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Zivenko A.V.,
Nakonechniy A.G., Motorkin D.Y.
Admiral Makarov National University of
Shipbuilding, Nikolaev
(Ukraine)
Level measurement principles &
sensors
Introduction. Measuring
the level of liquids in various reservoirs is a widespread and varied problem in industry. The actuality of level
measurement of liquids increases with the expansion of the automation, implementation of specialized control and accounting
systems. The most characteristic problems, connected
with the level measurement are:
-
In the chemical, oil & gas
industries – level measurement of various liquids in reservoirs and pipes, mass
& volume accounting during storage and commodity
operations, leakage control and emergency alarm.
-
Measurement of the volume and mass of
liquid products during their transportation/consumption in the reservoirs of
various vehicles (tank trucks, tank wagons, etc.). There are
special tasks in marine and shipbuilding industries, such
as measurement of liquid cargo parameters (including oils, LNG, fuels, discharge water, sludge, etc.), monitoring
of dock’s/vessel’s parameters (roll, trim, draught).
-
In agricultural and food industries,
environmental protection – measurement of level of various liquid products,
waste water managements, level measurements on the rivers or irrigation
channels, etc.
Many level
measurement methods have been developed, but only some of them have been widely
implemented. All of the level measurement techniques can be divided into three
groups: techniques which use effects associated with the
propagation of electromagnetic or acoustic waves – radiometric, laser, ultrasonic
(sonic), microwave; techniques which use principles not connected with the
propagation of waves through the controllable media – hydrostatic, float, capacitive,
conductive, pneumatic; and combined techniques – which use effects associated
with the propagation of electromagnetic/acoustic waves with special
constructions (e.g. – magnetostrictive level sensors). Each
of these techniques has their own advantages and drawbacks described next.
Laser level sensors deduce
the transit time of a laser beam reflected from the surface of the liquid. Basic advantages of such level sensors: non-contact measurement; laser sensors
are capable of providing very accurate results; laser
level sensors provide very quick response (important for some technological
processes). Basic drawbacks: risk of contamination (presence of dirt, dust or
any other type of coating on a laser level sensor can affect the performance of
these devices since it weakens the strength of the laser signal); can be used
for high-temperature or high-pressure applications (e.g. in reactor vessels),
but lasers must be used with specialized glass windows, which pass the laser
beam with minimal distortions, to isolate sensor from the process; cannot be
used for measuring the separation levels of various liquids (e.g. oil/water
separation level); expensive. Basic manufacturers of
laser level sensors are Laser Technology Inc., Dropsa, Sick.
Ultrasonic level sensors.
The principle of measurement is similar to laser level sensor. The sensors emit
high frequency acoustic waves that are reflected back to and detected by the
emitting transducer. The transit time of an ultrasonic signal is measured and
then the distance from level sensor to the surface of the liquid is calculated.
The ultrasonic sensors have wide popularity due to their relatively low price
and high functionality, they have such basic advantages: non-contact
measurement (or contact as an option); quick response; low energy consumption
and relatively low price. Basic drawbacks: speed of sound vary due to moisture,
temperature and pressures – it affects the accuracy of level sensor; it’s difficult
to use ultrasonic sensors if such factors as foam, steam, vapors and changes in
the concentration of the process material are present in your technological
process (wave guides are used to prevent errors caused by these factors). The
best-known manufacturers of ultrasonic level sensors are: Emerson, Siemens,
Endress Hauser, Kronhe, Vega.
Microwave
level sensors (also frequently described as "Radar"). Two basic principles
are used in this case: based on frequency modulated continuous wave techniques
(FMCW) and based on Time Domain Reflectometry (TDR) techniques. FMCW based systems
are noninvasive: electronic block on the top of the tank sends
down a linear frequency sweep. The reflected signal is delayed proportionally
to the distance to the level surface. The frequency of reflected signal is
different from that of the transmitted signal. The
frequency difference, which is calculated from the transmitting frequency and
the received frequency, is directly proportional to the measured distance.
Basic advantages of such technique: non-contact measurement; wide measurement
range and relatively small “dead zone”; high accuracy (generally, the higher
the frequency, the more accurate); operation in high pressure and
vacuum, high temperatures, dust (possible loss of accuracy). Basic drawbacks: expensive; dependent of tank internals; cannot be
used for measuring the separation levels of various liquids (e.g. oil/water
separation level); require special purging system
for cleaning, heating or cooling of antenna (condensation presence). The best-known manufacturers of FMCW-based level sensors are Emerson, Kronhe, Vega, Siemens, Endress Hauser, Finetek, Campbell Scientific.
Hydrostatic level sensors are
one of the most widely used for tank
gauging. These sensors are usually fitted to the tank bottom to measure the
differential pressure relative to the environment. The level of controllable homogeneous liquid is proportional to the pressure and density of the
measured liquid. Large
variations in temperature cause changes in liquids density that should be
accounted when the pressure is converted to level. The main advantages of this
technique are: relatively low cost; proven reliability; ease of installation
and use; broad media compatibility. Drawbacks: cannot be
used for measuring the separation levels of various liquids (e.g. oil/water
separation level); dependent on the density of the
medium; relatively low accuracy. Basic manufacturers of hydrostatic level
sensors: Emerson, Honeywell, Vega, General Electric, Kobold, Flowline, Endress
Hauser, Siemens, Fuji Electric.
