From Wikipedia, the free encyclopedia Level sensor
Level sensor
Level sensors detect the level of substances that flow, on the probe element. Vibrating level sensors are not
including liquids, slurries, granular materials, and pow- affected by dust, static-charge build-up from dielectric
ders. Fluids and fluidized solids flow to become essen- powders, or changes in conductivity, temperature, pres-
tially level in their containers (or other physical bound- sure, humidity or moisture content. Tuning-fork style vi-
aries) because of gravity whereas most bulk solids pile at bration sensors are another alternative. They tend to be
an angle of repose to a peak. The substance to be mea- less costly, but are prone to material buildup between the
sured can be inside a container or can be in its natur- tines.
al form (e.g., a river or a lake). The level measurement
can be either continuous or point values. Continuous lev- Rotating paddle
el sensors measure level within a specified range and de- Rotating paddle level sensors are a very old and estab-
termine the exact amount of substance in a certain place, lished technique for bulk solid point level indication. The
while point-level sensors only indicate whether the sub- technique uses a low-speed gear motor that rotates a
stance is above or below the sensing point. Generally the paddle wheel. When the paddle is stalled by solid mate-
latter detect levels that are excessively high or low. rials, the motor is rotated on its shaft by its own torque
There are many physical and application variables until a flange mounted on the motor contacts a mechan-
that affect the selection of the optimal level monitoring ical switch. The paddle can be constructed from a variety
method for industrial and commercial processes. The se- of materials, but tacky material must not be allowed to
lection criteria include the physical: phase (liquid, solid build up on the paddle. Build-up may occur if the process
or slurry), temperature, pressure or vacuum, chemistry, material becomes tacky because of high moisture levels
dielectric constant of medium, density (specific gravity) or high ambient humidity in the hopper. For materials
of medium, agitation (action), acoustical or electrical with very low weight per unit volume such as Pearlite,
noise, vibration, mechanical shock, tank or bin size and Bentonite or fly ash, special paddle designs and low-
shape. Also important are the application constraints: torque motors are used. Fine particles or dust must be
price, accuracy, appearance, response rate, ease of cali- prevented from penetrating the shaft bearings and mo-
bration or programming, physical size and mounting of tor by proper placement of the paddle in the hopper or
the instrument, monitoring or control of continuous or bin and using appropriate seals.
discrete (point) levels.
Admittance-type
Point and continuous level de- An RF Admittance level sensor uses a rod probe and RF
source to measures the change in admittance. The probe
tection for solids is driven through a shielded coaxial cable to eliminate
A variety of sensors are available for point level detection the effects of changing cable capacitance to ground.
of solids. These include vibrating, rotating paddle, me- When the level changes around the probe, a correspond-
chanical (diaphragm), microwave (radar), capacitance, ing change in the di-electric is observed. This changes
optical, pulsed-ultrasonic and ultrasonic level sensors. the admittance of this imperfect capacitor and this
change is measured to detect change of level.[1]
Vibrating point
These detect levels of very fine powders (bulk density: Point level detection of liquids
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 Pulse-Wave Ultrasonic (Non Invasive)
or greater). With proper selection of vibration frequency
The principle behind a Pulsed-Ultrasonic technology is
and suitable sensitivity adjustments, they can also sense
that the transmit signal consists of short bursts of ultra-
the level of highly fluidized powders and electrostatic
sonic energy. After each burst, the electronics looks for a
materials.
return signal within a small window of time correspond-
Single-probe vibrating level sensors are ideal for bulk
ing to the time it takes for the energy to pass through
powder level. Since only one sensing element contacts
the vessel. Only signal received during this window peri-
the powder, bridging between two probe elements is
od will qualify for additional signal processing. The dry
eliminated and media build-up is minimized. The vibra-
tion of the probe tends to eliminate build-up of material
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From Wikipedia, the free encyclopedia Level sensor
signal will not be received within this window, and there- Conductive
fore will be ignored.
