Level Switches 34

8/3/2019 Level Switches 34

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Level is measured at the position of the

interface between phases, where the phases

are liquid/gas, solid/gas, or immiscibleliquid/liquid and after reaching certain

level,switch turns the equipment ON & OFF.

Level is simply a measure of height.


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Direct

Indirect


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Direct methods employ physical

properties such as fluid motion and

buoyancy, as well as optical, thermal,

and electrical properties. Direct level

measurement does not require

compensation for changes in level

caused by changes in temperature.

Direct level measurements show the

actual level of the interface.


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Indirect level measurement involves

converting measurements of some other

quantity, such as pressure to level by

determining how much pressure is

exerted over a given area at a specific

measuring point, the height of thesubstance above that measuring point

can also be determined.


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A dipstick used to measure the amount of fuel

remaining in a tank. Dipsticks can also be usedto measure the quantity of liquid in an otherwiseinaccessible space, by inserting and removingthe stick and then checking the extent of itcovered by the liquid.


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Rotating paddle level sensorsare a very old and establishedtechnique for bulk solid pointlevel indication. The techniquerequires a low speed gear motorthat rotates a paddle wheel.When the paddle is stalled by

solid materials, the motor isrotated on its shaft by its owntorque until a flange mounted onthe motor contacts a mechanical

switch


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A sight glass or water gauge is atransparent tube through which the operatorof a tank or boiler can observe the level ofliquid contained within. Simple sight glasses

may be just a plastic or glass tubeconnected to the bottom of the tank at oneend and the top of the tank at the other. Thelevel of liquid in the sight glass will be the

same as the level of liquid in the tank.Today, however, sophisticated floatswitches have replaced sight glasses inmany such applications.


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It is used to measure liquidlevels in the tank in which a floatrests on the surface of liquid andfollows the changing level of

liquid.


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Ultrasonic transmitters work on theprinciple of sending a sound wave from apeizo electric transducer to the contents of

the vessel. The device measures the lengthof time it takes for the reflected sound waveto return to the transducer. A successfulmeasurement depends on reflection fromthe process material in a straight line backto the transducer


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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 zonealong the length of the base strip, and a

resistance wire or ribbon helically wound

around the insulated base strip, with thehelical turns bridging the insulated edge

portions being spaced from the underlying

un-insulated zone of the base strip.


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Hook- Type level Indicator consists of a wire of

corrosion resistance alloy bent into U-shaped withone 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.


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Displacer level detector workson the Archimedes Principlewhich states, A body whollyor partially immersed in fluid isbuoyed up a force which isequal to weight of the fluiddisplaced. By detecting the

apparent weight of immerseddisplacer the level of the fluidis measured


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High accuracy

Reliable for clean liquids

Can be mounted internally or externally

Externally mounted can be disconnect

from main unit for maintenance

Adoptable to liquid interfacemeasurement


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Key questions to ask before selecting a level measurementsensor:

1. Are you measuring a liquid or solid?

2. What is the applications temperature and pressure

ranges?

3. Is point level or continuous measurement required?

4. What level measurement range do you need?


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5. Is the measured material electricallyconductive?

6. Will the material coat or build up on surfaces?

7. Does turbulence, foam, or vapor occur at the

surface of the liquid?

8. Will you need contact or non-contact levelmeasurement?

9. What kind of output do you needanalog,relay, digital display, etc.?


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Switches

A switch is an electrical deviceusually electromechanical used tocontrol continuity between two

points


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Types of SwitchesHand switches: Hand Switches are

actuated by human touch.Process Switches: Process Switches areactuated by changes in some physical

process(temperature ,level , flow etc.)Limit Swiches:Limit Switches are actuatedby machine motion.


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Liquid Level SwitchesA floating object can be used to actuate aswitch mechanism when the liquid level in

an tank rises past a certain point.


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Nuclear Level SwitchesBoth source and detector are outside

of the vessel, with no intrusion at allexcept the radiation flux itself. Theradioactive sources used are fairly

weak and pose no immediate healththreat to operations or maintenancepersonnel.


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Float Level SwitchesAll float operated liquid level controls operate

on the basic buoyancy principle which states"the buoyancy force action on an object isequal to the mass of liquid displaced by theobject."


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Difference Between

Sensor,Switch & transducerSensor:Tell us the value

Switch: Turns something ON or OFF

Transducer: A sensor that can send asignal


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The many level technologies available in the market place

today all work. If they did not work, companies would not

be able to stay in business manufacturing them, and other

companies would not be buying them. The important thing

to remember is that all technologies work when applied

properly. Special consideration must always be given to the

principle of operation, and where the limitations of each

technology lie. The correct selection of a technology and

then the subsequent correct application of the technology

will make the level measurement a successful one.

Level Switches 34

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