Measurement and Control--LEVEL MEASUREMENT



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Overview

Level measurement is defined as the measurement of the position of an interface between two media. These media are typically gas and liquid, but they also could be two liquids. The first method of level measurement, a few thousands of years ago, consisted of a graduated stick that was referenced to an arbitrary datum line. In more recent times, the glass gage was developed as an evolution of the U-tube principle (this is described further in section 6 on pressure measurement). Eventually, level measurement was used on pressurized tanks by connecting the upper end of the tube to the vessel. With equal pressure in the tube and the vessel, the liquid level in the tube was at the same point as the level in the tank.

Over the years, level measurement technology has evolved, and highly accurate and reliable devices are now on the market. New principles of measurement are being introduced, and existing principles are continuously improved upon. Many parameters need to be considered when applying level-measuring devices, depending on the type of level measurement selected.

Ignoring such parameters may result in a measurement with a high error or one with a short life span. Like any item of instrumentation and control, level-measuring devices should be installed where they can be easily accessed for inspection and maintenance.

Level measurement is a key parameter that is used for reading process values, for accounting needs, and for control. Of the typical flow, level, temperature, and pressure measurements, flow tends to be the most difficult, but level follows closely behind. This section provides some of the basic knowledge plant personnel need to select the correct level-measuring device.

Classification

Level devices operate under different principles. They can be classified into three main categories that measure:

  • • the position (height) of the surface
  • • the pressure head
  • • the weight of the material through load cells

Load Cells

Mechanical lever scales, which provided 0.25 percent accuracy at best, were the typical load measuring device before the advent of the strain-gage-based cell. These mechanical devices were also expensive and complex in design and maintenance. In contrast, today's strain-gage load cells can attain accuracies of 0.03 percent of full scale, have simple designs, are relatively inexpensive, and are easy to calibrate.

The strain gage itself is bonded to a beam or other structural member, which bends slightly under the applied weight. This deformation changes the electrical resistance in one of the legs of the Wheatstone bridge, and the electronics convert that change into a weight. For more information about the Wheatstone bridge, refer to figure 11 (next section).

Units of Measurement

The 0 to 100 linear scale represents a percentage of level. It is the most commonly used scale for measuring liquid level. In some cases, the level measurement is converted into volume (e.g., gallons or liters) to provide a more meaningful indication.

Measurement of Solids

The level of solids often must be measured because of the continuous increase in the processing of solids and industry's need to comply with regulations.

When plants are measuring the level of solids, sensors located near the bottom of a bin may need to be protected from falling material when the bin is being filled. Where rods and probes are implemented, users must assess the impact of forces and abrasive effects. In addition, solid material often arches or forms "rat holes," which sometimes makes measuring such material's level difficult. In these environments, vibrators may need to be strategically mounted on the bins to break down those bridges. Proper location of the sensor is essential if they are to operate well.

The top level of solids material in a bin is rarely horizontal since most solids have an angle of repose. Therefore, the location of the measurement point should provide a representative aver age of the overall level, and in some cases several probes may have to be used for this purpose.

There are other specific requirements of solids measurements. For example, plants should keep in mind that, depending on the type of level sensor they use, the dielectric constant will vary for solids as the moisture content increases. Measuring devices that rely on such parameters may give the wrong reading.

The most common continuous types of measurements for solids are radiation, weighing, and ultrasonic. For on/off measurement, the most common types are diaphragm, rotating paddle, capacitance, and vibrating rod.

Weighing, which is performed by using load cells, is still one of the most common and reliable methods for measuring the contents of a tank. The advantages of weighing methods are that they are completely non-contacting and are relatively inexpensive to maintain. However, they have a higher initial cost than typical level sensors, and in existing installations, they may necessitate costly modification of the tank's construction.

Comparison Table

TBL. 1 summarizes the main types of level measurement with respect to a set of common parameters and can be used as a guide for selecting the appropriate method. The information presented in the table indicates typical values. Vendors may have equipment that exceeds the limits shown.

