Level sensor
Encyclopedia
Level sensors detect the level
of substances that flow, including liquids, slurries
, granular materials, and powders
. Fluids and fluidized solids flow to become essentially level in their containers (or other physical boundaries) because of gravity whereas most bulk solids pile at an angle of repose to a peak. The substance to be measured can be inside a container or can be in its natural form (e.g., a river or a lake). The level measurement can be either continuous or point values. Continuous level sensors measure level within a specified range and determine the exact amount of substance in a certain place, while point-level sensors only indicate whether the substance is above or below the sensing point. Generally the latter detect levels that are excessively high or low.
There are many physical and application variables that affect the selection of the optimal level monitoring method for industrial and commercial processes. The selection criteria include the physical: phase
(liquid, solid or slurry), temperature
, pressure
or vacuum
, chemistry
, dielectric constant
of medium
, density
(specific gravity) of medium, agitation (action), acoustical or electrical noise
, vibration
, mechanical shock
, tank or bin size and shape. Also important are the application constraints: price, accuracy, appearance, response rate, ease of calibration
or programming, physical size and mounting of the instrument, monitoring or control of continuous or discrete (point) levels.
), microwave
(radar
), capacitance, optical, pulsed-ultrasonic and ultrasonic level sensors.
Single-probe vibrating level sensors are ideal for bulk powder level. Since only one sensing element contacts the powder, bridging between two probe elements is eliminated and media build-up is minimized. The vibration of the probe tends to eliminate build-up of material on the probe element. Vibrating level sensors are not affected by dust, static-charge build-up from dielectric powders, or changes in conductivity, temperature, pressure, humidity or moisture content. Tuning-fork style vibration sensors are another alternative. They tend to be less costly, but are prone to material buildup between the tines.
, Bentonite
or fly ash
, special paddle designs and low-torque motors are used. Fine particles or dust must be prevented from penetrating the shaft bearings and motor by proper placement of the paddle in the hopper or bin and using appropriate seals.
. The probe is driven through a shielded coaxial cable to eliminate the effects of changing cable capacitance to ground. When the level changes around the probe, a corresponding change in the di-electric is observed. This changes the admittance of this imperfect capacitor and this change is measured to detect change of level.
Float-type sensors can be designed so that a shield protects the float itself from turbulence and wave motion. Float sensors operate well in a wide variety of liquids, including corrosives. When used for organic solvents, however, one will need to verify that these liquids are chemically compatible with the materials used to construct the sensor. Float-style sensors should not be used with high viscosity (thick) liquids, sludge or liquids that adhere to the stem or floats, or materials that contain contaminants such as metal chips; other sensing technologies are better suited for these applications.
A special application of float type sensors is the determination of interface level in oil-water separation systems. Two floats can be used with each float sized to match the specific gravity of the oil on one hand, and the water on the other. Another special application of a stem type float switch is the installation of temperature or pressure sensors to create a multi-parameter sensor. Magnetic float switches are popular for simplicity, dependability and low cost.
. These sensors are suitable for use with highly viscous liquids such as grease, as well as water-based and corrosive liquids. This has the additional benefit of being a relatively low cost technique for point level monitoring.
Conductive level sensors use a low-voltage, current-limited power source applied across separate electrodes. The power supply is matched to the conductivity of the liquid, with higher voltage versions designed to operate in less conductive (higher resistance) mediums. The power source frequently incorporates some aspect of control, such as high-low or alternating pump control. A conductive liquid contacting both the longest probe (common) and a shorter probe (return) completes a conductive circuit. Conductive sensors are extremely safe because they use low voltages and currents. Since the current and voltage used is inherently small, for personal safety reasons, the technique is also capable of being made “Intrinsically Safe
” to meet international standards for hazardous locations
. Conductive probes have the additional benefit of being solid-state devices and are very simple to install and use. In some liquids and applications, maintenance can be an issue. The probe must continue to be conductive. If buildup insulates the probe from the medium, it will stop working properly. A simple inspection of the probe will require an ohmmeter connected across the suspect probe and the ground reference.
Typically, in most water and wastewater wells, the well itself with its ladders, pumps and other metal installations, provides a ground return. However, in chemical tanks, and other non-grounded wells, the installer must supply a ground return, typically an earth rod.
level sensors are used for non-contact level sensing of highly viscous liquids, as well as bulk solids. They are also widely used in water treatment applications for pump control and open channel flow measurement. The sensors emit high frequency (20 kHz to 200 kHz) acoustic waves that are reflected back to and detected by the emitting transducer.
Ultrasonic level sensors are also affected by the changing speed of sound
due to moisture, temperature, and pressures. Correction factors can be applied to the level measurement to improve the accuracy of measurement.
