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Assignment Report
Report Title: Level Sensors
Subject: Industrial Instrumentation
Submitted By:
2007-Chem-101
Submitted To:
Dr. Naveed Ramzan
University of Engineering & Technology,
Lahore
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Abstract
A wide variety of level measurement systems are available to
address the broad spectrum of applications, accuracy needs,
installation requirements, and practices. Measurement technologies
are made available in different versions to address a wide range of
measurement needs or sometimes to address just one specific
application. This subsection will attempt to define some of the
general selection considerations of many available technologies,
the general forms of these technologies, and some of their general
advantages and disadvantages. As always, one must consult the
specifications from the various manufacturers for specific products
and users experiences in different installations to truly determine
their applicability to measurement situations. The family of level
measurement systems can be divided into many categories: liquids or
solids level measurement, point or continuous level measurement,
electromechanical or electrical/electromagnetic level measurement,
or contacting or noncontacting /nonintrusive level measurement.
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Table of Contents Introduction
............................................................................................................................................
4
Classification
...........................................................................................................................................
4
Types of Sensors
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5
Differential Pressure
...........................................................................................................................
5
Displacement
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6
Float Level Sensors
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7
Ultrasonic / Sonic
................................................................................................................................
8
Weight and Cable
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9
Sight Glass
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9
Radioactive (Nuclear)
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10
Bubbler
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11
Vibration
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12
Rotating Paddle Level Sensors
..........................................................................................................
13
Diaphragm
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13
Resistance Tape
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14
Hook- Type Level Sensor
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16
Level Measurement Sensor Selection
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16
References
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17
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Introduction 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. Level
sensors detect the level of substances that flow, including
liquids, slurries, granular materials, and powders. All such
substances flow to become essentially level in their containers (or
other physical boundaries) because of gravity. 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, 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.
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.
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Types of Sensors Electromechanical level measurement and
detection systems Floats for level detection and measurement of
liquids Displacers for level detection and measurement of liquids
Level detection of solids using rotating paddles Level measurement
of liquids and solids using plumb bob Electronic/electromagnetic
energy level measurement and detection systems Level detection of
liquids by use of conductivity Level detection of liquids by use of
vibrating forks resonance or rod attenuation Level detection of
solids by use of vibrating fork or rod attenuation Level detection
of liquids by use of ultrasonic gap Level detection of liquids by
use of thermodispersion Level measurement of liquids by use of
bubblers Level measurement of liquids by use of hydrostatic
pressure Ultrasonic level detection and measurement of liquids and
solids Capacitance level detection and measurement of liquids and
solids Radar level detection and measurement of liquids and solids
Level detection and measurement of liquids and solids by use of
time-domain reflectometry Level measurement of liquids by use of
magnetostrictive Level measurement of liquids by use of laser Level
detection and measurement of liquids and solids by use of
radiometric Level measurement of liquids and solids by use of
weighing Level detection by use of optics Level detection in
liquids by use of ultrasonic tank resonance [7]
Differential Pressure Differential-pressure level measurement,
also known as hydrostatic, is based on the height of the liquid
head. 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). [1] 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
liquidhence 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 vendors instructions when
setting the zero and span values. [1] Advantages/Disadvantages
Differential-pressure level measuring devices are easy to install
and have a wide range of applications. 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). They are simple and accurate.
Calibration of differential-pressure measuring
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devices is simple. Adjustments to zero, elevation/suppression,
and span are easy, and no special tools are required.
Differential-pressure measuring devices are affected by changes in
density. They can only be used for liquids with fixed specific
gravity. 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. Parts of the instrument
are exposed to the process fluid, while the outside leg is
susceptible to freezing. [1]
Figure 1: Differential Pressure
Displacement A displacer , 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. [1] Advantages/Disadvantages Displacers
are simple, dependable, and accurate. They may be mounted
internally or externally. These level measurement can only be used
for liquids with fixed specific gravity, where accuracy is not
required. A suitable drain is provided at the low point and a vent
valve at the highest point. Displacers are difficult to calibrate
and have many mechanical components. Therefore, displacer,
linkages, or levers should be free to move. Boiling liquid may
cause violent agitation at the liquid surface, so stilling wells
may be required where turbulence exists. the accuracy is also
affected by coating, buildup, or dirt. [1]
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Figure 2: Displacer
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. [2] 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. [2] 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. [2]
Advantages/Disadvantages Floats work well with clean liquids and
are accurate and adaptable to wide variations in fluid densities.