TBL. 1 Level measurement comparison

Notes for TBL. 1:

  • 1. With the proper design
  • 2. Limited by dP cell range. Filled systems are limited to 400°F (205°C) and 2000 psig (14 MPag)
  • 3. Good for non-freezing liquids only (unless heat tracing is used)
  • 4. Between high and low points, some may extend to 15 ft. (5 m), beyond that it must be built in sections
  • 5. Contact output only
  • 6. Limited by float movement
  • 7. 0.1% in some units with temperature compensation
  • 8. Optional to -40°F (-40°C) with heater
  • 9. S for glass and N for armored (magnetic)
  • 10. Must be fitted with optional/additional electronic sensing equipment (e.g., beam breaker)
  • 11. Armored/magnetic type to 3,000 psig (21MPa), bulls eye 10,000 psig (70MPag) max
  • 12. Unlimited with multiple units (multiple sources may be used for wide ranges)
  • 13. Water cooled detectors will handle temperatures to 3000°F (1600°C) - they are required for temperatures >140°F (60°C)
  • 14. Limited only by pressure transmitter range
  • 15. Temperature must be above dew point of purge gas
  • 16. Must be conductive
  • 17. Interface between conductive and non-conductive liquids/slurries
  • 18. Conductive path is required (dielectric constant greater than 19.0)
  • 19. Limited by probe materials (for electronics; 15-180°F (-26-82°C))
  • 20. Limited to selected materials - typically full vacuum to 2300 psig (16 MPag)
  • 21. Non-transparent liquids
  • 22. Yes, if measuring light absorption level of different materials
  • 23. Non-transparent foams
  • 24. On wet powders the vibrating fork may have the tendency to generate a cavity around itself, affecting performance
  • 25. Point; unlimited/continuous; from 6 in. (0.15 m) and up
  • 26. Dependent on diaphragm construction
  • 27. Theoretically limited by length of tape and sensitivity of sensor to pressure changes
  • 28. Will withstand up to 2200°F (1200°C) with special protective equipment
  • 29. Depends on foam density (foam may absorb signal) and signal strength

Differential Pressure (or Pressure/Static Head)

Principle of Measurement:

Differential-pressure level measurement, also known as "hydrostatic," is based on the height of the liquid head (the U-tube principle).

Level measurement in open tanks is based on the formula that the pressure head is equal to the liquid height above the tap multiplied by the specific gravity of the fluid being measured. In closed tanks, the true level is equal to the pressure measured at the tank bottom minus the static pressure above the liquid surface. To compensate for that static pressure, a leg is connected from the tank top to the low side of the differential pressure transmitter. Two options are available: dry leg and wet leg.

In dry leg applications, it is expected that the low side will remain empty (i.e., no condensation). If condensation takes place, an error will occur because a pressure head will be created on the low side. This error is avoided by intentionally filling the low side with a liquid-hence the term wet leg.

Where filled systems (with diaphragm seals) are used between the transmitter and the tank, calibration of the transmitter should allow for the specific gravity of the fill fluid. The user should refer to the vendor's instructions when setting the zero and span values.

FIG. 1 Differential-pressure level measurement.

Application Notes:

Differential-pressure measuring devices are easy to install and have a wide range of measurement. With proper modifications, such as extended diaphragm seals and flange connections, these instruments will handle hard-to-measure fluids (e.g., viscous, slurries, corrosive, hot). In addition, they are simple and accurate. Calibrating differential-pressure measuring devices is straightforward. Adjustments to zero, elevation/suppression, and span are easy, and no special tools are required.

On the other hand, differential-pressure measuring devices are affected by changes in density.

They should be used only for liquids with fixed specific gravity or where errors due to varying specific gravities are acceptable or compensated for. Pressure gages can be used to measure tank level because the static head pressure equals the density of the fluid multiplied by the height of the liquid head. Note that changes in liquid density due to changes in temperature will introduce errors.

Differential-pressure devices are susceptible to dirt or scale entering the tubing (in small process connections), which can easily plug them. In addition, parts of the instrument may be exposed to the process fluid, while the outside leg may be susceptible to freezing. These problems can be overcome with the proper design.

Where feasible, differential-pressure measuring instruments should be isolated from the process by a shutoff valve so the instrument can be removed without affecting the operation.

Where there is a possibility of condensation in the low-pressure impulse line, the plant should consider using filling tees. Differential-pressure devices often require the use of a constant head on the external (reference) leg. Keep in mind that the fill fluid should be compatible with the process fluid.

Zero Suppression and Elevation

When transmitters are mounted below the high side tap, a zero point adjustment is required.

