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What is meant by level measurement? |
Determination of material level
in a vessel is termed as level measurement. |
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Why is it required? |
| It is necessary to store
adequate amounts of material in a vessel without
unnecessary wastage. The input and output
devices such as pumps, and material conveyors
need to operate without damage and with minimum
wastage of power. Also it is necessary to know
the quantity stored in the vessel at any moment
of time for inventory control. In a processing
plant, the various machines should neither be
under nor over fed. Accurate level measurement
at critical points in the process ensures
optimal production. It avoids spillage of
hazardous material and ensures plant and
environment safety. |
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How is it done
broadly? |
Basically level measurement is
of two type:
- Point Level measurement.
- Continuous Level measurement.
Point level measurement also known as limit
switching, determines the desired point level by
means of a switching function either in the form
of a potential free relay output contact or the
open collector of a transistor. The switching
function can then be used for controlling the
filling and emptying operations of the vessel.
Continuous Level measurement determines the
level of material in the vessel at any moment of
time and displays the same in percent or in any
other distance or volumetric units as desired.
It also outputs a 4 to 20 mA current signal for
connecting to recording or controlling devices
such as PLC and PID controllers. Relay output
contacts that can be programmed to indicate
Alarm conditions at desired critical levels are
also provided.
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What types of devices are used for measurement
in general? |
Level measuring devices fall
into two main categories:
- Invasive (contact type)
- Non-invasive (non-contact type)
As the words suggest, the invasive type
device comes in contact with the material in the
vessel whereas the non-invasive type does not.
The invasive type therefore can be used in
applications where the material contact does not
damage the device and the device does not
contaminate the material. Where such
possibilities exists the non-
invasive is used. Normally the invasive devices
are cheaper than the non-invasive ones.
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What does a level measuring system comprise? |
Let us first consider an
invasive level measuring system. It comprises a
level sensing probe that can either be mounted
from the top or side of the vessel. When the
probe comes in contact with the material in the
vessel the physical event is converted to an
electrical signal either directly or by an
electronic insert in the probe head. The signal
after primary processing is transmitted to the
evaluation unit which converts it to a suitable
output, either, as a change in the state of a
relay contact or an analogue 4 to 20 mA output
for Alarm annunciation / control / recording.
For limit switching applications the probe can
be either mounted from top or side of the
vessel. For continuous level monitoring however,
the probe is always mounted from the top.
In the case of non-invasive level measuring
system the probe is normally mounted from the
top of the vessel. In certain applications it
can also be mounted from the side. The probe
does not come in contact with the material
directly but through some form of energy
exchange. The level data after being primarily
processed in the electronic insert in the probe
head is transmitted to the evaluation unit
wherein it is further processed and available as
analogue 4 to 20 mA and relay / open collector
transistor output for Alarm annunciation /
control / recording. |
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How is the physical
level converted to an electrical signal? |
| The physical change in level is
converted to an electrical signal by utilizing a
suitable device as interface. Such a device is
called a Transducer. The transducer converts
energy in one form into another.
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What are the
different principles on which level detectors
are based? |
The choice of a Transducer used
for any particular level detector application
depends on the characteristics of the service
material and the conditions in the vessel and
its environment. Since various types of
materials are stored in a processing plant under
varied conditions inside and outside the vessel,
a universal solution is never possible. Leading
technologies of level detection today are based
on the following principles:
Conductivity, RF Capacitance, RF Admittance,
Electromechanical, Hydrostatic, Ultrasonic,
Microwave and Nuclear.
A deep study of the various parameters of the
particular application is most essential for
making the right choice. Normally, level
instrument manufacturers are aware of the
limitations of their products, and given the
site conditions in detail they are the right
people who can recommend the appropriate
measuring system. |
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What terminology is
used for communicating level measurement
concepts in general? |
POINT LEVEL: Material
level distance as measured from the vessel’s
bottom.
POINT LEVEL DETECTION: Level detection at
the desired height from the bottom of the vessel
in terms of a switching function.