Once commissioned, however, the process fluid measured must
maintain its density if repeatability is required. Float Switches
are available and are capable of fail safe operation in extreme
process conditions. [2]
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Floats are affected by changes in product density since the
displacement of the body is equal to the weight of the fluid
displaced. If the specific gravity changes, then the weight of the
displaced material changes, thus changing the calibration. This is
especially problematic in interface measurements, where both
liquids increase or decrease density, while the signal is
proportional to the density difference.[2]
Figure 3: Float
Ultrasonic / Sonic Ultrasonic transmitters work on the principle
of sending a sound wave from a peizo electric transducer to the
contents of the vessel. The device measures the length of time it
takes for the reflected sound wave to return to the transducer. A
successful measurement depends on reflection from the process
material in a straight line back to the transducer. [3] Advantages/
Disadvantages The main advantages of ultrasonic level
instrumentation are that the transducer does not come into contact
with the process material, they have no moving parts and a single
top of vessel entry makes leaks less probable than fully wetted
techniques.[3] There are various influences that affect the return
signal. Things such as powders, heavy vapors, surface turbulence,
foam and even ambient noise can affect the returning signal.
Temperature can also be a limiting factor in many process
applications. Ultrasonic devices will not operate on vacuum or high
pressure applications. [3]
Figure 4 Sonic/Ultrasonic
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Weight and Cable With the weight and cable device , 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. [1] Advantages/Disadvantages Weight and cables are
accurate devices, and they are only momentarily in contact with the
process material prevents product from building up on the weight.
They can have mechanical problems, such as hang-up and friction.
They must be activated in order to measure, and they have no signal
transmission capability. In outdoor use, 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. [1]
Figure 5: Weight and Cable
Sight Glass A sight glass or water gauge is a transparent tube
through which the operator of a tank or boiler can observe the
level of liquid contained within. Simple sight glasses may be just
a plastic or glass tube connected to the bottom of the tank at one
end and the top of the tank at the other. The level of liquid in
the sight glass will be the same as the level of liquid in the
tank. Today, however, sophisticated float switches have replaced
sight glasses in many such applications.[4]
Advantages/Disadvantages Gages are used as a local indicator for
open or pressurized vessels. They must be accessible and located
within visual range. Gages are cheap and provide direct-reading
measurement. However, they are not suitable for dark liquids and
dirty fluids will prevent the liquid level from being viewed. They
can be easily damaged or broken. Glass gages should not be used to
measure hazardous liquids. Reflex gages are used for low- and
medium-pressure applications. For high-pressure applications, or
where the fluid is toxic, magnetic-type armored gages are used.
When installing such devices, good lighting is
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required. In installations where the gage is at a lower
temperature than the process, condensation may occur on the walls,
making the reading difficult. [1]
Figure 6: Sight Glass
Radioactive (Nuclear) With the radioactive (nuclear) device , 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.
[1] Advantages/Disadvantages Radioactive level measurement is
external device. 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. 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
radioactive source. 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. They are expensive and they are difficult to calibrate.
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. [1]
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Figure 7: Radioactive
Bubbler In a bubbler , 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 rota meter with a needle
valve, is required to provide a constant airflow of about 0.2 to
2.0 scfh (0.005 to 0.05 m3/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
pressure, and the low side measures the vessels top pressure, if it
is not vented to the atmosphere. [1] Advantages/Disadvantages The
bubbler offers low cost and easy maintenance. It can be operate
without electrical power. It can be used on pressurized or
unpressurized vessels. Variations in density will affect the
bubblers reading. Bubblers can become coated or plugged by process
fluid residue or dirt. The cost of purging fluid is ongoing, and
the purge gas can introduce unwanted components into the process.
The materials of construction for the bubbler must be compatible
with the process it is used in, and the bubblers 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 rodding. [1]
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Figure 8: Bubbler
Vibration Vibration devices 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/ft (12.8
kg/m) or greater. When the level drops below the fork, the
vibrating frequency is again in effect, and the relay is reversed.