This is called "zero suppression." Zero suppression occurs when a liquid head causes the pres sure reading in the impulse line to increase, causing a hydrostatic head. This head must be compensated for to avoid an error in measurement (see FIG. 2A). Zero elevation is basically the opposite of zero suppression. It is used in differential-pressure level measurement when a hydrostatic head called a wet leg is applied to the low side. This decreases the transmitter output, making a "zero elevation" necessary (see FIG. 2B).

Major equipment vendors provide users with the necessary calculations for the correct zero adjustment based on the transmitter's position and on the specific gravities of the fill fluid and the process fluid. Smart transmitters simplify zero suppression/elevation calculations.

====

FIG. 2A Example of Zero Suppression calculations for an open vessel.

Fluid SG = 0.9 Span = 100? x 0.9 = 90? WC Suppression = 20? x 0.9 = 18? WC Range = 18 to 90? WC Calibration:

1- calibrate transmitter from 0 to 90? WC (= span) 2- apply 18? WC to the high side 3- adjust zero until transmitter output reads 4mA Note: Do not adjust the span Zero is now set at 18? WC Span is set at 90-degree WC DP Transmitter 100 " 20 " LOW SIDE HIGH SIDE; Max. Level Min. Level

====

FIG. 2B Example of Zero Elevation calculations for a closed vessel with a wet leg.

===

Valve Manifolds

Manifolding first emerged with the development of differential-pressure measurement. Three valve and five-valve manifolds were assembled and piped using separate components. The three- and five-valve manifolds were unitized in the 1960s, and today are quite often sold as a component of the differential-pressure transmitter. The principal advantages of a unitized manifold are fewer leak points in the final installation, reduced material and labor cost, and reduced space requirements.

Most valve manifolds are threaded to the process tubing or piping. However, in plants where the process fluid is hazardous, the impulse line should be welded to the manifold. Welding makes repair and replacement expensive and difficult. Welding is generally available in two types: butt-weld and socket-weld. For additional information about line connections, refer to the section titled "Valve Bodies" in section 13.

Displacement

Principle of Measurement:

A displacer (see FIG. 3), which can be either partially or totally immersed, is restricted from moving freely with the liquid level. It transmits its change in buoyancy (mechanical force) to a transducer through a torque-tube unit. Sometimes the term float is used instead of displacer. However, the element does not actually float; it is submerged in the liquid being measured.

FIG. 3 Displacement.

Application Notes:

Displacers are simple, dependable, and accurate, and they can be mounted internally or externally. This type of level measurement should be used only for liquids with fixed specific gravity, where errors due to process variations are acceptable, and where a change in process condition will not create crystallization or solids.

External cage-type instruments are generally preferred. They are typically isolated by block valves and are heat traced and/or insulated (depending on the process fluid and on ambient conditions). The piping arrangement should be designed to prevent the formation of sediment on the bottom of the displacer cage. All components, including piping material and isolation valves, must be compatible with the process fluid. Typically, a suitable drain is provided at the low point and a vent valve at the highest point.

Displacers are difficult to calibrate and have numerous mechanical components. Therefore, it is important to ensure that the movement of the displacer, linkages, or levers is not restricted.

In addition, boiling liquid may cause violent agitation at the liquid surface, so stilling wells may be required where turbulence exists. Also, the element may be affected by coating, buildup, or dirt that can cling to the displacer.

Float

Principle of Measurement:

A float (see FIG. 4) consists of a hollow ball that rides freely on the surface of the liquid. Its position is a direct indication of level. The float is connected to an arm that operates a microswitch or a pointer and scale through a bearing. The spherical shape of the ball provides maximum volume-that is, maximum buoyancy-for its weight. For maximum sensitivity, the ball should be selected so it will sink to its largest (middle) section. This produces the largest force available to overcome friction and inertia of components.

FIG. 4 Float. FLOAT INDICATOR OR ELECTRONICS

Application Notes:

Float devices are low in cost and simple in design. They are also accurate and reliable. How ever, for turbulent liquids they require the use of stilling wells, they are physically large, and they are generally used for clean services only. To maintain the float's accuracy, liquids must have a fixed specific gravity. In addition, the float instrument is in contact with the process material, and buildup on the float will affect performance. Corrosion and chemical reactions are also a concern. The float's effective travel is limited by its construction; typically, the angle of measurement is limited to ±30 degrees from the horizontal.