Turbulence, foam, steam, chemical mists (vapors), and changes in the concentration of the process material also affect the ultrasonic sensor’s response. Turbulence and foam prevent the sound wave from being properly reflected to the sensor; steam and chemical mists and vapors distort or absorb the sound wave; and variations in concentration cause changes in the amount of energy in the sound wave that is reflected back to the sensor. Stilling wells and wave guides are used to prevent errors caused by these factors.
Proper mounting of the transducer is required to ensure best response to reflected sound. In addition, the hopper, bin, or tank should be relatively free of obstacles such as weldments, brackets, or ladders to minimise false returns and the resulting erroneous response, although most modern systems have sufficiently "intelligent" echo processing to make engineering changes largely unnecessary except where an intrusion blocks the "line of sight" of the transducer to the target. Since the ultrasonic transducer is used both for transmitting and receiving the acoustic energy, it is subject to a period of mechanical vibration known as “ringing”. This vibration must attenuate (stop) before the echoed signal can be processed. The net result is a distance from the face of the transducer that is blind and cannot detect an object. It is known as the “blanking zone”, typically 150mm – 1m, depending on the range of the transducer.
The requirement for electronic signal processing circuitry can be used to make the ultrasonic sensor an intelligent device. Ultrasonic sensors can be designed to provide point level control, continuous monitoring or both. Due to the presence of a microprocessor and relatively low power consumption, there is also capability for serial communication from to other computing devices making this a good technique for adjusting calibration and filtering of the sensor signal, remote wireless monitoring or plant network communications. The ultrasonic sensor enjoys wide popularity due to the powerful mix of low price and high functionality.
Capacitance level sensors excel in sensing the presence of a wide variety of solids, aqueous and organic liquids, and slurries. The technique is frequently referred to as RF for the radio frequency signals applied to the capacitance circuit. The sensors can be designed to sense material with dielectric constant
s as low as 1.1 (coke and fly ash) and as high as 88 (water) or more. Sludges and slurries such as dehydrated cake and sewage slurry (dielectric constant approx. 50) and liquid chemicals such as quicklime (dielectric constant approx. 90) can also be sensed. Dual-probe capacitance level sensors can also be used to sense the interface between two immiscible liquids with substantially different dielectric constants, providing a solid state alternative to the aforementioned magnetic float switch for the “oil-water interface” application.
Since capacitance level sensors are electronic devices, phase modulation and the use of higher frequencies makes the sensor suitable for applications in which dielectric constants are similar. The sensor contains no moving parts, is rugged, simple to use, and easy to clean, and can be designed for high temperature and pressure applications. A danger exists from build-up and discharge of a high-voltage static charge that results from the rubbing and movement of low dielectric materials, but this danger can be eliminated with proper design and grounding.
Appropriate choice of probe materials reduces or eliminates problems caused by abrasion and corrosion. Point level sensing of adhesives and high-viscosity materials such as oil and grease can result in the build-up of material on the probe; however, this can be minimized by using a self-tuning sensor. For liquids prone to foaming and applications prone to splashing or turbulence, capacitance level sensors can be designed with splashguards or stilling wells, among other devices.
A significant limitation for capacitance probes is in tall bins used for storing bulk solids. The requirement for a conductive probe that extends to the bottom of the measured range is problematic. Long conductive cable probes (20 to 50 meters long), suspended into the bin or silo, are subject to tremendous mechanical tension due to the weight of the bulk powder in the silo and the friction applied to the cable. Such installations will frequently result in a cable breakage.
A common application of economical infrared-based optical interface point level sensors is detecting the sludge/water interface in settling ponds. By using pulse modulation techniques and a high power infrared diode, one can eliminate interference from ambient light, operate the LED at a higher gain, and lessen the effects of build-up on the probe.
An alternate approach for continuous optical level sensing involves the use of a laser. Laser light is more concentrated and therefore is more capable of penetrating dusty or steamy environments. Laser light will reflect off most solid, liquid surfaces. The time of flight can be measured with precise timing circuitry, to determine the range or distance of the surface from the sensor. Lasers remain limited in use in industrial applications due to cost, and concern for maintenance. The optics must be frequently cleaned to maintain performance.
Microwave sensors are executed in a wide variety of techniques. Two basic signal processing techniques are applied, each offering its own advantages: Time-Domain Reflectometry (TDR) which is a measurement of time of flight divided by the speed of light, similar to ultrasonic level sensors, and Doppler systems employing FMCW techniques. Just as with ultrasonic level sensors, microwave sensors are executed at various frequencies, from 1 GHz to 30 GHz. Generally, the higher the frequency, the more accurate, and the more costly. Microwave is also executed as a non-contact technique, monitoring a microwave signal that is transmitted through the medium (including vacuum), or can be executed as a “radar on a wire” technique. In the latter case, performance improves in powders and low dielectric media that are not good reflectors of electromagnetic energy transmitted through a void (as in non-contact microwave sensors). But the same mechanical constraints exist that cause problems for the capacitance (RF) techniques mentioned previously.