[1] Advantages/Disadvantages Vibration units have no moving parts,
are rugged and reliable, are good for low-density materials, and
require little maintenance. They cannot be used in vibrating bins,
especially if the two frequencies are close. 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. [1]
Figure 9: Vibration
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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. 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.[3] Advantages/Disadvantages A paddle wheel is
inexpensive, simple, and reliable. It is susceptible to shock,
vibration, and damage by falling material. Paddle wheels generally
require some protection(e.g., a protective baffle) from material
charging from the top. Hang-ups or material buildup on the paddle
will affect the device performance.
Figure 10: Paddle Wheel
Diaphragm The diaphragm is a point measurement device. The
process materials (or hydrostatic pressure) apply pressure on a
diaphragm, which in turn actuates a switch. [1]
Advantages/Disadvantages The diaphragm is reliable, easy to
maintain. Coating may affect the flexing of the diaphragm, and
abrasive material may affect its performance. 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 75mm.) above any sediment in the vessel bottom to prevent
dirt from building up at the diaphragm. [1]
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Figure 11: Diaphragm
Resistance Tape The resistance-tape material level sensor with
which the invention is employed is sold commercially and is known
as a "Metritape" sensor, The Metritape sensor comprises an
elongated metallic base strip having electrical insulation on the
edges and back of the strip to define an un-insulated zone along
the length of the base strip, and a resistance wire or ribbon
helically wound around the insulated base strip, with the helical
turns bridging the insulated edge portions being spaced from the
underlying un-insulated zone of the base strip. This sensor
structure is enclosed within a continuous polymeric or other
protective sleeve to provide a clean and dry inner chamber for the
sensor. The sensor is disposed within a tank or vessel containing
the liquid or fluent material, the level of which is to be
monitored. The pressure of the material surrounding the immersed
sensor causes the deflection of the enclosing sleeve and helical
turns in the immersed portion of the sensor into engagement and
electrical contact with the underlying base strip, such that an
electrical resistance proportional to material level is
provided.[5] Advantages/Disadvantages A resistance tape will handle
corrosive liquids and slurries. It must contact the material and is
susceptible to moisture getting inside the tape. 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. [1]
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Figure 12: Resistance tape
Laser There are two types of laser measurement 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. [1] Advantages/Disadvantages 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. Lasers are
relatively expensive, though still better then radioactive
(nuclear) types.[1]
Figure 13: Laser
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Hook- Type Level Sensor Hook- Type level Indicator consists of a
wire of corrosion resistance alloy bent into U-shaped with one arm
longer than the other. The shorter arm is pointed with 60 degrees
while the longer is attached to a slider on a Vernier scale, which
moves over the main scale and shows the reading. [6] In this type
of sensor the hook is pushed below the surface of the liquid whose
level is to be measured and gradually raised until the point is
just about to break through the surface. This principle is also
utilized in measuring point manometer in which measuring point
consists of a steel point fixed with the point upward underneath
the water surface. [6]
Level Measurement Sensor Selection General considerations in
level measurement technology selection
Density and viscosity Chemical composition Ambient temperature
Process temperature Process pressure Regulated environments Process
agitation Vapor, mist, and dust Interfaces and gradients Process
conductivity and dielectric constants Vibration Material buildup or
stickiness Static charge Humidity/moisture Repeatability, stability
and accuracy requirements
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References 1. Battikha, N. E., The Condensed Handbook of
Measurement and Control; Publisher
ISA: The Instrumentation, Systems, and Automation Society
(2006); 3rd Ed., pg. 99-
121
2. http://www.iceweb.com.au/technical.html (retrieved on
05-11-2010)
3. http://www.iceweb.com.au/Level/LevelWeb.htm (retrieved on
05-11-2010)
4. http://en.wikipedia.org/wiki/Sight_glass#cite_note-bell
(retrieved on 06-11-2010)
5. Edwin P. Dews, N.H. William.E.Pierce, D. Ehrenfried, (1990),
United States Patent, p
6
6. Singh, S. K., Industrial Instrumentation and Control; McGraw
Hill Publishing
Company Limited; 2nd edition, pg. 225; 226
7. McMillan, Gregory K. , Process/Industrial Instruments and
Controls Handbook;
McGraw Hill Publishing Company Ltd; 5th Ed. , pg. 155 Section
4
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