HIGH LEVEL: Maximum level at which
“vessel full” indication is required.
LOW LEVEL: Minimum level at which “vessel
empty” indication is required.
HIGH HIGH LEVEL: Above maximum expected
level.
LOW LOW LEVEL: Below minimum expected
level.
PUMP CONTROL LOGIC: As applied to
overhead tank level control, the pump is STOPPED
if the level is equal to or greater than the
maximum desired level and does not RESTART until
the level is equal to or less than the minimum
desired level. Thus the level is maintained
between the minimum and maximum point levels.
As applied to sump level control, the pump is
STOPPED when the level is equal to or less than
the minimum desired level and RESTARTED when it
is equal to or greater than the maximum desired
level. Thus the level is maintained between the
minimum and maximum desired levels.
INDEPENDENT SWITCHING: For overhead tank
level indication, the relay is de-energized when
level is equal to or more than the desired
maximum level initiating a high level Alarm. For
sumps, the relay is de-energized when level is
equal to or less than minimum level initiating a
low level Alarm.
SEQUENTIAL PUMP CONTROL: For dewatering
applications, the pumps are sequentially started
as the level starts increasing. The pumps remain
ON until the level is equal to or less than the
minimum desired level.
SWITCHING DELAY: In applications which
require single point switching for liquids, one
often encounters ripples and waves on the liquid
surface as a result of inflow of liquid into the
tank. Such turbulences can cause frequent
switching ON and OFF of the controlled device,
close to the point level, as a result of touch
and go of the crest and troughs of the surface
waves. An adjustable switching delay can be set
optimally to avoid the frequent switching, at
the same time keeping the response time within
the acceptable limits. The same result is
possible by using a pump control logic that
gives a dead band, which is slightly more than
the expected wave amplitude. In the dead band
method, delay is not needed.
DELAY MODES: Switching delays can be
selected to occur either after the probe is
covered with material or when the probe is
uncovered by the material or under both
conditions. Also delays can be inhibited during
calibration.
DELAY ADJUSTMENT: Provision is made for
adjusting the switching delays up to 20 Sec.
This is sufficient for most of the applications.
SENSITIVITY ADJUSTMENT: This adjusts the
initial conditions such that the switching
occurs when the material covers a known amount
of the probe length for a particular type
material. This adjustment is to be made after
installation to compensate for the initial
conditions. Sensitivity adjustment is a very
important factor for proper operation of the
level measurement system. A higher sensitivity
can lead to instability whereas a lower
sensitivity can lead to non-detection of level.
Sensitivity therefore must be set optimally as
per application requirements.
HYSTERISIS: The term is applied to a
switching function. The region between the ON
and OFF of a switch where nothing happens or
which may be termed as a dead band is called
Hysterisis. As applied to level measurement this
is the region between two point levels within
which the level is undetected or the controller
takes no action. Single point controllers have a
fixed hysterisis whereas for two point pump
control action the hysterisis is variable and
can be set as desired.
FAIL SAFE: In the event of a failure of
the level controller or the failure of the power
supply to the instrument the output relay
contacts must revert to the fail-safe state.
That is High level controller should not cause
overflow and the low level controller should not
cause an empty tank condition. This means the
alarm condition must be initiated in the
de-energized condition of the relay. This
ensures that in the event of failure the relay
will be de-energized and will not cause the
undesired condition to occur.
FAIL SAFE SELECT: It will be observed
that the relay output conditions required for
High level and Low level fail-safe are just the
opposite. For universality of operation fail
safe selection becomes a necessity. Thus it is
not necessary to stock two different type of
instruments as spare.
LEVEL MEASUREMENT SYSTEM: This consists
of probe, electronic insert (pre-amplifier),
evaluation unit, and interconnecting cable.
SYSTEM FAULT INDICATOR: In the event of a
system fault an LED indicator glows. A number of
faults are possible in a level measuring system.