Microwave-based sensors are not affected by fouling of the microwave-transparent glass or plastic window through which the beam is passed nor by high temperature, pressure, or vibration. These sensors do not require physical contact with the process material, so the transmitter and receiver can be mounted a safe distance from the process, yet still respond to the presence or absence of an object. Microwave transmitters offer the key advantages of ultrasonics: the presence of a microprocessor to process the signal provides numerous monitoring, control, communications, setup and diagnostic capabilities. Additionally, they solve some of the application limitations of ultrasonics: operation in high pressure and vacuum, high temperatures, dust, temperature and vapor layers. One major disadvantage of microwave or radar techniques for level monitoring is the relatively high price of such sensors.
Because of the degree of accuracy possible with the magnetostrictive technique, it is popular for “custody-transfer” applications. It can be permitted by an agency of weights and measures for conducting commercial transactions. It is also frequently applied on magnetic sight gages. In this variation, the magnet is installed in a float that travels inside a gage glass or tube. The magnet operates on the sensor which is mounted externally on the gage. Boilers and other high temperature or pressure applications take advantage of this performance quality.
The choice of float and stem materials depends on the liquid in terms of chemical compatibility as well as specific gravity and other factors that affect buoyancy. These sensors work well for liquid level measurements in marine, chemical processing, pharmaceuticals, food processing, waste treatment, and other applications. With the proper choice of two floats, resistive chain level sensors can also be used to monitor for the presence of an interface between two immiscible liquids whose specific gravities are more than 0.6, but differ by as little as 0.1 unit.
Since these sensors sense increasing pressure with depth and because the specific gravities of liquids are different, the sensor must be properly calibrated for each application. In addition, large variations in temperature cause changes in specific gravity that should be accounted for when the pressure is converted to level. These sensors can be designed to keep the diaphragm free of contamination or build-up, thus ensuring proper operation and accurate hydrostatic pressure level measurements.
For use in open air applications, where the sensor cannot be mounted to the bottom of the tank or pipe thereof, a special version of the hydrostatic pressure level sensor can be suspended from a cable into the tank to the bottom point that is to be measured. The sensor must be specially designed to seal the electronics from the liquid environment. In tanks with a small head pressure (less than 100 INWC), it is very important to vent the back of the sensor gauge to atmospheric pressure. Otherwise, normal changes in barometric pressure will introduce large error in the sensor output signal. In addition, most sensors need to be compensated for temperature changes in the fluid.
Air bubbler systems contain no moving parts, making them suitable for measuring the level of sewage, drainage water, sewage sludge, night soil
, or water with large quantities of suspended solids. The only part of the sensor that contacts the liquid is a bubble tube which is chemically compatible with the material whose level is to be measured. Since the point of measurement has no electrical components, the technique is a good choice for classified “Hazardous Areas”. The control portion of the system can be located safely away, with the pneumatic plumbing isolating the hazardous from the safe area.
Air bubbler systems are a good choice for open tanks at atmospheric pressure and can be built so that high-pressure air is routed through a bypass valve to dislodge solids that may clog the bubble tube. The technique is inherently “self-cleaning”. It is highly recommended for liquid level measurement applications where ultrasonic, float or microwave techniques have proved undependable.
in a continuous casting
process of steelmaking. The water-cooled mold is arranged with a source of radiation, such as Cobalt-60 or Cesium-137, on one side and a sensitive detector such as a scintillometer
on the other. As the level of molten steel rises in the mold, less of the gamma radiation is detected by the sensor. The technique allows non-contact measurement where the heat of the molten metal makes any contact technique impractical.
Level
Level or levels may refer to:-Places:*Levél, Győr-Moson-Sopron, Hungary*Level, Ohio, United States*Level Valley*Levels, West Virginia-Engineering-related:*Floor, or storey, of a building or a mine...
of substances that flow, including liquids, slurries
Slurry
A slurry is, in general, a thick suspension of solids in a liquid.-Examples of slurries:Examples of slurries include:* Lahars* A mixture of water and cement to form concrete* A mixture of water, gelling agent, and oxidizers used as an explosive...
, granular materials, and powders
Wiktionary
Wiktionary is a multilingual, web-based project to create a free content dictionary, available in 158 languages...