Generally, the float is not installed directly on top of pressurized vessels. If it is, the vessel may have to be taken out of service in order to do maintenance on the float. For this reason, external cage-type instruments are preferred. They are isolated from the process vessel by isolating valves. The movement of the float, linkages, or levers must not be restricted.

Sonic/Ultrasonic

Principle of Measurement:

Sonic and ultrasonic sensors (see FIG. 5) consist of a transmitter that converts electrical energy into acoustical energy and a receiver that converts acoustical energy back into electrical energy. In sonic sensors, the unit uses the echo principle and emits pulses that have an approximate frequency of 10 kHz. After each pulse, the sensor detects the reflected echo. The trans mitted and return time of the sonic pulse is relayed electronically and converted into a level indication. The principle for ultrasonic sensors is the same, except that the operating frequency is about 20 kHz or higher.

FIG. 5 Sonic/ultrasonic. TIMED GAIN CONTROL WAVE SHAPING LOGIC AND DISPLAY GENERATOR AND TRANSMITTER TIMING GENERATOR TRANSDUCERS TRANSMITTED BURST ELAPSED TIME PROPORTIONAL TO DISTANCE OBJECT BEING SENSED RECEIVER AND AMPLIFIER RECEIVED BURST (ECHO)

Application Notes:

Sonic and ultrasonic devices are noncontacting, reliable, and accurate. They penetrate high humidity, are cost effective, have no moving parts, and are unaffected by changes in density, conductivity, or composition.

However, strong industrial noise or vibration at the unit's operating frequency will affect performance, and in some designs, dusts tend to give false signals. In addition, coating may affect these devices' performance since deposit buildup on the probe (or the membrane) will attenuate the signal. For this reason, the unit should not come into contact with the process fluid.

Users should compare the maximum process temperature and pressure with the limits of the sensor.

Sonic and ultrasonic devices cannot be used to measure the level of foam because the sound signal is absorbed by foam. Also, since the operation of these devices depends on the speed of sound, they will not work in a vacuum. Various factors can affect the speed of sound and so the instrument's accuracy, vapor concentration, pressure, temperature, relative humidity, and the presence of other gases/vapors. Frequently, temperature compensation may be required to avoid variations in accuracy.

The plant should follow the manufacturer's installation recommendations carefully. Users should consider the effect of the process material (since the sensor's thin membrane corrodes easily) as well as the effects of spurious echoes. Such echoes must be avoided to prevent errors in the signal. The beam divergence is typically between 8 to 15 degrees (compared to 0.3 degrees for a laser type), and it produces an increasing footprint as the distance increases. No braces, stiffeners, or other cross-members should lie in the path of the ultrasonic beam. Also, most operating span ranges will not measure levels of less than 1 ft (0.3 m). In closed flat-top tanks, it may be necessary to reduce the transmit repetition so that respective echoes have enough time to die out. In some cases, a sound-absorbing layer may have to be installed to the underside of the tank top.

Tape (Float and Tape)

Principle of Measurement:

Tape devices (see FIG. 6) are similar to floats. A tape connects a float on one end and a counterweight on the other. The counterweight moves up and down a graduated scale located outside the tank. The counterweight is used to keep tension on the tape as the float rises and falls with the level. Where the tape is replaced with a chain, the chain is engaged in a sprocket.

For pressurized tanks, the connection is sealed through a magnetic link.

This level-measuring instrument is rarely used for signal transmission. It is generally used for local indication only. For maximum sensitivity, users should select a spherical float so it will sink to its largest (i.e., its middle) section. This produces the largest forces available to over come friction and inertia of components.

FIG. 6 Float and tape. FLOAT WEIGHT; SCALE

Application Notes:

Tapes are accurate. However, they can have mechanical problems such as hang-up and friction.

Also, material buildup on the float will affect performance. In applications where high accuracy is required, compensation for the varying specific gravity may be necessary. In addition, the tape's mechanical parts should be protected from possible weather interference.

The sensor in the tank should be located close to a manway and sufficiently distant from agitation and from incoming or discharging lines to minimize the effect of turbulence. Stilling wells are often used if the vessel is agitated.

