Ch 2_type of Measurement in Industrial Application J5800

8/3/2019 Ch 2_type of Measurement in Industrial Application J5800

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This topic introduces to the types ofmeasurement which includes fluidmeasurement, fluid level measurement, fluidflow measurement and temperaturemeasurement using given examples.

TYPE OF MEASUREMENT IN

INDUSTRIAL APPLICATION


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FLUID PRESSURE MEASUREMENT


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BOURDON TUBE PRESSURE GAUGE

The Bourdon tube pressure gauge is probably themost popular pressure sensor.

Basic Bourdon tubes are made from metal alloys such

as stainless steel or brass.They consist of a tube of elliptical or oval cross

section, sealed at one end.

There are various shapes of bourdon tube, includinghelical, spiral and twisted.

A common design is the C-shape as shown in figure.


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Principle operation..

Here the tube is at atmospheric pressure. Whenincreased pressure is applied to the open end, itdeflects outwards (tries to straighten) in proportionto the pressure inside the tube (the outside of the tube

remains at atmospheric pressure).

As the pressure decreased, the tube starts to return toits atmospheric pressure position.

Compared to other elastic pressure sensors thedeflection produced by Bourdon tubes is large.


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The bourdon tubepressure gauge, consistof a bourdon tubeconnected to a pointer.

The pointer moves overa calibrated scale.

When pressure isapplied, the movement

of the tube is fairlysmall, so to increase themovement of thepointer it is

mechanically amplified.


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Some designs of bourdon tube pressure gauge tend tobe fairly inexpensive because they are mass produced.

They are suitable for use with both liquids and gases,

are used in a wide variety of applications, bothindustrial and domestic.

Applications range from type pressure gauges,measuring the pressure in pneumatically controlled

tools and machines, to pipeline pressure in chemicalplants.


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BELLOWS

Bellows are differential pressuresensing devices mainly used in lowpressure range of about 0 to 1000Pascal.

Figure shows a set of metallicbellows, held in inside a protective

casing.

The bellow are made of a thincopper alloy tube pressed into acorrugated shape.

This is sealed at one end, with asmall hole at the other end.

When pressure is applied via thehole, the bellows expand a distance.

This displacement can be calibrated

in terms of pressure.


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Figure shows a single chamber bellows. It consists oftwo circular metal diaphragms connected back to

back.

These devices, commonly referred to as capsules, givesmaller displacements than multi-chamber bellows.


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Bellow usually produce small displacement. Theseneed to be amplified, and displacement sensors suchas LVDTs and potentiometers are often used toconvert the displacement into an electrical signal.


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Bellows should not be used in an environment wherethey may be subjected to vibration or shock.

Their accuracy is also affected by temperature

changes.An example of the use of bellows is in control

applications, for closing valves in a pipe when criticalpressured is reached.


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LIQUID LEVEL MEASUREMENT


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There are 3 type of direct mechanical method of liquidlevel measurement using:

Sight glass

Dipstick Float operated gauges


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SIGHT GLASS

The sight glass shown is a simple and inexpensivemethod of measuring the level of a liquid in acontainer.

it is similar in principle to the measuring cylinder, butallows opaque materials to be used for the tankconstruction and hence stronger or less expensivedesigns.


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The precision and accuracy of the result depends onthe skill of the reader, and how accurate and precisethe scale gradations are.

The type of liquid in the tank and the diameter of thesight glass will also affect the accuracy and precisionof this method.

Hence sight glasses may be used in applications where

high precision and accuracy are not vital, such as in oilstorage tanks, or domestics kettles.


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DIPSTICK

The dipstick is a simpleand inexpensive (if notprecise) method ofdetermining liquid

level.The dipstick consists of

a thin rod withgraduated scale, which

in normal use isinserted vertically into atank so that its lowerend is submerged below

the level of the liquid.


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This measuring instrument is essentially the same asthe calibration scale on the side of the measuring

vessel.

Instead of being permanently attached to the side ofthe container, the scale is on a removable probe, orstick.

Such as dipsticks used in petrol tankers, the scale is

calibrated in volume to show the amount delivered.In the case of the dipsticks used in car engine, the

scale usually has only two marks MAX & MIN.