Conductive level sensors are ideal for the point level de-
tection of a wide range of conductive liquids such as wa-
Magnetic and mechanical float ter, and is especially well suited for highly corrosive liq-
The principle behind magnetic, mechanical, cable, and uids such as caustic soda, hydrochloric acid, nitric acid,
other float level sensors involves the opening or closing ferric chloride, and similar liquids. For those conductive
of a mechanical switch, either through direct contact liquids that are corrosive, the sensor’s electrodes need to
with the switch, or magnetic operation of a reed. With be constructed from titanium, Hastelloy B or C, or 316
magnetically actuated float sensors, switching occurs stainless steel and insulated with spacers, separators or
when a permanent magnet sealed inside a float rises or holders of ceramic, polyethylene and Teflon-based mate-
falls to the actuation level. With a mechanically actuated rials. Depending on their design, multiple electrodes of
float, switching occurs as a result of the movement of a differing lengths can be used with one holder. Since cor-
float against a miniature (micro) switch. For both mag- rosive liquids become more aggressive as temperature
netic and mechanical float level sensors, chemical com- and pressure increase, these extreme conditions need to
patibility, temperature, specific gravity (density), buoy- be considered when specifying these sensors.
ancy, and viscosity affect the selection of the stem and Conductive level sensors use a low-voltage, current-
the float. For example, larger floats may be used with limited power source applied across separate electrodes.
liquids with specific gravities as low as 0.5 while still The power supply is matched to the conductivity of the
maintaining buoyancy. The choice of float material is al- liquid, with higher voltage versions designed to operate
so influenced by temperature-induced changes in spe- in less conductive (higher resistance) mediums. The pow-
cific gravity and viscosity – changes that directly affect er source frequently incorporates some aspect of control,
buoyancy.[2] such as high-low or alternating pump control. A conduc-
Float-type sensors can be designed so that a shield tive liquid contacting both the longest probe (common)
protects the float itself from turbulence and wave mo- and a shorter probe (return) completes a conductive cir-
tion. Float sensors operate well in a wide variety of liq- cuit. Conductive sensors are extremely safe because they
uids, including corrosives. When used for organic sol- use low voltages and currents. Since the current and volt-
vents, however, one will need to verify that these liquids age used is inherently small, for personal safety reasons,
are chemically compatible with the materials used to the technique is also capable of being made “Intrinsically
construct the sensor. Float-style sensors should not be Safe” to meet international standards for hazardous lo-
used with high viscosity (thick) liquids, sludge or liquids cations. Conductive probes have the additional benefit of
that adhere to the stem or floats, or materials that con- being solid-state devices and are very simple to install
tain contaminants such as metal chips; other sensing and use. In some liquids and applications, maintenance
technologies are better suited for these applications. can be an issue. The probe must continue to be conduc-
A special application of float type sensors is the de- tive. If buildup insulates the probe from the medium, it
termination of interface level in oil-water separation sys- will stop working properly. A simple inspection of the
tems. Two floats can be used with each float sized to probe will require an ohmmeter connected across the
match the specific gravity of the oil on one hand, and suspect probe and the ground reference.
the water on the other. Another special application of a Typically, in most water and wastewater wells, the
stem type float switch is the installation of temperature well itself with its ladders, pumps and other metal instal-
or pressure sensors to create a multi-parameter sensor. lations, provides a ground return. However, in chemical
Magnetic float switches are popular for simplicity, de- tanks, and other non-grounded wells, the installer must
pendability and low cost. supply a ground return, typically an earth rod.
Pneumatic
Pneumatic level sensors are used where hazardous con-
Sensors for both Point Level
ditions exist, where there is no electric power or its use Detection or Continuous Moni-
is restricted, and in applications involving heavy sludge
or slurry. As the compression of a column of air against a toring of Solids and Liquids
diaphragm is used to actuate a switch, no process liquid
contacts the sensor’s moving parts. These sensors are Ultrasonic
suitable for use with highly viscous liquids such as Ultrasonic level sensors are used for non-contact level
grease, as well as water-based and corrosive liquids. This sensing of highly viscous liquids, as well as bulk solids.
has the additional benefit of being a relatively low cost They are also widely used in water treatment applica-
technique for point level monitoring. tions for pump control and open channel flow measure-
ment. The sensors emit high frequency (20 kHz to
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From Wikipedia, the free encyclopedia Level sensor
200 kHz) acoustic waves that are reflected back to and Capacitance
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 measure-
ment.