Weight and Cable

Principle of Measurement:

With the weight and cable device (see FIG. 7), a cable or tape is attached to a weight that descends into the tank. This motion is activated by a timer. When the weight makes contact with the surface of the material, the motor automatically reverses direction and retrieves the weight at about 1 ft/s (0.3 m/s). During descent, pulses are generated and displayed on a counting unit, which indicates either material stored or available filling capacity.

FIG. 7 Weight and cable.

Application Notes:

Weight and cables are accurate devices, and the fact that they are only momentarily in contact with the process material prevents product from building up on the weight. However, they can have mechanical problems, such as hang-up and friction. Also, they must be activated in order to measure, and they have no signal transmission capability.

In outdoor installations, measures should be taken to protect the mechanical parts of the level measuring instruments from possible weather interference. Stilling wells are often used if the vessel is agitated.

Gage

Principle of Measurement:

A gage, also known as a "sight glass" or "manometer" (see FIG. 8), operates on the U-tube principle. There are three basic types of gages: glass, reflex, and magnetic.

The glass type consists of two glass sections, in between which is the fluid to be measured.

The reflex type consists of a single glass with cut prisms. Light is refracted for the vapor portion of the column and is shown generally as a light color. Light is absorbed for the liquid portion of the column and is shown generally as a dark color.

Magnetic-type gages have a float that lies inside a nonmagnetic chamber. The float contains a magnet, which rotates wafers 180 degrees as the level increases or decreases. The rotated wafers present the opposite face with a different color.

FIG. 8 Sight glass. VESSEL LIQUID SHUTOFF VALVE LEVEL GLASS CALIBRATED SCALE LIQUID LEVEL; DRAIN VALVE

Application Notes:

Gages are used as a local indicator for open or pressurized vessels. They must be accessible and located within visual range. In certain services, such as steam drum service, glass gages must conform to local code requirements (e.g., ASME Power Boiler Code). Gages are low in cost and provide direct-reading measurement. However, they are not suitable for dark liquids (except if the magnetic type is used), and dirty fluids will prevent the liquid level from being viewed.

On safe applications, glass gages can be used. However, they can be easily damaged or broken.

Glass gages should not be used to measure hazardous liquids. Reflex gages are permissible for low- and medium-pressure applications. For high-pressure applications, or where the fluid is toxic, magnetic-type armored gages should be used. However, this type should be kept away from magnetic fields.

For safety reasons, the length of glass gages between process connections should not exceed 4 ft (1.25 m). In addition, to perform maintenance on glass gages, isolating valves are required to facilitate the removal of the gage glass. Drain and vent valves also are frequently installed.

These isolating valves must be implemented in accordance with the piping specifications. In addition, glass tubes are sometimes provided with ball check valves so the process connection shuts off in the event the glass tube breaks.

When installing such devices, good lighting is required. Sometimes an illuminator may be required in dark areas. In installations where the gage is at a lower temperature than the process, condensation may occur on the walls, making the reading difficult.

Radioactive (Nuclear)

Principle of Measurement:

With the radioactive (nuclear) device (see FIG. 9), a radioactive source radiates through the vessel. The gamma quantum is seen by the radiation detector (such as a Geiger counter) and is transformed into a signal. When the vessel is empty, the count rate is high. The radioactive source holder is designed to direct a collimated beam of radiation toward the tank and to be shielded in all other directions so as to reduce the radiation levels to below the legal limit.

The strength of the sensed radiation depends on the thickness of the vessel wall, the distance between the source and detector, and the density and thickness of the measured material. The radiation source generally has a half-life of 30 years; therefore, corrections for source decay are rarely required.

FIG. 9 Radioactive (nuclear).

Application Notes:

Radioactive level measurement is external to the vessel. It can be added or removed without disturbing the process. Radioactive (nuclear) devices are highly reliable, non-contacting devices with no moving parts. They are unaffected by temperature, pressure, and corrosion, and their mode of failure is limited and predictable.

However, radioactive (nuclear) devices require special engineering and licensing for the application they are used with, and extreme care is required when locating and installing the radio active source. The manufacturer's recommendations must be closely followed, and the manufacturer should be consulted to obtain optimum results and maximum safety. Operator exposure to radiation must be minimized, and therefore, plants may need shielding lead plates at the source or detector.

Radioactive (nuclear) units are expensive to install and operate in order to maintain their compliance with regulations. Special care must be exercised when installing them, which drives their cost up, and they are difficult to calibrate. Before installing such a device, the user should keep in mind that the plant will need a special license and training.

