAL

Containers can be made from plastic, glass, metal, steel and stainless steel, among other things. There is no real standard for the materials containers are made from, though the material used infl uences decisions affecting what mediums can be contained and what type of sensor can be used per container. For example, a capacitive sensor cannot sense through containers made from stainless steel and a “see-through” ultrasonic sensor designed for this function must be used.

SizeThe size of a container is based on the application it is being used for. The size can range from very tall/short in height to very large/small in diameter, and includes cylinders, funnels, and other shapes designed for specifi c applications. The size greatly infl uences the types of sensor used, as well as the reaction time of the sensor and the rate of fl uid change. Tall (height) and small (diameter) containers also have a faster rate of change than shorter/larger containers, which affects the sensor’s reaction ability.

A sensor’s applicability can also be affected by the size of the container. For instance, if a container is very tall with a small diameter some sensors will not work to sense levels within this container. Probes will not fi t inside the container and an ultrasonic “see-through” sensor will not mount fl ush to the container wall.

Shape

A container’s shape can also vary greatly depending on the application. From very tall/short in height to very small/large in diameter, the size is totally dependant on the application and user’s demands. Shapes include pressurized containers and cylinders, silos, funnels, and just about anything you can imagine to be used to contain something.

Special properties

This includes instances where atypical container types are used to accommodate certain industry or medium (material being sensed) considerations. One such instance is double-jacketing containers for the food and beverage industry. This method is similar to a double-broiler, where a pressurized “jacket” is built around the actual container and is used to heat, cool and control the conditions of the liquid contained inside. It is impossible to detect liquid level within the container through the jacket, without somehow removing a section of the jacket so as to position a sensor (for instance, an ultrasonic “see-through” sensor) against the container wall to detect the level inside. It is extremely expensive to retrofi t an existing container to do this. Note that industrial pressurized containers also entail extremely expensive retrofi tting associated with recertifi cation and downtime.


MEDIA PROPERTIES

The gamut of materials being sensed is practically insurmountable. Basically, if you can imagine a material, it is being sensed. From fi ne powder like fl our and makeup [powders], to large broken rocks and boulders in a quarry, the type of media sensed has a profound impact on the type of sensor that can be used. Instances that produce a large charge of static electricity may render run-of-the-mill sensors inoperable. Media, like dust, can also permeate the sensor causing damage, and in some cases, extreme environmental wear can harm a sensor.

DryGrain elevators produce fi ne dust particles in the air that can result in explosive conditions. To function in this volatile environment, a sensor must be designed for explosion proof and/or intrinsically safe environments. These sensors must be extremely rugged and able to resist extreme temperatures and fl ammability, in addition to being specifi ed for hazardous locations.

Fine powders can also affect a sensor’s operation. False outputs are possible if the powder permeates the sensor’s housing. If fi ne powder accumulates on the outside of the sensor and is somehow affected by moisture (water, condensation, etc.), the ensuing buildup can inhibit the sensing fi eld and prevent a sensor from properly functioning.

Elaborating on the quarry scenario, the size and weight of the media being sensed may also pose issues for the common sensor. When gravel, rocks or large stone (even boulders) are dumped and poured into a container, the sensor can be damaged by mere contact. It’s easy to imagine media of this nature destroying or rendering a sensor in this environment useless. Sensors must be designed for extreme wear and harmful environments, in addition to careful placement (mounting), to withstand these conditions. Further, these conditions may also produce dust that, as previously mentioned, can harm the sensor.

The rapid transfer of plastic pellets in hoppers can cause high static electricity. When the transfer occurs for extended periods of time, as often found in said applications, a high amount of static electricity causes electromagnetic interference with the internal components of the sensor resulting in sensor failure. High static electricity in these applications can also cause the plastic to stick together, generating a false reading by the sensor, i.e. pellets “clumping” together can cause error in level detection.


LiquidsPerhaps liquids produce the most adverse and challenging conditions for sensors. The term liquid is not an accurate statement to encompass all the media needed to be measured in this category. The better phrase? Anything that is not dry. From fruit cocktail to honey, to wax and liquid nitrogen, there is simply no norm. Stemming from that fact, there is really not a norm for sensor application in this realm, though there are ways to help one determine which is best suited per application.

The viscosity of the media has a large impact on the type of sensor able to be used per application. Depending on the viscosity, the media may cling to the sides of the container or, when using a probe, cling to the actual probe and prevent the sensor from providing an accurate reading. Depending on the thickness of the media, some sensors will work better than others. For instance, probes work better for sensing honey, as the honey “pulls” off a probe more quickly than the sides of container, therefore one would generally not use an ultrasonic “see-through” sensor to sense honey.