. Fluids and fluidized solids flow to become essentially level in their containers (or other physical boundaries) because of gravity whereas most bulk solids pile at an angle of repose to a peak. The substance to be measured can be inside a container or can be in its natural form (e.g., a river or a lake). The level measurement can be either continuous or point values. Continuous level sensors measure level within a specified range and determine the exact amount of substance in a certain place, while point-level sensors only indicate whether the substance is above or below the sensing point. Generally the latter detect levels that are excessively high or low.
There are many physical and application variables that affect the selection of the optimal level monitoring method for industrial and commercial processes. The selection criteria include the physical: phase
Phase (matter)
In the physical sciences, a phase is a region of space , throughout which all physical properties of a material are essentially uniform. Examples of physical properties include density, index of refraction, and chemical composition...
(liquid, solid or slurry), temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
, pressure
Pressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...
or vacuum
Vacuum
In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...
, chemistry
Chemistry
Chemistry is the science of matter, especially its chemical reactions, but also its composition, structure and properties. Chemistry is concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds....
, dielectric constant
Dielectric constant
The relative permittivity of a material under given conditions reflects the extent to which it concentrates electrostatic lines of flux. In technical terms, it is the ratio of the amount of electrical energy stored in a material by an applied voltage, relative to that stored in a vacuum...
of medium
Medium
- Communication :* Medium , storage and/or transmission tools used to store and deliver information or data* Transmission medium, in physics and telecommunications, any material substance which can propagate waves or energy...
, density
Density
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...
(specific gravity) of medium, agitation (action), acoustical or electrical noise
Noise
In common use, the word noise means any unwanted sound. In both analog and digital electronics, noise is random unwanted perturbation to a wanted signal; it is called noise as a generalisation of the acoustic noise heard when listening to a weak radio transmission with significant electrical noise...
, vibration
Vibration
Vibration refers to mechanical oscillations about an equilibrium point. The oscillations may be periodic such as the motion of a pendulum or random such as the movement of a tire on a gravel road.Vibration is occasionally "desirable"...
, mechanical shock
Shock (mechanics)
A mechanical or physical shock is a sudden acceleration or deceleration caused, for example, by impact, drop, kick, earthquake, or explosion. Shock is a transient physical excitation....
, tank or bin size and shape. Also important are the application constraints: price, accuracy, appearance, response rate, ease of calibration
Calibration
Calibration is a comparison between measurements – one of known magnitude or correctness made or set with one device and another measurement made in as similar a way as possible with a second device....
or programming, physical size and mounting of the instrument, monitoring or control of continuous or discrete (point) levels.
Point and continuous level detection for solids
A variety of sensors are available for point level detection of solids. These include vibrating, rotating paddle, mechanical (diaphragmDiaphragm
-Optics and photography:* Diaphragm , a stop in the light path of a lens, having an aperture that regulates the amount of light that passes* Diaphragm shutter, a type of leaf shutter consisting of a number of thin blades in a camera-Acoustics:...
), microwave
Microwave
Microwaves, a subset of radio waves, have wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz and 300 GHz. This broad definition includes both UHF and EHF , and various sources use different boundaries...
(radar
Radar
Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...
), capacitance, optical, pulsed-ultrasonic and ultrasonic level sensors.
Vibrating point
These detect levels of very fine powders (bulk density: 0.02 g/cm3 – 0.2 g/cm3), fine powders (bulk density: 0.2 – 0.5 g/cm3), and granular solids (bulk density: 0.5 g/cm3 or greater). With proper selection of vibration frequency and suitable sensitivity adjustments, they can also sense the level of highly fluidized powders and electrostatic materials.Single-probe vibrating level sensors are ideal for bulk powder level. Since only one sensing element contacts the powder, bridging between two probe elements is eliminated and media build-up is minimized. The vibration of the probe tends to eliminate build-up of material on the probe element. Vibrating level sensors are not affected by dust, static-charge build-up from dielectric powders, or changes in conductivity, temperature, pressure, humidity or moisture content. Tuning-fork style vibration sensors are another alternative. They tend to be less costly, but are prone to material buildup between the tines.
Rotating paddle
Rotating paddle level sensors are a very old and established technique for bulk solid point level indication. The technique uses a low-speed gear motor that rotates a paddle wheel. When the paddle is stalled by solid materials, the motor is rotated on its shaft by its own torque until a flange mounted on the motor contacts a mechanical switch. The paddle can be constructed from a variety of materials, but tacky material must not be allowed to build up on the paddle. Build-up may occur if the process material becomes tacky because of high moisture levels or high ambient humidity in the hopper. For materials with very low weight per unit volume such as PearlitePearlite
Pearlite is often said to be a two-phased, lamellar structure composed of alternating layers of alpha-ferrite and cementite that occurs in some steels and cast irons...