Some of the faults are; probe short, probe open,
probe insulation failure, probe element broken,
electronic insert failure, interconnecting cable
open or short, wrong connections etc. In simpler
designs, in case of any fault or faults
occurring an indicator glows. The fault has to
be diagnosed manually. In advanced systems the
diagnosis is done automatically.
PROBE: It is a device that fits at a strategic
position on the vessel and responds in some way
to a level change.
ELECTRONIC INSERT: It consists of
electronic circuitry and fits in the probe head.
It converts the level change to a suitable
electrical signal for onward transmission to the
evaluation unit. Generally it derives power from
the evaluation unit for its operation.
EVALUATION UNIT: It accepts the signal
from the probe and processes it further to
obtain the desired output such as a relay
contact or an analogue voltage or current signal
for alarm annunciation or control or for
connecting to a recorder or PLC. It is sometimes
mounted integral to the probe or sometimes
remote; as per application requirements.
INTERCONNECTING CABLE: The cable used to
connect the electronic insert to the evaluation
unit. This carries the power for operation of
the electronic insert and the signal from the
electronic insert to the evaluation unit. This
cable can be of the ordinary type having 4,3,2
cores as necessary or it can be screened to
reduce electromagnetic interference. The
manufacturer’s recommendation should be followed
as regards the type of cable. Generally armored
cables are used in plant environments or the
cables are run through separate metallic
conduits away from high current carrying cables,
to reduce electromagnetic interference.
PROBE INACTIVE ZONE: The portion of the
probe that does not detect level change is
termed as inactive. Many a time the probe is
mounted to a nozzle, which is welded to the
vessel. There is always some gap between the
probe insertion and the nozzle. There is every
possibility for material to fill this gap. If
the portion of the probe insertion extending up
to the nozzle length is not made inactive then
there is every possibility that the probe will
respond to the material filling the gap and not
the actual level change. This portion is
therefore made inactive by using an extended
ground construction.
PROBE ACTIVE ZONE: This is that part of
the probe which responds to a level change.
VESSEL ACTIVE AND INACTIVE ZONES: The
efficiency of storage vessels is never 100%.
Ideally on evacuation, the vessel should be
completely empty. This does not happen in
practice. There is always some material sticking
to the sidewalls of the vessel. The thickness of
this coating depends not only on vessel
construction but also on characteristics of the
stored material and its moisture content. The
central axis of the vessel is the most active
zone. Probes should be mounted in such a way
that they always experience the material
movement and are always in the active zone. This
will ensure true level indication.
MATERIAL BUILDUP ON PROBE: Certain
materials have a tendency to stick to the vessel
walls and the probe. Material can form a bridge
between the vessel wall and the active portion
of the probe. This phenomena is termed as
material build up on probe. This has the effect
of indicating level even when the level is below
the desired point level. The effect is
pronounced in side-mounted probes. As mentioned
above using an inactive probe zone and extending
the active portion of the probe in the dynamic
portion of the vessel can avoid the situation.
RAT HOLING: In an electromechanical
vibration type level instrument used for very
light and fluffy service material there is a
tendency to form cavities as a result of the
displacement of material due to the force
exerted by the vibrating member. This can give
false indications. As the cavity resembles a rat
hole the phenomena is known as Rat holing.
COATING ON PROBE: Sticky material often
coats the probe surface. If the coating extends
from the active element to ground (probe
mounting), the probe will detect a level even
when the level is not there. If the material be
conductive then the problem is severe. Special
level measurement systems are available to offer
a reliable solution to such problems.
PROBE MOUNTING ARRANGEMENT: There are
various types of probe mounting arrangements
used that depend on the application requirement.
Generally the methods used are of the screwed or
flanged types that comply with various standards
such as ASA, ANSI, DIN, BS, IS, etc. The screwed
type generally complies with straight or tapered
pipe thread standards such as BS, NPT etc. Food
and Pharmaceutical applications need sanitary
standards using tri-clamp mechanisms.
ELECTROSTATIC DISCHARGE DEVICE: The
probes for RF Capacitance / Admittance systems
when used for level detection in dry insulating
material with low dielectric constants have a
tendency to store a static electric charge that
is created by friction due to material movement.