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FLOAT OPERATED GAUGES

There are 2 common forms of float operated gauge

The counterweight

The electrical version


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The counterweight operated float gauge

The movement of the float follows the changing liquidlevel and this is relayed to the pointer. The scale can

be calibrated in volume or mass.

Accurate readings can be obtained but this dependsupon scale length and the extent of scale graduations.


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Electrically operated float gauge

The float is designed to follow the level of the liquid.When the level changes, the movement of the floatproduces an angular movement of the slider along thepotentiometer.

This alters the potential difference, producing avoltage reading that is directly related to the level ofthe liquid.


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The scale of the voltmeter can be calibrated in volume,mass, or height.

Because the level signal is electrical, it may be

conditioned for remote recording or display, or usedas a feedback signal in a control system.


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There are 2 type of indirect liquid level measurementusing:

Pressure sensor

Pressure differential manometer


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Pressure sensors

In certain cases where weight or level measurementsare taken and an electrical output signal is required,pressure sensors may be used.

Various types may be used, but a typical examplewould be piezoresistive.

In a pressure sensor, pressure is used to deflect adiaphragm.

The diaphragm deflection generates an electricalsignal, via a device such as a resistance strain gauge,giving a signal corresponding to pressure.


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Pressure sensors are located in a container in whichthe height or depth of the liquid is to be measured.

Any change in the height or depth of the liquidproduces a proportional change in the sensor output.

Referring to figure, pressure P is:


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Pressure differential manometer

With a tank of liquid open to the atmosphere, thepressure difference can be measured between a pointnear the base of the tank and the atmosphere.

The result is then proportional to the height of liquidabove the pressure measurement point.


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With closed tank, the pressure difference has to bemeasured between a point near the bottom of the tankand in the gases above the liquid surface.

The pressure gauges used for such measurementstend to be diaphragm instruments.


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There are 2 types of the liquid level measurementusing electrical method:

Capacitance

Nucleonic gauge


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Capacitance probes

Because of theirinherently safeoperation, capacitanceprobes (sometimes

called capacitancegauges) are regularlyused to measure fuelquantity in aeroplanes.

The sensor comprises 2cylindrical tubes, oneinside the other, whichform a capacitor in the

fuel tank as shown in


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If the tank is filled with fuel, as in figure, thecapacitance will change since fuel has a higherdielectric value than air.


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If the tank is partly filled as in figure, the capacitancewill change proportionally with the level of fuel in thetank.


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As the fuel rises and falls then the added value ofcapacitance due to fuel, also rises and falls.

Any change in capacitance can be used to produce a

change in electrical output, which can then be used tooperated a fuel level or quantity indicator, or be linkedto a control device.

Because of the presence of the fuel, the design of the

capacitance probe needs to take into account criteriasuch as corrosion and leakage.


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Nucleonic gauge

One form of level indicator uses gamma radiationfrom a radioactive source, generally cobalt-60,casium-137 or radium-226.

A detector is placed on one side of the container andthe source on the other.

The intensity of the radiation depends on the amountof liquid between the source and detector and can be

used to determine the level of the liquid.


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Figure shows 2 possible arrangements.

With a compact source and extended detector, levelchanges over the length of the detector can be

determined.A compact source and a compact detector can be used

where small changes in a small range of level are to bedetected.


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Such methods can be used for liquids, slurries andsolids, and, since no elements of the system are in theliquid, for corrosive and high temperature liquids.


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FLUID FLOW MEASUREMENT


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The types of fluid measurement are:

1. Variable area flow meter

2. Positive displacement flow meter

3. Electromagnetic flow meter

4. Ultrasonic flow meter


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VARIABLE AREA FLOW METER(ROTAMETER)

A typical variable flowmeter is shown infigure. It consists of aclear, tapered tube

containing a float.It mounts vertically in

the pipeline carryingthe fluid.

Fluid flows through thegap between the floatand the tube and thiscreates a pressuredifference which forces


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To keep the float in the centre of the tube, the floatmay have small vanes so it rotates, or have a fixed

vertical guide rod running through the centre of it.

Used for measuring both liquid and gas flows.Need little recalibration or maintenance, but their

accuracy can be considerably affected by changes intemperature of the fluid they are measuring.


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Orifice plate flow meter

An orifice meter, asshown in figure,typically consists of adisc with a hole in thecentre which insertedinto the pipe.