The radioactive (nuclear) measuring device is applied where other types of measurement can not be used. On vessels larger than 30 ft (10 m) in diameter or on vessels with extremely thick walls, the source may have to be suspended vertically inside the vessel. Special shield containers are then required.

Bubbler (Dip Tube)

Principle of Measurement:

In a bubbler (see FIG. 10), a small amount of air (or inert gas) purge flows through a dip tube in the vessel. Sometimes, to provide rigidity, a stand pipe is used instead of a dip tube. The dip tube (or pipe) generally extends to about 3 inches (75 mm) from the bottom of the tank and is notched to keep the size of the air bubble small. The pressure that is required to force air bubbles from the bottom of the tube is the liquid head above the end of the tube. A purge meter, which consists of a rotameter with a needle valve, is required to provide a constant air flow of about 0.2 to 2.0 scfh (0.005 to 0.05 m^3 /hr). A pressure regulator located upstream of the purge meter provides a smooth operation. In plants where remote level indication is required, the high-pressure side of the differential-pressure transmitter measures the tube pres sure, and the low side measures the vessel's top pressure, if it is not vented to the atmosphere.

In some cases, liquid purge is used instead of air purge, usually in cases where air or other gases cannot be used. Generally, about 1 U.S. gal/h at 15 psi (4 L/h at 100 kpa) differential pressure is the maximum required liquid purge rate. In other cases, where level measurement is required only occasionally or where utilities are not available, a hand pump (instead of a constant air supply) can be connected to the dip tube in order to measure level.

FIG. 10 Bubbler. PRESSURE GAGE CALIBRATED IN LEVEL, ½ in. STEEL PIPE, ½ in. TUBE OR PIPE CLEAN OUT LIQUID LEVEL PRESSURE GAGE PRESSURE REGULATOR AIR SUPPLY FLOW REGULATOR BUBBLES OF ESCAPING AIR CLEARANCE IF THERE IS SEDIMENT

Application Notes:

The bubbler offers low cost and easy maintenance, it can operate without electrical power, and it can be used on pressurized or unpressurized vessels. However, variations in density will affect the bubbler's readout, and bubblers can become coated or plugged by process fluid residue or dirt. In addition, the cost of purging fluid is ongoing, and the purge gas can introduce unwanted components into the process. The introduction of a foreign material, usually instrument air, into the process should be acceptable. Otherwise, a special gas (or liquid) should be used instead. Also, if a vessel is emptied by pressurization, the liquid being measured may be forced up the dip tube/pipe, which causes an incorrect readout.

This measuring device is not an off-the-shelf item; some engineering is required. The materials of construction for the bubbler must be compatible with the process it is used in, and the bubbler's dip tube installation must be capable of withstanding the maximum air pressure that blockage causes. A tee piece at the top of the dip tube (or pipe) may be required to enable rod ding.

Capacitance

Principle of Measurement:

Capacitance level measurement (see FIG. 11) measures the changing electrical capacitance that occurs within the device as the level in the vessel varies. This device can be used for conductive or nonconductive fluids, but the dielectric constant of the fluid being measured must remain constant, unless a unit is used that compensates for dielectric variations. When plants apply the capacitance type of measurement, they must keep in mind that dry, nonconductive materials with a moisture content may become conductive.

If the material to be measured is nonconductive, the capacitor consists of two conductive plates (the probe and the vessel wall) that are separated by an insulator (the material being measured).

If the material being measured is conductive, an insulated probe is used. This insulation serves as the dielectric, and the material serves as one of the plates. Capacitors can also be used for quantitative analysis of water in oil down to 0.1 percent water.

Capacitor operation is affected by three factors; plate area, dielectric material, and plate spacing. Greater capacitance is obtained from a larger plate area, a higher dielectric constant, and less plate spacing. The relationship between these three factors is:

Capacitance = Plate Area Dielectric Constant /Distance Between Plates

FIG. 11 Capacitance. PROBE INSULATION TANK PROBE INSULATION TANK (A) CONDUCTIVE LIQUID (B) NON-CONDUCTIVE LIQUID

Application Notes:

Capacitance level measurement is an easy technique to install. It is simply designed with no moving parts, is unaffected by nonconductive buildup, and can be used for pressurized or unpressurized vessels. However, calibration may be time consuming. The unit is affected by changes to the material's dielectric constant and thus requires temperature compensation. In addition, conductive residue coating will affect the unit's performance.