Foamy media or those that produce foam when agitated also require special consideration when choosing a sensor. Not only is it extremely diffi cult for a sensor to detect foam through a container wall, the foam may also build-up on the container sides potentially causing sensor malfunction (i.e. foam dries on the container’s sides). If using an ultrasonic sensor positioned over the container to detect liquid level, foam can easily fl ow over the container because the sensor is programmed to sense the liquid, not the foam level.

Product build-up is also a large consideration in these applications, as wax, glue and other adhesives can build-up on the sides of the containers and interfere with the fl ow and the sensors reading. Non-homogenous liquids, for instance fruit cocktail, can lodge and jam fl oats on probes and also cause sensor failure. Note that some capacitive sensors are designed to ignore this buildup.

Media that is allowed to change states will also pose issues for sensor reading. For example, a sensor that is programmed to detect hot wax will not necessarily detect wax that has been allowed to cool. Further, media that is hot enough to produce vapors may also pose problems and false reads for some [ultrasonic] sensors, as the vapors impede on the pulse and/or range of the sensor.


APPLICATION CONDITIONS

The environment where an application is located is vital to consider prior to choosing sensors for level detection. Rugged and sanitary environments, temperature variables, and hazardous areas all require uniquely designed devices to withstand the rigors associated with each area.

SanitarySanitary and hygienic environments, like those found in food, beverage and pharmaceutical industries require absolute cleanliness. These environments must maintain sanitary standards, for instance the 3A standard for dairy applications. Some of these environments, like breweries, are also subject to frequent washdowns by high-pressure and possibly caustic liquids to ensure cleanliness.

Sensors must be capable of operating in these applications, in addition to being rated for use in sanitary locations. Sensors are made from material compliant with these locations, and are sometimes fi tted with special mounting mechanisms for increased usability.

TemperatureAll sensors have temperature ratings; therefore a particular sensor’s feasibility for an application is dependent on its rating and the temperature within the environment. Some sensors are better for extreme cold/heat better than others. As previously mentioned in this paper, ultrasonic sensors are affected by heat waves, therefore an ultrasonic sensor would not be effective if mounted above a media producing heat waves.

Some applications entail requirements that make sensor use unfeasible, for instance when frying potato chips in 400 degree oil. The oil produces heat waves that prevent ultrasonic sensors use, the sides of the metal container are too hot for ultrasonic “see-through” or capacitive sensors to be used, and the agitation caused by the dipping/frying action prevents accurate detection by other sensors. Another example includes a liquid being held at the freezing point actually freezing. Once frozen, the sensor will no longer be able to detect the liquid, as liquid and ice are measured (detected) differently.

AgitationMany level detection applications involved some measure of mixing and/or agitation to the media, and a sensor in this environment must be able to function around said disturbances.


Different processes negate the use of different sensors in these applications. If air is brought into the liquid at a high speed, for instance when mixing bread dough, air pockets will form within the media and potentially cause a sensor’s misread. Further, low/high speed mixing may cause agitation that will cause [probe] sensors to false output.

Filling and mixing simultaneously can make nearly every sensor produce a false reading, for example when large/aggressive fi lling cause vortex formation. Further, when large quantities of fi ll cause spray, the sensor may pick up the spray and begin to measure the spray rather than level.

CONCLUSION

Many types of level detection devices are available to aid in the determinant of media within containers. These devices are implemented to ensure reliability of processes, and prevent potential errors that could be caused from the former method of level detection: a person visually monitoring the operation. The time that must be dedicated for human monitoring in these applications can be ineffi cient and costly. Not to mention, the monotonous action or a simple distraction can lead to errors.

This paper has begun to identify the numerous sensing tools used to detect level, and some of the instances where they may be implemented. This should serve as a general guide for sensor’s use in these applications, and inform one about the complexities involved therein. As previously mentioned, the sheer variety of applications and materials being sensed requires close examination to make an informed decision in regard to sensing options. It is good practice to consult a sensing specialist, if on-staff, or other persons/resources well versed on this subject prior to sensor purchase.

In some situations, it may be found that sensors are unfeasible for a particular application. In this case, a manufacturer may choose to use human monitoring, or examine other measurement methods. For instance, if level is being monitored to prevent pump “run-dry” or fl ow rate, other (pressure/fl ow) sensing options can be implemented elsewhere in the line.