The sensing element being insulated from ground
acts as an efficient capacitor that stores the
charge that can build up to several Volts. This
can cause permanent damage to the electronics
connected to the probe. To avoid this from
happening, a static discharge device is used to
limit the build up on probe to a safe limit.
GALVANIC ISOLATION: Analogue 4 to 20 ma
output from continuous indicators (level
transmitters) are connected to PLC or to
recorders and indicators. Normally one end of
the PLC is connected to plant grounded. The
internal power supply of the PLC is used to
source the 4 to 20 ma loop of various types of
transmitters such as RTDs, Thermistors etc. that
do not contain internal power sources. Wherever
separate sources are used in a transmitter there
is every possibility that the two supplies may
interact causing damage or wrong indication due
to aiding or opposition of the two sources. The
Galvanic isolation separates the two grounds and
so avoids the unwanted interaction.
LOOP RESISTANCE: In 4 to 20 ma current
output there is a limit to the maximum value of
the loop resistance, which amongst other
factors, depends upon the source voltage.
Normally the maximum limit ranges from 400 to
1000 Ohms. There is no lower limit. It could be
0 Ohms too.
LEVEL TRANSMITTER: It is a device that
converts physical level change into an
electrical current say 4 to 20 ma. Normally a
Span and Zero adjustment are provided for
calibration.
PULSE FREQUENCY MODULATION (PFM): Level
data from the probe and electronic insert is
sent as frequency-modulated pulses, which
have the advantage of noise immune long distance
transmission.
RF TRANSMISSION: In RF Admittance type
level switches utilizing driven shield technique
to eliminate the cable influence and coating
problems, Radio Frequency is directly
transmitted to the probe. No electronic insert
is required in the probe head. This method
however uses a special screened cable with a
drain wire and a special 3-electrode probe. The
transmission distance is limited to about 15
mtrs. for the Dot display model and about 50
mtrs. for the Bar graph display model.
DC VOLTAGE TRANSMISSION: In some of the
older types of level switches a 3 to 12 V DC
transmission is used. The distance limitation
with these types depends on the DC resistance of
the cable used. Normally a cable with resistance
per core not exceeding 25 Ohms does not cause
appreciable loss of signal. As the lower voltage
is lifted to 3 V, noise amplitudes below 3 V do
not affect the signal.
AC TRANSMISSION: In Vibrating Fork
instruments, transmission between Fork and
evaluation unit is by AC pulses at the resonant
frequency of the Fork. Distance between the Fork
and the Evaluation unit is limited to around 10
mtrs. maximum, and that too by means of special
independently shielded screened cables. |
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What is the
architecture of an invasive probe in general? |
 The
architecture of an extended ground capacitance
type probe is shown in the fig. It consists of a
cast Aluminum probe head containing an
electronic insert, a cable entry, two neoprene
seals, one for the cover and one for the bottom,
a mounting bush with appropriate threads, an
extended ground arrangement using a pipe, Teflon
part insulation and an SS sense element. The
probe head is of weatherproof construction
suitable for outdoor installation. The mounting
bush is SS or MS plated with BSP or NPT threads
for screwing into a nozzle that is welded on to
the vessel. For flanged mounting a slip on
flange having appropriate threads can be screwed
on to the mounting bush before installation. The
extended ground portion is inactive and must
have a minimum length equal to the nozzle
length. The insulation separating the sense
element from the ground can be either of the
part or full type. The one shown in the Fig.1 is
part, generally suitable for non-conducting
service material. If the material is conductive
then full insulation is necessary. PTFE (Teflon)
does not react with a wide spectrum of material
It can stand wide variations of temperature and
its non-sticky character make it ideal for use
in general purpose standard probes. The sensing
element is normally made of a material that does
not react with the service material. SS being a
material that does not react with a wide variety
of service materials is ideal for use in general
purpose
standard probes. |
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What is meant by
weatherproof construction and what are ingress
protection standards? |
| Instruments meant for outdoor
installations have to face the harsh outdoor
environment. They need to have a weatherproof
construction. Certain standards have been
defined for weather proofness. They are termed
as IP followed by a number. Egs. IP20, IP40,
IP55, IP65, IP67. IP means ingress protection.