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 en-
sure 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 Capacitance level sensors excel in sensing the presence
most modern systems have sufficiently "intelligent" echo of a wide variety of solids, aqueous and organic liquids,
processing to make engineering changes largely unnec- and slurries. The technique is frequently referred to as RF
essary except where an intrusion blocks the "line of for the radio frequency signals applied to the capacitance
sight" of the transducer to the target. Since the ultrason- circuit. The sensors can be designed to sense material
ic transducer is used both for transmitting and receiving with dielectric constants as low as 1.1 (coke and fly ash)
the acoustic energy, it is subject to a period of mechani- and as high as 88 (water) or more. Sludges and slurries
cal vibration known as “ringing”. This vibration must at- such as dehydrated cake and sewage slurry (dielectric
tenuate (stop) before the echoed signal can be processed. constant approx. 50) and liquid chemicals such as quick-
The net result is a distance from the face of the transduc- lime (dielectric constant approx. 90) can also be sensed.
er that is blind and cannot detect an object. It is known Dual-probe capacitance level sensors can also be used to
as the “blanking zone”, typically 150mm – 1m, depending sense the interface between two immiscible liquids with
on the range of the transducer. substantially different dielectric constants, providing a
The requirement for electronic signal processing cir- solid state alternative to the aforementioned magnetic
cuitry can be used to make the ultrasonic sensor an intel- float switch for the “oil-water interface” application.
ligent device. Ultrasonic sensors can be designed to pro- Since capacitance level sensors are electronic de-
vide point level control, continuous monitoring or both. vices, phase modulation and the use of higher frequen-
Due to the presence of a microprocessor and relatively cies makes the sensor suitable for applications in which
low power consumption, there is also capability for seri- dielectric constants are similar. The sensor contains no
al communication from to other computing devices mak- moving parts, is rugged, simple to use, and easy to clean,
ing this a good technique for adjusting calibration and fil- and can be designed for high temperature and pressure
tering of the sensor signal, remote wireless monitoring applications. A danger exists from build-up and dis-
or plant network communications. The ultrasonic sensor charge of a high-voltage static charge that results from
enjoys wide popularity due to the powerful mix of low the rubbing and movement of low dielectric materials,
price and high functionality. 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 mate-
rials 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, capac-
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From Wikipedia, the free encyclopedia Level sensor
itance level sensors can be designed with splashguards or are applied, each offering its own advantages: Time-Do-
stilling wells, among other devices. main Reflectometry (TDR) which is a measurement of
A significant limitation for capacitance probes is in time of flight divided by the speed of light, similar to ul-
tall bins used for storing bulk solids. The requirement trasonic level sensors, and Doppler systems employing
for a conductive probe that extends to the bottom of the FMCW techniques. Just as with ultrasonic level sensors,
measured range is problematic. Long conductive cable microwave sensors are executed at various frequencies,
probes (20 to 50 meters long), suspended into the bin or from 1 GHz to 30 GHz. Generally, the higher the frequen-
silo, are subject to tremendous mechanical tension due to cy, the more accurate, and the more costly. Microwave is
the weight of the bulk powder in the silo and the friction also executed as a non-contact technique, monitoring a
applied to the cable. Such installations will frequently re- microwave signal that is transmitted through the medi-
sult in a cable breakage. um (including vacuum), or can be executed as a “radar
on a wire” technique. In the latter case, performance im-
Optical interface proves in powders and low dielectric media that are not
Optical sensors are used for point level sensing of sed- good reflectors of electromagnetic energy transmitted
iments, liquids with suspended solids, and liquid-liquid through a void (as in non-contact microwave sensors).