Authored by Karen Keller

D15 TURCK Inc. 3000 Campus Drive Minneapolis, MN 55441 Application Support: 1-800-544-PROX Fax: (763) 553-0708 www.turck.com

CapacitiveSensors

Industrial Products and their Dielectric Constants

Material Dielectric Constant

Epoxy resin 2.5 - 6

Ethanol 24

Ethyl bromide 4.9

Ethylene Chloride 10.5

Ethylene Dichloride 11.0

Ethylene glycol 38.7

Ethylene Oxide 14.0

Ferrous Oxide 14.2

Fired Ash 1.5

Flour 2.5 - 3.0

Formic Acid 59.0

Freon R22 & 502, liquid 6.1

Gasoline 2.2

Glass 3.1 - 10

Glass, raw material 2.0 - 2.5

Glycerine 47

Hexane 1.9

Hydrochloric Acid 4.6

Hydrogen cyanide 95.4

Hydrogen peroxide 84.2

Ice, -5C 2.85

Ice, -18C 3.16

Isobutylamine 4.5

Lime, shell 1.2

Marble 8.0 - 8.5

Melamine resin 4.7 - 10.2

Methane, liquid 1.7

Methanol 33.6

Mica, white 4.5 - 9.6

Milk, powdered 3.5 - 4

Nitrobenzene 36

Neoprene 6 - 9


ABS resin, pellet 1.5 - 2.5

Acetic Acid 4.1

Acetone 19.5

Acetyl bromide 16.5

Acrylic resin 2.7 - 4.5

Air 1.0

Alcohol, industrial 16 - 31

Alcohol, isopropyl 18.3

Ammonia 15 - 25

Aniline 5.5 - 7.8

Aqueous solutions 50 - 80

Asbestos 3.0

Ash (fly) 1.7

Bakelite 3.6

Barley powder 3.0 - 4.0

Benzene 2.3

Benzyl acetate 5

Butane 1.4

Cable sealing compound 2.5

Calcium carbonate 9.1

Carbon Dioxide 1.6

Carbon tetrachloride 2.2

Celluloid 3.0

Cellulose 3.2 - 7.5

Cement 1.5 - 2.1

Cement powder 5 - 10

Cereal 3 - 5

Charcoal 1.2 - 1.8

Chlorine, liquid 2.0

Coke 1.1 - 2.2

Corn 5 - 10

Ebonite 2.7 - 2.9

TURCK Inc. 3000 Campus Drive Minneapolis, MN 55441 Application Support: 1-800-544-PROX Fax: (763) 553-0708 www.turck.com D16

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IndustrialAutomation

Industrial Products and their Dielectric Constants


Rubber 2.5 - 35

Salt 6.0

Sand 3 - 5

Shellac 2.0 - 3.8

Silicon dioxide 4.5

Silicone rubber 3.2 - 9.8

Silicone varnish 2.8 - 3.3

Soybean 2.8

Styrene resin 2.3 - 3.4

Sugar 3.0

Sugar, granulated 1.5 - 2.2

Sulfur 3.4

Sulfuric acid 84

Teflon, PCTFE 2.3 - 2.8

Teflon, PTFE 2.0

Toluene 2.3

Trichloroethylene 3.4

Urea resin 6.2 - 9.5

Urethane 3.2

Vaseline 2.2 - 2.9

Vinyl Chloride 2.8

Water 48 - 88

Wax 2.4 - 6.5

Wood, dry 2 - 7

Wood, pressed board 2.0 - 2.6

Wood, wet 10 - 30

Xylene 2.4

Zinc Oxide 1.7

Zirconium Oxide 12.5

Zirconium Silicate 5.0


Nylon 4 - 5

Oil, for transformer 2.2 - 2.4

Oil, paraffin 2.2 - 4.8

Oil, peanut 3.0

Oil, petroleum 2.1

Oil, soybean 2.9 - 3.5

Oil, turpentine 2.2

Paint 5 - 8

Paraffin 1.9 - 2.5

Paper 1.6 - 2.6

Paper, hard 4.5

Paper, oil saturated 4.0

Perspex 3.2 - 3.5

Petroleum 2.0 - 2.2

Phenol 9.9 - 15

Phenol resin 4.9

Polyacetal (Delrin) 3.6

Polyamide (nylon) 2.5

Polycarbonate 2.9

Polyester resin 2.8 - 8.1

Polyethylene 2.3

Polypropylene 2.0 - 2.3

Polystyrene 3.0

Polyvinyl Chloride resin 2.8 - 3.1

Porcelain 4.4 - 7

Press board 2 - 5

Propane, liquid 1.6

Propylene, liquid 11.9

Quartz glass 3.7

Rice, dry 3.5

unraveling the complexities of level detection · turck inc. 3000 campus drive minneapolis, mn...

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