, Bentonite
Bentonite
Bentonite is an absorbent aluminium phyllosilicate, essentially impure clay consisting mostly of montmorillonite. There are different types of bentonite, each named after the respective dominant element, such as potassium , sodium , calcium , and aluminum . Experts debate a number of nomenclatorial...
or fly ash
Fly ash
Fly ash is one of the residues generated in combustion, and comprises the fine particles that rise with the flue gases. Ash which does not rise is termed bottom ash. In an industrial context, fly ash usually refers to ash produced during combustion of coal...
, special paddle designs and low-torque motors are used. Fine particles or dust must be prevented from penetrating the shaft bearings and motor by proper placement of the paddle in the hopper or bin and using appropriate seals.
Admittance-type
An RF Admittance level sensor uses a rod probe and RF source to measures the change in admittanceAdmittance
In electrical engineering, the admittance is a measure of how easily a circuit or device will allow a current to flow. It is defined as the inverse of the impedance . The SI unit of admittance is the siemens...
. The probe is driven through a shielded coaxial cable to eliminate the effects of changing cable capacitance to ground. When the level changes around the probe, a corresponding change in the di-electric is observed. This changes the admittance of this imperfect capacitor and this change is measured to detect change of level.
Pulse-Wave Ultrasonic (Non Invasive)
The principle behind a Pulsed-Ultrasonic technology is that the transmit signal consists of short bursts of ultrasonic energy. After each burst, the electronics looks for a return signal within a small window of time corresponding to the time it takes for the energy to pass through the vessel. Only signal received during this window period will qualify for additional signal processing. The dry signal will not be received within this window, and therefore will be ignored.Magnetic and mechanical float
The principle behind magnetic, mechanical, cable, and other float level sensors involves the opening or closing of a mechanical switch, either through direct contact with the switch, or magnetic operation of a reed. With magnetically actuated float sensors, switching occurs when a permanent magnet sealed inside a float rises or falls to the actuation level. With a mechanically actuated float, switching occurs as a result of the movement of a float against a miniature (micro) switch. For both magnetic and mechanical float level sensors, chemical compatibility, temperature, specific gravity (density), buoyancy, and viscosity affect the selection of the stem and the float. For example, larger floats may be used with liquids with specific gravities as low as 0.5 while still maintaining buoyancy. The choice of float material is also influenced by temperature-induced changes in specific gravity and viscosity – changes that directly affect buoyancy.Float-type sensors can be designed so that a shield protects the float itself from turbulence and wave motion. Float sensors operate well in a wide variety of liquids, including corrosives. When used for organic solvents, however, one will need to verify that these liquids are chemically compatible with the materials used to construct the sensor. Float-style sensors should not be used with high viscosity (thick) liquids, sludge or liquids that adhere to the stem or floats, or materials that contain contaminants such as metal chips; other sensing technologies are better suited for these applications.
A special application of float type sensors is the determination of interface level in oil-water separation systems. Two floats can be used with each float sized to match the specific gravity of the oil on one hand, and the water on the other. Another special application of a stem type float switch is the installation of temperature or pressure sensors to create a multi-parameter sensor. Magnetic float switches are popular for simplicity, dependability and low cost.
Pneumatic
Pneumatic level sensors are used where hazardous conditions exist, where there is no electric power or its use is restricted, and in applications involving heavy sludge or slurry. As the compression of a column of air against a diaphragm is used to actuate a switch, no process liquid contacts the sensor's moving partsMoving parts
The moving parts of a machine are those parts of it that move. Machines comprise both moving and fixed parts. The moving parts have controlled and constrained motions....
. These sensors are suitable for use with highly viscous liquids such as grease, as well as water-based and corrosive liquids. This has the additional benefit of being a relatively low cost technique for point level monitoring.
Conductive
Conductive level sensors are ideal for the point level detection of a wide range of conductive liquids such as water, and is especially well suited for highly corrosive liquids such as caustic soda, hydrochloric acid, nitric acid, ferric chloride, and similar liquids. For those conductive liquids that are corrosive, the sensor’s electrodes need to be constructed from titanium, Hastelloy B or C, or 316 stainless steel and insulated with spacers, separators or holders of ceramic, polyethylene and Teflon-based materials. Depending on their design, multiple electrodes of differing lengths can be used with one holder. Since corrosive liquids become more aggressive as temperature and pressure increase, these extreme conditions need to be considered when specifying these sensors.Conductive level sensors use a low-voltage, current-limited power source applied across separate electrodes. The power supply is matched to the conductivity of the liquid, with higher voltage versions designed to operate in less conductive (higher resistance) mediums. The power source frequently incorporates some aspect of control, such as high-low or alternating pump control. A conductive liquid contacting both the longest probe (common) and a shorter probe (return) completes a conductive circuit. Conductive sensors are extremely safe because they use low voltages and currents. Since the current and voltage used is inherently small, for personal safety reasons, the technique is also capable of being made “Intrinsically Safe
Intrinsic safety
Intrinsic safety is a protection technique for safe operation of electronic equipment in explosive atmospheres and under irregular operating conditions. The concept was developed for safe operation of process control instrumentation in hazardous areas, particularly North Sea gas platforms...