The standards define the smallest particle size
that should not enter the enclosure under
specified conditions. The details of test
conditions and severity are mentioned in ISI
specification literature. Independent
laboratories authorized for environmental
testing issue certificates signifying up to what
level of material ingression the enclosure can
withstand. |
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What is meant by a
flameproof enclosure? How are flameproof
standards defined? |
| An enclosure is supposed to be
flame proof, if, after placing it in an
environment of specified air gas mixture, a
spark created to ignite the air gas mixture
within the enclosure, does not cause the flame
to emerge and ignite the air gas mixture outside
the enclosure. The main criteria in the design
of such enclosures are to provide sufficient
flame path and maintain the specified air gap
between the mating surfaces, thus avoiding the
emergence of the flame from within the
enclosure. Also the material of construction and
the enclosure wall thickness is such that it can
sustain the internal detonation pressure in the
event of its occurrence. The withstanding
pressure depends upon the internal volume of the
enclosure. During manufacture each enclosure is
hydraulically tested to ensure its ability to
withstand such pressures. The gasses encountered
in Industrial applications are categorized into
groups I, IIA, IIB and IIC. Details of gasses
contained in the groups can be obtained from IS
specification for flameproof enclosures of
electrical apparatus (IS:2148).
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What is termed as a
hazardous zone in a plant? |
| The area in which inflammable
gas or vapors are present is called a hazardous
zone. Hazardous zones are categorized as Zone0,
Zone1, and Zone2 depending upon the severity of
the hazard. Zone0 refers to continuous hazard.
Zone1 to intermittent hazard under working
conditions and zone2 refers to hazard under
fault condition only. Those areas that do not
fall under any of these categories are referred
to as non-hazardous. Certain norms have to be
followed in the choice and installation
procedures of instruments meant for the various
zones. Recommendations can be found in the
relevant standard specifications. |
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What is intrinsic
safety? |
| Equipment that will not cause
fire hazard even under defined fault conditions
within the equipment is called intrinsically
safe. Basically the energy input to such
equipment is restricted to a low value so that
under the defined fault conditions there is no
likelihood of the occurrence of a spark. |
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Methods, intrinsic
safety and the use of flameproof enclosure
ensure safety from fire hazard. How do they
compare? |
| Intrinsically safe instruments
do not need expensive enclosures and are cheaper
as compared to flameproof systems. It is
possible to calibrate intrinsically safe
instruments even in hazardous areas since there
is no restriction in opening the enclosure and
doing the settings. They are often the first
choice if available. The design of intrinsically
safe circuits depends upon the availability of
low power devices and circuit design skills. It
may not be possible to restrict power to the
circuits in all the cases. Flameproof systems
use the standard circuits and neither depends on
special low power devices, nor it is necessary
to restrict the power to the instrument. Their
operation and performance is not hampered due to
restriction of power input, as in the case of
intrinsically safe units. The flame proof
housing design is however bulky and costly and
the instrument cover cannot be opened in the
hazardous area for wiring, calibration, or
maintenance when the plant is in operation. |
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How does a level
switch based on the conductivity principle
function? |
| The level switch can only be
used to detect the level of conductive liquids
like water whose conductivity is more than the
specified value. For standard applications this
should greater than 25 micro siemens. A part
insulated conductivity probe is installed on the
vessel. The length of the probe is defined by
the point level at which the alarm is desired.
