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Level sensor

Contents- Introduction.

- awad

-Types of level measurements.

- Types of level Sensor .

- Conclusion .

- References.

Introduction

The process tanks are widely used for many purposes such

as mixing processes and Interim storage of working fluid, as

well as treatment

and avoiding instaility errors of flowing

fluid.

so

The controlling of the tank level is an important function in

these applications.

!irstly that we need an e"uipment to represent the li"uid level in

the control system These e"uipment are called sensor


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In the language word sensor means # device which is used to

recordsomething that has changes with time ".

In industrial side sensor means a device used to convertphysicalehavior or phenomena to electrical signal

level sensor

It is a sensor used to convert the level of fluid in tank or

reservoir toelectrical signal $analogue signal or digital signal%

Types of level measurements.

i)- open tank measurement .

it is the level measurement when the tank without roof$open

to atmospheric%.

&m' (m ) S.* $inch water column %

+here &m maximum pressure. (m maximum head. S.* li"uid specificgravity

ii)- close tank measurement.

it is the measurement when the tank has roof , in this cause there

aretwo pressures , one caused y weight of fluid and the other

form the gas etween the fluid service and roof.

&m'&gas (m ) S.*

$in w.c%+here

&m maximum pressure. (m maximum head. S.* li"uid specific

gravity&gas, gas pressure

iii)-open tank with sealed uid.

In this cause the level sensor is supplied with seal fluid that its

level isproportional to the level of the li"uid .


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Span'$ &.seal - &.li"min% - $&.seal - &.li"max %

P.liq

' (m ) S.*$li"uid%

P.seal ' (m ) S.*$seal%

+here

(m max li"uid level. (.seal max seal level.

P.liqma! max pressure of li"uid . &.seal max pressure of seal.

Types of Level sensor

- !loat operated type.

- Solid proe type.

- /ltrasonic type.

- 0icrowave radar type.

- 1ydrostatic type.


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- "loat operated level sensor type#-There are two types of float level sensor

2%- float switch level sensor $ digital work principle # low or full 3%

This kind is used for alarming applications which gives twosignals, signal that indicates low level $ switch is off % when levelis falling and signal indicates high level $ switch is on %when levelis rising y assuming that the switch is normally op

4%- continues float level sensor $analogue work principle %

This type measure the level to determine the exact amount ofsustance in a continuous manner

which gives the level of fluid in every state y converting thedisplacement of floating part to e"uivalent electrical signal .

This type consist of floating part attached to metal rod and thereis variale resistor which its kno is mounted on this rod.

5s the floating part displaced the position of the resistor kno willchange which in turn change the resistor value.

This change is sensed y proper circuit such as whetstone


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ridge and then translated to electrical signal .

R= K*L

+here

R - resistance.

(- length of displacement. 6 - caliration constant.

5dvantages -

1as simple construction which easy to maintain

(ong life stand

7isadvantages ,-

The sensor must e has length of a tank $the sensor and the tank

are the same inlength %, so its used in small tanks.


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- $olid pro%e level sensortype&capacitive type)#The principle of capacitive level measurement is

ased on change ofcapacitance.5n insulated electrode acts as one plate ofcapacitor and the tankwall $or reference electrode ina non-metallic vessel% acts as the other plate thecapacitance depends on the fluid level so theempty tank has a lower capacitance while a filled tankhas a higher capacitance.

5 simple capacitor consists of two electrode plateseparated y asmall thickness of an insulator suchas solid, li"uid, gas, or vacuum. This insulator is alsocalled as dielectric.8alue of capacitance depends on dielectric which isused, alsodepends on the area of the plate anddistance etween the plates.+hen electrical signal is applied etween theconductive proe andthe vessel wall.The signal caused very low current flow through thedielectric $process material in the tank %

+hen the level in the tank drops, the dielectricconstant drops causing a drop in the capacitancereading and then drops in current flow.the capacitance is $from the proe to the vessel wall%

This change of level is detected y internal circuit andtranslated into electrical signal .