” to meet international standards for hazardous locations
Electrical Equipment in Hazardous Areas
In electrical engineering, a hazardous location is defined as a place where concentrations of flammable gases, vapors, or dusts occur. Electrical equipment that must be installed in such locations is especially designed and tested to ensure it does not initiate an explosion, due to arcing contacts...
. Conductive probes have the additional benefit of being solid-state devices and are very simple to install and use. In some liquids and applications, maintenance can be an issue. The probe must continue to be conductive. If buildup insulates the probe from the medium, it will stop working properly. A simple inspection of the probe will require an ohmmeter connected across the suspect probe and the ground reference.
Typically, in most water and wastewater wells, the well itself with its ladders, pumps and other metal installations, provides a ground return. However, in chemical tanks, and other non-grounded wells, the installer must supply a ground return, typically an earth rod.
Ultrasonic
UltrasonicUltrasound
Ultrasound is cyclic sound pressure with a frequency greater than the upper limit of human hearing. Ultrasound is thus not separated from "normal" sound based on differences in physical properties, only the fact that humans cannot hear it. Although this limit varies from person to person, it is...
level sensors are used for non-contact level sensing of highly viscous liquids, as well as bulk solids. They are also widely used in water treatment applications for pump control and open channel flow measurement. The sensors emit high frequency (20 kHz to 200 kHz) acoustic waves that are reflected back to and detected by the emitting transducer.
Ultrasonic level sensors are also affected by the changing speed of sound
Speed of sound
The speed of sound is the distance travelled during a unit of time by a sound wave propagating through an elastic medium. In dry air at , the speed of sound is . This is , or about one kilometer in three seconds or approximately one mile in five seconds....
due to moisture, temperature, and pressures. Correction factors can be applied to the level measurement to improve the accuracy of measurement.
Turbulence, foam, steam, chemical mists (vapors), and changes in the concentration of the process material also affect the ultrasonic sensor’s response. Turbulence and foam prevent the sound wave from being properly reflected to the sensor; steam and chemical mists and vapors distort or absorb the sound wave; and variations in concentration cause changes in the amount of energy in the sound wave that is reflected back to the sensor. Stilling wells and wave guides are used to prevent errors caused by these factors.
Proper mounting of the transducer is required to ensure best response to reflected sound. In addition, the hopper, bin, or tank should be relatively free of obstacles such as weldments, brackets, or ladders to minimise false returns and the resulting erroneous response, although most modern systems have sufficiently "intelligent" echo processing to make engineering changes largely unnecessary except where an intrusion blocks the "line of sight" of the transducer to the target. Since the ultrasonic transducer is used both for transmitting and receiving the acoustic energy, it is subject to a period of mechanical vibration known as “ringing”. This vibration must attenuate (stop) before the echoed signal can be processed. The net result is a distance from the face of the transducer that is blind and cannot detect an object. It is known as the “blanking zone”, typically 150mm – 1m, depending on the range of the transducer.
The requirement for electronic signal processing circuitry can be used to make the ultrasonic sensor an intelligent device. Ultrasonic sensors can be designed to provide point level control, continuous monitoring or both. Due to the presence of a microprocessor and relatively low power consumption, there is also capability for serial communication from to other computing devices making this a good technique for adjusting calibration and filtering of the sensor signal, remote wireless monitoring or plant network communications. The ultrasonic sensor enjoys wide popularity due to the powerful mix of low price and high functionality.
Capacitance
Capacitance level sensors excel in sensing the presence of a wide variety of solids, aqueous and organic liquids, and slurries. The technique is frequently referred to as RF for the radio frequency signals applied to the capacitance circuit. The sensors can be designed to sense material with dielectric constantDielectric constant
The relative permittivity of a material under given conditions reflects the extent to which it concentrates electrostatic lines of flux. In technical terms, it is the ratio of the amount of electrical energy stored in a material by an applied voltage, relative to that stored in a vacuum...
s as low as 1.1 (coke and fly ash) and as high as 88 (water) or more. Sludges and slurries such as dehydrated cake and sewage slurry (dielectric constant approx. 50) and liquid chemicals such as quicklime (dielectric constant approx. 90) can also be sensed. Dual-probe capacitance level sensors can also be used to sense the interface between two immiscible liquids with substantially different dielectric constants, providing a solid state alternative to the aforementioned magnetic float switch for the “oil-water interface” application.