The sense electrode is connected to a low
voltage AC supply derived from the evaluation
unit (about 12 V). For metallic tanks, the tank
body is connected to ground, which in turn
connects to the low voltage circuit ground
through a suitable resistance. The voltage
across this resistance is monitored by the
circuitry in the evaluation unit. When the level
is below the tip of the probe, there is no flow
of electric current and therefore no voltage
across the resistance. When the liquid covers
the probe tip there is a flow of current via the
conductive liquid and the resistance. The
voltage across the series resistance is used to
switch a relay after due amplification and
signal processing. The output contacts of the
relay can be used for alarm indication or
control.
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Why is low AC
supply is used on the conductivity probe? Why
not DC? |
| Firstly the use of a low voltage
supply on the probe reduces the risk of a shock
hazard. Secondly AC on probe prevents
deterioration of the sense electrode due to
electrolysis. |
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Can the
conductivity type level switch be used to detect
level in an insulated vessel like a PVC tank? |
| Yes it can be used. It is
however necessary to install an extra probe
whose length is more than the sense element so
that it always remains dipped in the liquid and
serves as a ground electrode. A current can now
flow between the two probes via the conductive
liquid when both the probes are dipped in the
liquid. |
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Can the
conductivity type level switch be used to detect
the level of slurries? |
| Yes it can be used. Top mounting
arrangement is preferred for conductive slurries
that have a tendency to coat. PTFE part
insulation with SS sensing element is generally
used. If the coating dries up and becomes
non-conductive then the probe tip needs
cleaning. |
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Can the
conductivity level switch be used to detect the
level of industrial waste containing traces of
oil? |
| No. As far as possible the use
of conductivity type level switch should be
avoided for such applications. The oil can coat
the electrode tip and insulate it thus avoiding
detection of the level. A better solution is a
capacitance type or a vibration type system. |
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How is capacitance
principle used to measure level? |
A storage vessel is made to
behave like an electrical capacitor by
installing an insulated electrode on it. The
sense electrode and the tank wall form the two
electrodes of the capacitor with the material as
the dielectric. The capacitance ‘C’ of a
parallel plate capacitor can be calculated by
the following formula:
Where ‘C’ is the capacitance in Farads,
‘K’ is the dielectric constant, ‘A’
is the electrode area and ‘D’ is the
distance between the electrodes. After
installation of the probe, area of electrode and
the distance between them remain constant. With
a change in level the dielectric varies from air
to that of the material. The dielectric constant
for air is 1 whereas that of other materials is
always more than 1. A change in the level
therefore causes a change in the value of the
capacitance. The measurement of the capacitance
therefore affords a method of measuring the
level indirectly.
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How is the value of
vessel capacitance measured? |
There are various methods by
which the value of the capacitor can be
measured. The suitability of the method depends
on a number of factors such as; range of
capacitance values to be measured, end purpose
of measurement, (that is limit switching or
continuous indication), type of output required
(digital or analog) etc. Normally for level
measurement capacitance values range from 30 to
1200 Pf for switching applications whereas 30 to
5000 Pf for continuous measurement. Following
methods are popular: -
- Probe capacitance forms one arm of a
bridge circuit having a radio frequency (RF)
source. Frequencies used are between 100 KHz
and 1Mhz.
- Probe capacitance forms an element of
the frequency determining circuit in an
Oscillator.
- Probe capacitance forms a
feedback-controlling element in a critically
tuned Oscillator.
- Probe capacitance forms the charging
element in a constant current charging
Circuit.
- Probe capacitance forms an element that
shifts the phase in relation with a
Reference waveform.
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What are the
limitations of using a capacitive level
measuring system? |
Capacitive level measuring
systems work satisfactorily where the electrical
characteristics of the service material do not
vary with time. As observed from the above
formula, once the probe is installed the
distance between the electrodes and the area of
the electrode remain stable. What can vary with
time is the dielectric constant of the material.
Dielectric changes can occur due to changing
moisture content, changing geometric
configuration in the case of lumpy material,
temperature variations in certain substances
prone to change with temperature, blended
substances where the ratio of ingredients
determining the mix, change. It can also change
due to incorrect probe insulation that varies
with temperature or has a tendency to absorb
moisture. It can vary as a result of build up of
static charge on the probe or due to floating
particles carrying a static charge in the
vicinity of the probe. Other conditions
responsible for a change in the probe
capacitance with time could be due to change in
the environmental condition inside the vessel
such as fluidization. The physical
characteristics of the material such as flow
characteristics, stickiness, coating and build
up tendency, hygroscopicity and viscosity can
also affect the reliability of measurement.