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C=KA /D

'here #

C- capacitance .

(-area of plat and the wall of tank.

-permittivity of air*-dielectric constant (-area of plats

(dvanta+es #-

-,as hi+h accuracy ran+e.

-Can %e used in uids of hi+h viscosity

isadvanta+es #--dicult to maintains

-e!pensive

-the uid must %e in steady sate


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- /ltrasonic level sensor type

ultrasonic level sensors consists of a transmitted ,receiver,gate controlunit, clock pulse generator, counter down,timing and control unit.

To measure the level with an ultrasonic sensor the moduleshould emounted aove the tank.

+hen the ultrasonic signal is transmitted ,the moduleoutputs a pulse.

+hen the echo is received another pulse is generatedusing an external counter you can measure the timeetween two pulses and then the distance of the li"uidthrough this e"uation.


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D= 0.5*V * t

+here9

7'distance of the level

8'velocity of sound ::2.; m


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- /icrowave radar level sensor type#Radar level measurement is ased on the principle of

measuring thetime re"uired for the microwave pulse and its

reflected echo to makea complete return trip etween thenon-contacting sensor and thesensed material level.

Then internal circuit converts this plus signal to electrical

analoguesignal which will indicates the level of fluid .

Radar level transmitters range etween ;.= *1> and 4? *1>

D=c*N/2*f

'here

C01 @2345 &li+ht speed) 6 70num%er ofrepeats

8f0 the di9erence %etween two frequencies


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5dvantages of this type is the same as ultra sonic type

7isadvantages

-Axpensive

-Bot accurate if the li"uid is not steady state .

-,ydrostatic level sensortype#

"or many years hydrostatic pressure

measurement has %een the most importantmeasurin+ principle in continuous levelmeasurement.

'hat is meant %y hydrostatics:

,ydrostatic is the wei+ht &force) of uid actin+

downwards with earth +ravity on speci;c area

'here it means pressure


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This pressure is direct proportional to theactin+ force &wei+ht of uid )and indirectwith the area of the tank

P0"


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So as the height increase the hydrostatic pressure increase Incase of closed tank the hydrostatic pressure is

&4' .g.h -&2

p4 ' hydrostatic pressure .

p2' pressure of the enclosed gas in the tank .

' density of the fluid .

g ' gravity $ earth acceleration % .

h ' height of the li"uid column.

-Types of hydrostatic pressure level sensors-

pressure sensor in the hydrostatic to main types

-e!ternal hydrostatic pressure sensor.

-internal hydrostatic pressure sensor.


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Classification y structure

2%- ON/OFF level sensor t!e#

in this type we can o%tain a simple di+ital si+nalwith two level conditions as ma!imum level or

minimum level for openin+ and closin+ valve or

for alarmin+ indication.

$ensor response to the pressure variation %y the linear

mechanical sprin+

The mathematical e!pression of sprin+

F=K x

+here

! - The applied force on the piston

D- the displacement of piston 6- spring constant

4%- "o#$l%t&n' (#rost%t&c !ress$re level sensor)

in this type we can otain an analog signal expressesInstantaneous value of tank li"uid level using varialeimpedance of the sensitive memer which responses thehydrostatic pressure y variation of flexile resistingdiaphragm .


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constr$ct&on )

a%- diaphragm

%-pressure sensor

$strain gauge and +heatstone ridge %.

c%-amplifier

d%-8 to I converter

e%- the ody $ cover %

Concl$s&on )


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(evel sensor It is a sensor used to convert the level of fluid intank or reservoir to electrical signal

There are many types of level sensors which its ideal of

operation depending on different principles ut the mostcommon and the est types are

/ltrasonic ecause it is modern and accurate

1ydrostatic level sensor type ecause it can e used in manyapplications and ecause is has long using life .

(evel sensors give us the aility to know how much the fluidlevel in the tank to let the storage processing under control.

Ey using level sensor$on


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