Since capacitance level sensors are electronic devices, phase modulation and the use of higher frequencies makes the sensor suitable for applications in which dielectric constants are similar. The sensor contains no moving parts, is rugged, simple to use, and easy to clean, and can be designed for high temperature and pressure applications. A danger exists from build-up and discharge of a high-voltage static charge that results from the rubbing and movement of low dielectric materials, but this danger can be eliminated with proper design and grounding.
Appropriate choice of probe materials reduces or eliminates problems caused by abrasion and corrosion. Point level sensing of adhesives and high-viscosity materials such as oil and grease can result in the build-up of material on the probe; however, this can be minimized by using a self-tuning sensor. For liquids prone to foaming and applications prone to splashing or turbulence, capacitance level sensors can be designed with splashguards or stilling wells, among other devices.
A significant limitation for capacitance probes is in tall bins used for storing bulk solids. The requirement for a conductive probe that extends to the bottom of the measured range is problematic. Long conductive cable probes (20 to 50 meters long), suspended into the bin or silo, are subject to tremendous mechanical tension due to the weight of the bulk powder in the silo and the friction applied to the cable. Such installations will frequently result in a cable breakage.
Optical interface
Optical sensors are used for point level sensing of sediments, liquids with suspended solids, and liquid-liquid interfaces. These sensors sense the decrease or change in transmission of infrared light emitted from an infrared diode (LED). With the proper choice of construction materials and mounting location, these sensors can be used with aqueous, organic, and corrosive liquids.A common application of economical infrared-based optical interface point level sensors is detecting the sludge/water interface in settling ponds. By using pulse modulation techniques and a high power infrared diode, one can eliminate interference from ambient light, operate the LED at a higher gain, and lessen the effects of build-up on the probe.
An alternate approach for continuous optical level sensing involves the use of a laser. Laser light is more concentrated and therefore is more capable of penetrating dusty or steamy environments. Laser light will reflect off most solid, liquid surfaces. The time of flight can be measured with precise timing circuitry, to determine the range or distance of the surface from the sensor. Lasers remain limited in use in industrial applications due to cost, and concern for maintenance. The optics must be frequently cleaned to maintain performance.
Microwave
Microwave sensors are ideal for use in moist, vaporous, and dusty environments as well as in applications in which temperatures vary. Microwaves (also frequently described as RADAR), will penetrate temperature and vapor layers that may cause problems for other techniques, such as ultrasonic. Microwaves are electromagnetic energy and therefore do not require air molecules to transmit the energy making them useful in vacuums. Microwaves, as electromagnetic energy, are reflected by objects with high conductive properties, like metal and conductive water. Alternately, they are absorbed in various degrees by dielectric or insulating mediums such as plastics, glass, paper, many powders and food stuffs and other solids.Microwave sensors are executed in a wide variety of techniques. Two basic signal processing techniques are applied, each offering its own advantages: Time-Domain Reflectometry (TDR) which is a measurement of time of flight divided by the speed of light, similar to ultrasonic level sensors, and Doppler systems employing FMCW techniques. Just as with ultrasonic level sensors, microwave sensors are executed at various frequencies, from 1 GHz to 30 GHz. Generally, the higher the frequency, the more accurate, and the more costly. Microwave is also executed as a non-contact technique, monitoring a microwave signal that is transmitted through the medium (including vacuum), or can be executed as a “radar on a wire” technique. In the latter case, performance improves in powders and low dielectric media that are not good reflectors of electromagnetic energy transmitted through a void (as in non-contact microwave sensors). But the same mechanical constraints exist that cause problems for the capacitance (RF) techniques mentioned previously.
Microwave-based sensors are not affected by fouling of the microwave-transparent glass or plastic window through which the beam is passed nor by high temperature, pressure, or vibration. These sensors do not require physical contact with the process material, so the transmitter and receiver can be mounted a safe distance from the process, yet still respond to the presence or absence of an object. Microwave transmitters offer the key advantages of ultrasonics: the presence of a microprocessor to process the signal provides numerous monitoring, control, communications, setup and diagnostic capabilities. Additionally, they solve some of the application limitations of ultrasonics: operation in high pressure and vacuum, high temperatures, dust, temperature and vapor layers. One major disadvantage of microwave or radar techniques for level monitoring is the relatively high price of such sensors.