The installation must ensure that the probe is not in contact with the tank walls. If the application requires an insulated probe, users must take care during installation to prevent damage to the probe's insulating material.

Conductivity

Principle of Measurement:

Conductivity level measurement (see FIG. 12) works as follows: when material contacts the probe, a low-voltage electrical path is completed between the container wall and the probe, which actuates a relay. For nonconductive containers, the path is between the level probe and a reference probe.

Application Notes: Conductivity measuring devices are easy to install, have no moving parts, are relatively simple and low in cost, and can be used on pressurized or unpressurized vessels. However, they pro vide only a point measurement, and they are susceptible to coating by nonconductive materials. In DC circuits, the conductivity unit may cause electrolytic corrosion at the probe (whereas AC circuits prevent electrolytic plating).

FIG. 12 Conductivity. ELECTRODES; ELECTRODES NON-CONDUCTIVE CONTAINER NON-CONDUCTIVE CONTAINER; CONDUCTIVE FLUID

When implementing a conductivity device, users should consider that the unit may cause sparking as the liquid level rises to reach the probe. Intrinsically safe designs are available if they are required.

Thermal

Principle of Measurement:

Typically, thermal devices consist of a heater element next to a temperature switch. When the liquid rises above the switch, it dissipates the heat, and the temperature switch activates (or deactivates).

Application Notes:

A thermal level switch offers low cost, uses semiconductor electronics with no moving parts, is sensitive, and has a simple and reliable design. However, such devices are sensitive to coating or caking materials, and they provide point measurement only. Also, they cannot be used where heating will affect product quality.

Radar

Principle of Measurement:

The radar (see FIG. 14) is similar to the sonic and ultrasonic unit, but operates at a frequency of about 24 GHz.

FIG. 13 Thermal. THERMOWELL, TANK WALL HEAT SOURCE AND SENSOR CONTROL CIRCUIT HOUSING WIRES TO CONTROL UNITS AND DC POWER SUPPLY, LT

FIG. 14 Radar.

Application Notes:

The radar is easy to install and provides reliable noncontact measurement. It provides touch free indication without special licensing (as is required for nuclear units) and will "see through" vessels made of plastic. Transducers that are mounted outside a metal vessel must be provided with a nonmetallic window since radar transducers will not penetrate metal. However, they will penetrate material with a low dielectric constant such as plastic, wood, glass, and the like.

Radar units are expensive. In addition, spurious reflections from metal objects will cause interference and affect the radar's performance. However, the 24 GHz signal provides a relatively narrow bandwidth (when compared to ultrasonic devices), and when well aimed, it should avoid tank obstacles such as tank walls, baffle plates, and agitators.

Beam Breakers

Principle of Measurement:

The beam breaker (see FIG. 15) is also known as a "photometric" or "light beam." Its basic components are a light source and a receiver (photocell) that accepts the light beam and measures it. The light travels in a straight line until it is intercepted by an object (such as the liquid level in a tank). The light beam is broken or reflected by the level in the vessel, as detected by the receiver.

Application Notes:

The beam breaker offers a low cost solution and can be used for pressurized or unpressurized vessels. It is also easy to apply, is of simple construction, and is unaffected by gravity. How ever, sensitivity adjustment is available only in some units, and residue coating will affect the beam breaker's performance. In addition, beam breakers have a limited range and are affected by changes in reflectivity.

FIG. 15

Two types of beam breakers. SOURCE RECEIVER SOURCE (A) REFLECTED BEAM (B) BROKEN BEAM RECEIVER

When applying such devices, the designer should consider the effect of liquid drops or condensation since they will deflect the beam and affect performance. In addition, on clear liquids it may be difficult to interrupt the light beam (and get an indication). In some cases, it may be necessary to shield the light receiver from outside light sources to avoid the introduction of measurement errors.

Vibration

Principle of Measurement:

Vibration devices (see FIG. 16) consist of a tuning fork that vibrates at its natural resonant frequency by a piezoelectric crystal, which is located at the base of the probe. When the vibrating fork contacts a material, either dry or in suspension (20% minimum), the vibration frequency is altered, which switches a relay. The material needs to have a bulk density of 0.9 lb/ft³ (12.8 kg/m³) or greater. When the level drops below the fork, the vibrating frequency is again in effect, and the relay is reversed.