interfaces. These sensors sense the decrease or change in But the same mechanical constraints exist that cause
transmission of infrared light emitted from an infrared problems for the capacitance (RF) techniques mentioned
diode (LED). With the proper choice of construction ma- previously.
terials and mounting location, these sensors can be used Microwave-based sensors are not affected by fouling
with aqueous, organic, and corrosive liquids. of the microwave-transparent glass or plastic window
A common application of economical infrared-based through which the beam is passed nor by high tempera-
optical interface point level sensors is detecting the ture, pressure, or vibration. These sensors do not require
sludge/water interface in settling ponds. By using pulse physical contact with the process material, so the trans-
modulation techniques and a high power infrared diode, mitter and receiver can be mounted a safe distance from
one can eliminate interference from ambient light, op- the process, yet still respond to the presence or absence
erate the LED at a higher gain, and lessen the effects of of an object. Microwave transmitters offer the key advan-
build-up on the probe. tages of ultrasonics: the presence of a microprocessor to
An alternate approach for continuous optical level process the signal provides numerous monitoring, con-
sensing involves the use of a laser. Laser light is more trol, communications, setup and diagnostic capabilities.
concentrated and therefore is more capable of penetrat- Additionally, they solve some of the application limita-
ing dusty or steamy environments. Laser light will reflect tions of ultrasonics: operation in high pressure and vacu-
off most solid, liquid surfaces. The time of flight can be um, high temperatures, dust, temperature and vapor lay-
measured with precise timing circuitry, to determine the ers. One major disadvantage of microwave or radar tech-
range or distance of the surface from the sensor. Lasers niques for level monitoring is the relatively high price of
remain limited in use in industrial applications due to such sensors.
cost, and concern for maintenance. The optics must be
frequently cleaned to maintain performance. Continuous level measurement
Microwave of liquids
Microwave sensors are ideal for use in moist, vaporous,
and dusty environments as well as in applications in Magnetostrictive
which temperatures vary. Microwaves (also frequently Magnetostrictive level sensors are similar to float type
described as RADAR), will penetrate temperature and va- sensors in that a permanent magnet sealed inside a float
por layers that may cause problems for other techniques, travels up and down a stem in which a magnetostrictive
such as ultrasonic. Microwaves are electromagnetic en- wire is sealed. Ideal for high-accuracy, continuous level
ergy and therefore do not require air molecules to trans- measurement of a wide variety of liquids in storage and
mit the energy making them useful in vacuums. Mi- shipping containers, these sensors require the proper
crowaves, as electromagnetic energy, are reflected by ob- choice of float based on the specific gravity of the liquid.
jects with high conductive properties, like metal and When choosing float and stem materials for magne-
conductive water. Alternately, they are absorbed in var- tostrictive level sensors, the same guidelines described
ious degrees by dielectric or insulating mediums such as for magnetic and mechanical float level sensors apply.
plastics, glass, paper, many powders and food stuffs and Magnetostrictive level and position devices charge
other solids. the magnetostrictive wire with electrical current, when
Microwave sensors are executed in a wide variety the field intersects the floats’ magnetic field a mechani-
of techniques. Two basic signal processing techniques cal twist or pulse is generated, this travels back down the
4
From Wikipedia, the free encyclopedia Level sensor
wire at the speed of sound, like ultrasound or radar the thereof, a special version of the hydrostatic pressure lev-
distance is measured by time of flight from pulse to re- el sensor can be suspended from a cable into the tank
turn pulse registry. the time of flight corresponds to the to the bottom point that is to be measured. The sensor
distance from the sensor detecting the return pulse. must be specially designed to seal the electronics from
the liquid environment. In tanks with a small head pres-
Because of the degree of accuracy possible with the mag- sure (less than 100 INWC), it is very important to vent the
netostrictive technique, it is popular for “custody-trans- back of the sensor gauge to atmospheric pressure. Other-
fer” applications. It can be permitted by an agency of wise, normal changes in barometric pressure will intro-
weights and measures for conducting commercial trans- duce large error in the sensor output signal. In addition,
actions. It is also frequently applied on magnetic sight most sensors need to be compensated for temperature
gages. In this variation, the magnet is installed in a float changes in the fluid.