Magnetostrictive
Magnetostrictive level sensors are similar to float type sensors in that a permanent magnet sealed inside a float travels up and down a stem in which a magnetostrictive wire is sealed. Ideal for high-accuracy, continuous level measurement of a wide variety of liquids in storage and shipping containers, these sensors require the proper choice of float based on the specific gravity of the liquid. When choosing float and stem materials for magnetostrictive level sensors, the same guidelines described for magnetic and mechanical float level sensors apply.Because of the degree of accuracy possible with the magnetostrictive technique, it is popular for “custody-transfer” applications. It can be permitted by an agency of weights and measures for conducting commercial transactions. It is also frequently applied on magnetic sight gages. In this variation, the magnet is installed in a float that travels inside a gage glass or tube. The magnet operates on the sensor which is mounted externally on the gage. Boilers and other high temperature or pressure applications take advantage of this performance quality.
Resistive chain
Resistive chain level sensors are similar to magnetic float level sensors in that a permanent magnet sealed inside a float moves up and down a stem in which closely spaced switches and resistors are sealed. When the switches are closed, the resistance is summed and converted to current or voltage signals that are proportional to the level of the liquid.The choice of float and stem materials depends on the liquid in terms of chemical compatibility as well as specific gravity and other factors that affect buoyancy. These sensors work well for liquid level measurements in marine, chemical processing, pharmaceuticals, food processing, waste treatment, and other applications. With the proper choice of two floats, resistive chain level sensors can also be used to monitor for the presence of an interface between two immiscible liquids whose specific gravities are more than 0.6, but differ by as little as 0.1 unit.
Hydrostatic pressure
Hydrostatic pressure level sensors are submersible or externally mounted pressure sensors suitable for measuring the level of corrosive liquids in deep tanks or water in reservoirs. For these sensors, using chemically compatible materials is important to assure proper performance. Sensors are commercially available from 10mbar to 1000bar.Since these sensors sense increasing pressure with depth and because the specific gravities of liquids are different, the sensor must be properly calibrated for each application. In addition, large variations in temperature cause changes in specific gravity that should be accounted for when the pressure is converted to level. These sensors can be designed to keep the diaphragm free of contamination or build-up, thus ensuring proper operation and accurate hydrostatic pressure level measurements.
For use in open air applications, where the sensor cannot be mounted to the bottom of the tank or pipe thereof, a special version of the hydrostatic pressure level sensor can be suspended from a cable into the tank to the bottom point that is to be measured. The sensor must be specially designed to seal the electronics from the liquid environment. In tanks with a small head pressure (less than 100 INWC), it is very important to vent the back of the sensor gauge to atmospheric pressure. Otherwise, normal changes in barometric pressure will introduce large error in the sensor output signal. In addition, most sensors need to be compensated for temperature changes in the fluid.
Air bubbler
An air bubbler system uses a tube with an opening below the surface of the liquid level. A fixed flow of air is passed through the tube. Pressure in the tube is proportional to the depth (and density) of the liquid over the outlet of the tube.Air bubbler systems contain no moving parts, making them suitable for measuring the level of sewage, drainage water, sewage sludge, night soil
Night soil
Night soil is a euphemism for human excrement collected at night from cesspools, privies, etc. and sometimes used as a fertilizer. Night soil is produced as a result of a waste management system in areas without community infrastructure such as a sewage treatment facility, or individual septic...
, or water with large quantities of suspended solids. The only part of the sensor that contacts the liquid is a bubble tube which is chemically compatible with the material whose level is to be measured. Since the point of measurement has no electrical components, the technique is a good choice for classified “Hazardous Areas”. The control portion of the system can be located safely away, with the pneumatic plumbing isolating the hazardous from the safe area.
Air bubbler systems are a good choice for open tanks at atmospheric pressure and can be built so that high-pressure air is routed through a bypass valve to dislodge solids that may clog the bubble tube. The technique is inherently “self-cleaning”. It is highly recommended for liquid level measurement applications where ultrasonic, float or microwave techniques have proved undependable.
Gamma ray
A nuclear level gauge or gamma ray gauge measures level by the attenuation of gamma rays passing through a process vessel. The technique is used to regulate the level of molten steelSteel
Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...
in a continuous casting
Continuous casting
Continuous casting, also called strand casting, is the process whereby molten metal is solidified into a "semifinished" billet, bloom, or slab for subsequent rolling in the finishing mills. Prior to the introduction of continuous casting in the 1950s, steel was poured into stationary molds to form...
process of steelmaking. The water-cooled mold is arranged with a source of radiation, such as Cobalt-60 or Cesium-137, on one side and a sensitive detector such as a scintillometer
Scintillometer
A scintillometer is a scientific device used to measure small fluctuations of the refractive index of air caused by variations in temperature, humidity, and pressure. It consists of an optical or radio wave transmitter and a receiver at both ends of an atmospheric propagation path...
on the other. As the level of molten steel rises in the mold, less of the gamma radiation is detected by the sensor. The technique allows non-contact measurement where the heat of the molten metal makes any contact technique impractical.