Application Notes:

Vibration units have no moving parts, are rugged and reliable, are good for low-density materials, and require little maintenance. However, they should not be used in vibrating bins, especially if the two frequencies are close. In addition, product buildup will affect the performance

of vibration units, the switch setting cannot be readily changed, and vibration units typically require protection from materials that are charged from the top.

FIG. 16 Vibration.

Paddle Wheel

Principle of Measurement:

In a paddle wheel (see FIG. 17) a motor keeps the paddle rotating. When the material rises and prevents the paddle's rotation, a switch is actuated.

FIG. 17 Paddle wheel. ROTATING PADDLE TANK WALL STEEL HOUSING ELECTRIC MOTOR TO POWER SOURCE AND AMPLIFIER MOUNTING PLATE

Application Notes:

A paddle wheel is inexpensive, simple, and reliable. However, it is susceptible to shock, vibration, and damage by falling material. Therefore, paddle wheels generally require some protection (e.g., a protective baffle) from material charging from the top. In addition, hang-ups or material buildup on the paddle will affect the device's performance, and material bridging around the switch will give an erroneous state.

Diaphragm

Principle of Measurement:

The diaphragm (see FIG. 18) is a point measurement device. The process materials (or hydrostatic pressure) apply pressure on a diaphragm, which in turn actuates a switch.

Application Notes:

The diaphragm is reliable, easy to maintain, and available for different applications. However, coating may affect the flexing of the diaphragm, and abrasive material may affect its performance. In addition, the accuracy of the unit is affected by changes in specific gravity.

The diaphragm must be in contact with the material. It should be at least 2 to 3 in. (50 to 75 mm.) above any sediment in the vessel bottom to prevent dirt from building up at the diaphragm.

FIG. 18 Diaphragm. WIRES TO

CONTROL RELAYS MICRO-SWITCH MOUNTING PLATE TANK WALL DIAPHRAGM SYNTHETIC RUBBER REINFORCED WITH NYLON "DRIVING" MAGNET "DRIVEN" MAGNET

Resistance Tape

Principle of Measurement:

Resistance tapes (see FIG. 19) function as follows: as the level rises in the tank, the resistance element is shorted to the conductive probe (due to liquid pressure), affecting loop resistance. The unit measures the loop resistance and provides an indication of level.

Application Notes:

A resistance tape will handle corrosive liquids and slurries. However, it must contact the material and is susceptible to moisture getting inside the tape. Users may therefore need to use a desiccant, which entails additional maintenance. In addition, resistance tape devices are affected by changes in specific gravity, are not suitable for flammable atmospheres, and are neither accurate nor rugged. They require careful engineering and careful installation. Plants may need to use stilling if turbulence exists.

FIG. 19 Resistance tape. RESISTANCE ELEMENT SHORTED BELOW SURFACE BY WEIGHT OF PROCESS MATERIAL; MATERIAL LEVEL IN SILO SEALED OUTER JACKET RESISTANCE ELEMENT CONDUCTIVE BASE STRIP

Laser

Principle of Measurement:

There are two types of laser measurement (see FIG. 20): pulsed and continuous wave (frequency modulated). In industrial applications, the pulsed-type is the most common because of its range and ability to penetrate through vapors and dust.

The pulsed-type laser operates as follows: its transmitter emits a continuous series of pulses at a target. The time taken by each pulse to travel from the transmitter to the target (e.g., the liquid surface) and back is measured and converted into distance.

The continuous wave laser consists of a transmitter that emits a continuous laser beam at the target. When the beam hits the target, phase-shifting occurs. Based on the degree of phase shift and on other constant parameters such as wave frequency, the device determines the distance of the target and therefore level.

Application Notes:

Laser transducers mounted outside a metal vessel can measure level through a process-rated sight glass. This means the laser unit can be accessed without having to interrupt the process.

Laser-type level measurement uses an extremely short wavelength and produces a very narrow beam. These features provide very good accuracy and non-contact measurement for difficult applications. However, lasers are relatively expensive, though still better then radioactive (nuclear) types.

FIG. 20 Laser. LASER SIGHT GLASS


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