Capacitive level measurement systems have a very
broad spectrum of applications. They can afford
a cheaper and reliable solution to many
applications provided the various factors
mentioned above are given due consideration. |
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Can the
limitations be overcome by innovative designs? |
Yes, if not all but some of the
limits have been extended by the use of
innovative designs utilizing the latest
developments in the technology.
- Utilizing a 3-electrode probe structure,
incorporating an active shield, a coat
immunizing circuitry, and the admittance
principle of measurement, has minimized
material coating and build up problems.
- A discharge device connected between the
sense electrode and ground protects the
circuit components from failure due to high
voltage static charge accumulation on the
sense electrode.
- A special probe comprising a separate
electrode that is permanently dipped in the
liquid serves as a measurement device that
continuously monitors the change in the
dielectric constant of the liquid. This
signal is used to compensate for the
dielectric changes of the material. The
system therefore needs no re-calibration.
- Microprocessor based systems
incorporating intelligent digital
communication between the electronic insert
in the probe head and the evaluation unit
improve the signal quality to the evaluation
unit even at fairly large distances. The
interconnection is by 2-wire cable that is
used for powering the sensor electronics as
well as for signal transmission. The cost of
the installation is thus reduced. The
self-checking feature and signal validation
method increase the reliability further.
- For high temperature operation, probes
are designed with either PTFE or ceramic
insulation with either cooling fins or stand
offs that keeps the sensor electronics
within its operating limits.
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Is there any other
way to reduce the dependence of level
measurement on changing electrical
characteristics of the material? |
Yes, if we use a principle that
is not affected by the electrical
characteristics of the material.
Electromechanical devices are not affected by
electrical characteristics of the service
material and the ingenious use of such devices
could afford solutions difficult to solve by
capacitive systems.
The vibrating fork and vibrating rod are
reliable solutions for free flowing powders and
granular solids. The rotating paddle has a wider
spectrum as it can tolerate lumpy and sticky
solids and slurries as well. These instruments
are relatively simple and easy to install and
commission, as they need no elaborate
calibration procedure. A specially designed
vibrating fork is available for liquids, which
is more reliable than floats.
Electromechanical level measuring devices using
the plumbing method find use for continuous
level indication of material in deep Silos that
are 25 to 50 mtrs. tall. The method has good
accuracy and is reliable in dusty environment as
in cement storage and raw material Silo and in
Coal Bunkers. A lighter version of the device is
also suitable in plastic chip Silo where other
methods are not so reliable due to static charge
hazard.
There are tape and float methods utilizing the
follower or servo balancing principle that are
accurate and reliable for petrochemical
applications. Also instruments based on
magnetostrictive principle utilizing a magnet in
the float are very accurate and reliable for
detecting the level of liquids in shallow as
well as fairly deep tanks. |
|
What are
non-invasive methods of level measurements?
Where are they used? |
| In applications where the
contact with the material is prohibitive either
due to environmental conditions outside the
vessel or inside the vessel the non-invasive
methods are required. Basically energy is beamed
at the target material and the reflected wave is
detected. The time of flight is measured. If the
velocity of the wave is known the distance can
be calculated. Energy in different forms that
may suit any particular application can be used.
This gives rise to a number of types of level
measuring instruments based on various forms of
energies. There are instruments based on
transmission of pulse burst of sonic and
ultrasonic energy and the receipt of its echo.
There are instruments based on electromagnetic
energy using the microwave Radar principle.
There are instruments using coherent light based
on the Laser principle. There are instruments
based on Nuclear energy. Each of the methods has
their advantages and disadvantages. A judicious
choice should be made after giving due
consideration to the particular application.
|
