Level sensor
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There are many physical and application variables that affect the selection of the optimal level monitoring solution for industrial and / or commercial processes. The selection criteria include the physical: state (liquid, solid or slurry), temperature, pressure or vacuum, chemistry, dielectric constant of medium, density or specific gravity of medium, agitation, acoustical or electrical noise, vibration, mechanical shock, tank or bin size and shape; and 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;
This article discusses level sensing from the perspective of the state of the material - solid, liquid, and slurry-type - and how their physical and electrical properties may affect the performance of the sensor. Table 1 lists commonly available level sensors and their suitability for use with different states of materials, and Table 2 lists the physical and electrical characteristics one will need to evaluate before choosing a sensor or sensing technology for their application.
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[edit] Point and Continuous Level Detection of Solids Only
A variety of sensors are available for point level detection of solids. These include vibrating, rotating paddle, mechanical (diaphragm), microwave (radar), capacitance, optical, and ultrasonic level sensors.
[edit] Vibrating Point Level Sensors
Vibrating level sensors are designed for point level detection of very fine powders (bulk density: 0.02 g/cm - 0.2 g/cm), fine powders (bulk density: 0.2 - 0.5 g/cm), and granular solids (bulk density: 0.5 g/cm or greater). With proper selection of vibration frequency and suitable sensitivity adjustments, the level of highly fluidized powders and electrostatic materials can also be sensed.
Single-probe vibrating level sensors are ideal for highly static bulk powder environments. Since only one sensing element contacts the powder, bridging between two probe elements is eliminated and media build-up is minimized. Vibrating level sensor technology offers other advantages: The vibration of the probe itself tends to eliminate build up of material on the probe element; and they are not affected by dust, static-charge build-up from dielectric powders, or changes in conductivity, temperature, pressure or humidity/moisture content. Tuning fork style vibration sensors are another alternative. They tend to have a lower price point, but are prone to material buildup between the forks.
[edit] Rotating Paddle Level Sensors
Rotating paddle level sensors are a very old and established technique for bulk solid point level indication. The technique requires 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 bulk densities (very low weight per unit volume) such as Pearlite, Bentonite or fly ash, the weight of the material is insufficient to stop the paddle. For such difficult applications, special paddle designs and the use of lower-torque motors can be employed. In addition, 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 sealing technology.
[edit] Point Level Detection of Liquids Only
[edit] Magnetic and Mechanical Float Level Sensors
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.
[edit] Pneumatic Level Sensors
Pneumatic level sensors are indicated 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 parts. These sensors are suitable for use with highly viscous liquids such as grease, as well as water-based and corrosive liquids. It has the additional benefit of being a relatively low cost technique for point level monitoring.
[edit] Conductive (Electrode-Based) Level Sensors
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.
The technology behind conductive level sensing involves 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.
[edit] Sensors for both Point Level Detection or Continuous Monitoring of Solids and Liquids
[edit] Capacitance Level Sensors (also called RF)
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 constants as low as 1.1 (coke and fly ash) and as high as 88 (water) or more. Sludges and slurries such as dehydrated cock cake and sewage slurry (dielectric constant 50) and liquid chemicals such as quicklime (dielectric constant 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, 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, or 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.
[edit] Optical Interface Point Level Sensors
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 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.
[edit] Ultrasonic Level Sensors
Ultrasonic level sensors (sometimes called sonic) are ideal for non-contact level sensing of highly viscous liquids such as heavy oil, grease, latex, and slurries as well as bulk solids like cement, sand, grain, rice, and plastic pellets. The sensors emit high frequency, “ultra” sonic (20 kHz to 200 kHz) acoustic waves that are reflected back to and detected by the emitting transducer.
Since the speed of sound in air fluctuates with moisture level and temperature, ultrasonic level sensors are also affected by changing moisture levels and varying temperatures and pressures inside the hopper or container. But when ultrasonic sensors are used in conjunction with humidity and temperature sensors, or a distance reference, correction factors can be applied to the level measurement making the technology very accurate.
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 and/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 address some of the above constraints.
Proper mounting is important to ensure that sound waves are reflected perpendicularly back to the sensor. Otherwise, even slight misalignment of the sensor in relation to the process material reduces the amount of sound wave detected by the transducer. In addition, the hopper, bin, or tank should be free of obstacles such as weldments, brackets, or ladders to eliminate false returns and the resulting erroneous response. 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”.
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.
[edit] Microwave/ Radar Level Sensors
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 dielectric properties, like metal and conductive water. Alternately, they are absorbed in various degrees by low dieletric 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 mediums 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.
[edit] Continuous Level Measurement of Liquids Only
[edit] Magnetostrictive Level Sensors
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.
[edit] Resistive Chain Level Sensors
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.
Again, 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.
[edit] Hydrostatic Pressure Level Sensor
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.
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.
[edit] Air Bubbler Level Measurement Systems
Pneumatically based 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.
Numerous level sensing devices incorporating numerous technologies are available or can be adapted for a wide variety of applications. In addition to chemical compatibility, understanding and evaluating how the physical and electrical characteristics of the process material, listed in table 2, affects the operation of a sensor or sensing technology will assure trouble-free operation and long sensor life.