Float level sensors
operate by float movement with a change in level. Of course - density of a
float must be less than that of the liquid. There are two main types of such
level sensors: a) the position of a float, which is afloat, is sensed; b) the
change in weight is measured when a float is dipped partially into the liquid. Main
advantages: low cost; determination of interface level in oil-water separation
systems if additional float is used; another special application of float level
sensor is the installation of temperature or pressure sensors to create a
multi-parameter sensor. Drawbacks: dependent on the density of the
medium; heavy contamination causes failure of the level sensor. Basic manufacturers of float level sensors
are: ABB, GEMS, Musasino, Valcom, Nivelco, Kobold, Finetek, Magnetrol.
Magnetostrictive level sensors are similar to float type sensors but these sensors contain permanent
magnet inside a float situated on a stem in which a magnetostrictive wire is
sealed. Magnetic field of such special “float” magnetizes the wire axially.
Since the two magnetic fields are superimposed, around the float magnet a
torsion wave is generated which runs in both directions along the wire. The
time is measured between emission of the current pulse and arrival of the wave
at the probe head. The position of the float is determined on the basis of the
transit times. Magnetostrictive sensors accounted for 21% of the total sensors
used in industrial, power & manufacturing applications [1]. These sensors seems
to be ideal for high-accuracy, continuous level measurement of various “clean” liquids, relatively low cost,
but they require the proper choice of float based on the specific density of
the liquid and as for float-type sensors - heavy contamination causes failure
of the sensor.
Capacitive level sensors.
The
principle of capacitive level measurement is based on change of capacitance of
the partially dipped into the liquid capacitor (sensor). Capacitance of such
sensor depends on the level of the liquid. The main advantages of capacitive
level sensors are: low price and high reliability; wide measurement range; determination of interface level in oil-water separation systems
available for some liquids. Basic drawbacks: relatively low accuracy; dependent
on the liquid relative permittivity.
There are such
widely used point-level detection sensors as pneumatic and conductive. Pneumatic
level sensors are commonly used in hazardous conditions, this sensors are
suitable for use with highly viscous liquids such as grease. These sensors can
be used where there is no electric power or its use is restricted. Conductive level
sensors are usually used for the point-level detection of a wide range of
conductive liquids such as water, highly corrosive liquids (caustic soda,
hydrochloric acid, nitric acid, etc.).
Obviously basic important factors choosing the sensor for your application are
the initial cost and the accuracy of the level sensor. Fig. 1 can give you basic tips for selecting the right level sensor in terms of
cost and accuracy.
Fig.
1 – The initial cost for level sensors
However,
initial cost is only one aspect that must be considered when choosing level
sensor for your application – low initial cost may be outweighed by low
accuracy and high maintenance costs. It’s known that 51% of customers spending
are level sensor hardware, another part contain spending on installation and
calibration of level sensors, wiring, maintenance and software
programming/adaptation.
It should be pointed out that
other important aspect of choosing the level sensor is network connectivity and
especially ability of wireless data transfer. Nowadays users rely on a number
of different wired network interfaces, including HART,
Modbus, Profibus, etc. The choice of any particular interface is a crucial
problem for engineers, because it’s linked with all measurement and automation
equipment connected with the process. It’s convenient to use sensors with
wireless interfaces because of the reduction in cable costs, ease of
installation and increase in flexibility and scalability of the measurement
systems and automation solutions.
Conclusions. It
should be noted that there is no
universal level sensor, suitable for all practical applications. The dominance of any level sensor on the market is based on a low
product and maintenance cost, ease of installation and use, proven reliability
and ruggedness, broad media compatibility. Despite the robust growth in the use
of microwave/radar (contact and noncontact) devices the largest market share
during 2013 will be for hydrostatic pressure transmitters and float-type
sensors (magnetostrictive). The only
drawback of microwave sensors is their relatively high price, but due to evolution
of microwave electronics and DSP
methods this type of level sensors has great improvement potential.
References
1. Level Sensors Market: Global Forecast & Analysis
- Focus on Technologies [Contact Type (Magnetostrictive, Vibratory probe,
Hydrostatic, Magnetic float) and Non Contact Type (Ultrasonic, Radar, Optical,
Laser)], Applications and Geography (2013-2018) [Text] // Published
by Markets&Markets, January 2013. – 330 pages.
2. J
Timothy Shea
Market Levels Rising in
the Process Level Measurement and Inventory Tank Gauging Segments [Electronic resource] // Sensors magazine, June 1, 2012. URL:
http://www.sensorsmag.com/process-industries/market-levels-rising-process-level-measurement-and-inventory-10042.