that travels inside a gage glass or tube. The magnet op-
erates on the sensor which is mounted externally on the Air bubbler
gage. Boilers and other high temperature or pressure ap- An air bubbler system uses a tube with an opening below
plications take advantage of this performance quality. the surface of the liquid level. A fixed flow of air is passed
through the tube. Pressure in the tube is proportional to
Resistive chain the depth (and density) of the liquid over the outlet of
Resistive chain level sensors are similar to magnetic float the tube.
level sensors in that a permanent magnet sealed inside a Air bubbler systems contain no moving parts, making
float moves up and down a stem in which closely spaced them suitable for measuring the level of sewage,
switches and resistors are sealed. When the switches are drainage water, sewage sludge, night soil, or water with
closed, the resistance is summed and converted to cur- large quantities of suspended solids. The only part of the
rent or voltage signals that are proportional to the level sensor that contacts the liquid is a bubble tube which is
of the liquid. chemically compatible with the material whose level is
The choice of float and stem materials depends on the to be measured. Since the point of measurement has no
liquid in terms of chemical compatibility as well as spe- electrical components, the technique is a good choice for
cific gravity and other factors that affect buoyancy. Th- classified “Hazardous Areas”. The control portion of the
ese sensors work well for liquid level measurements in system can be located safely away, with the pneumatic
marine, chemical processing, pharmaceuticals, food pro- plumbing isolating the hazardous from the safe area.
cessing, waste treatment, and other applications. With Air bubbler systems are a good choice for open tanks
the proper choice of two floats, resistive chain level sen- at atmospheric pressure and can be built so that high-
sors can also be used to monitor for the presence of an pressure air is routed through a bypass valve to dislodge
interface between two immiscible liquids whose specific solids that may clog the bubble tube. The technique is
gravities are more than 0.6, but differ by as little as 0.1 inherently “self-cleaning”. It is highly recommended for
unit. liquid level measurement applications where ultrasonic,
float or microwave techniques have proved undepend-
Hydrostatic pressure able.
Hydrostatic pressure level sensors are submersible or ex-
ternally mounted pressure sensors suitable for measur- Gamma ray
ing the level of corrosive liquids in deep tanks or water A nuclear level gauge or gamma ray gauge measures level
in reservoirs. For these sensors, using chemically com- by the attenuation of gamma rays passing through a
patible materials is important to assure proper perfor- process vessel. The technique is used to regulate the level
mance. Sensors are commercially available from 10mbar of molten steel in a continuous casting process of steel-
to 1000bar. making. The water-cooled mold is arranged with a source
Since these sensors sense increasing pressure with of radiation, such as Cobalt-60 or Cesium-137, on one side
depth and because the specific gravities of liquids are dif- and a sensitive detector such as a scintillometer on the
ferent, the sensor must be properly calibrated for each other. As the level of molten steel rises in the mold, less
application. In addition, large variations in temperature of the gamma radiation is detected by the sensor. The
cause changes in specific gravity that should be account- technique allows non-contact measurement where the
ed for when the pressure is converted to level. These sen- heat of the molten metal makes any contact technique
sors can be designed to keep the diaphragm free of con- impractical.
tamination or build-up, thus ensuring proper operation
and accurate hydrostatic pressure level measurements.
For use in open air applications, where the sensor
See also
cannot be mounted to the bottom of the tank or pipe • List of sensors
5
From Wikipedia, the free encyclopedia Level sensor
References [2] Deeter. "Float Level Sensors".
http://www.deeterelectronicsinc.com/. Retrieved
[1] Sapcon Instruments. "Fly Ash Level Detection?". 2009-05-05.
http://www.sapconinstruments.com/articles/
article1.html. Retrieved 2008-05-05.
Retrieved from "http://en.wikipedia.org/w/index.php?title=Level_sensor&oldid=464461538"
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