The fluid emerges fromthe orifice plate as a

convergent streamwhich is projected intothe pipe.

The pressure difference

is measured between a


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POSITIVE DISPLACEMENT FLOW METER

The rotating lobe meter is a type of positivedisplacement meter.

The lobe positioned within the chamber at the right-

angles, will rotate synchronously when fluid flow.Each lobe will trap a set amount of liquid during part

of its rotation.


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For each complete rotation of the lobes a knownquantity of fluid passes through the chamber. At theend of each rotation a counter is incremented and,

because the quantity that passes through the chamber

is known, a measure of the total quantity deliveredcan be determined.


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Accurate results can be achieved using a rotating lobemeter, and they may be used, for example, in meteringfuel-oil deliveries to consumer.

If provided with power, the rotating lobe meter canalso work as a pump which delivers fluid and providesa flow reading at the same time.

These are used where accurate control of flow is

needed, for example, a dosing pump in a chemicaltreatment plant.


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Electromagnetic flow meter

A typical electromagnetic flow meter is shown infigure.

Two coils, connected in series, are mounted either

side of the pipe at right angles to the direction of flow.When the coils are energized by a current a small

magnetic field is created across the fluid.


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Two conductance probes are mounted to be in directelectrical contact with the fluid.

There are positioned at right angles to the fluid flow

and the magnetic field. These probe sense the e.m.finduced and this signal conditioned so it can be usedby external displays and recording instruments.


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AdvantagesdisadvantagesDo not disrupt the flow

Not affected by fluidscontaining suspended

matter (liquid cementor food pulp)

Wide applications fromthe flow measurement

of liquid metals toblood flow ratemonitoring.

Because of the weakmagnetic field themagnitude of theinduced e.m.f. is

extremely small, andthis may lead toproblem with Electricalnoise and Significant

errors in themeasurement.


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Ultrasonic flow meter

Figure shows a typical ultrasonic flow meter clamedonto a pipe.

The ultrasonic flow meter comprises an ultrasonic

transmitter, a reflector and a separate receivermounted a set distance away along the pipe.


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The transmitter produces a sound pulse, whichrebounds off the reflector and finally reaches thereceiver.

If the flow is zero, the time it takes from the pulsebeing transmitted to reaching the receiver iscontrolled only by the distance between thetransmitter and the receiver and the speed of sound inthe fluid.

If the fluid is flowing from the transmitter towards thereceiver then the sound pulse will travel faster.


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AdvantagesDisadvantagesHighly accurate andstable.

Can be used with many

liquids, conducting andnon-conducting

Can measure flowcontinuously in both

directionsDo not disrupt the flow

portable

Expensive

Particles of suspendedmatter in the liquid are

required for eitherultrasonic technique towork successfully

Cannot be used with

gases.


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THERMOCOUPLE

RESISTANCE THERMOMETER

THERMISTOR

TEMPERATURE MEASUREMENT


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THEMOCOUPLE

Has two dissimilar metals joined to form a closed-circuit.

A probe or protective sheath holts one junction. Thisis placed in substance whose temperature is to bemeasured.


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If one junction is kept at a different temperature fromthe other then a current will flow.

The magnitude and direction of the current dependsupon which metals are used and the temperature ofthe junctions.

The size of the resultant e.m.f. is small, usually in theorder of milivolts. A voltmeter at the cold end of the

circuit provides an output to the user, usuallycalibrated in temperature.


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Hot junction and cold junction

When a pair of different metals are formed in a loopwith two junctions at different temperatures, a currentwill flow whose value is related to the temperature.This is calledSeebeck effect .


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Metal X and Y are dissimilar, and junction 1 andjunction 2 are at different temperatures, T1 and T2.because of the seebeck effect, small e.m.f.s aregenerated across the junctions. The algebraic sum of 2

e.m.f.s causes a current to flow.


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If both of junction are at the same temperature, noe.m.f will be generated.

If the temperature of one junction starts to change,and not the other, an e.m.f will be generated and getlarger as the temperature difference between the 2

junction increases.

This is the basis of the thermocouple!


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The value of this e.m.f. E depends on the two metalsconcerned and the temperatures tof both junctions.Usually one junction is held at 0C and then, to areasonable extent, the following relationship holds:

Where a and b are constants for the metals

concerned.

i l d f ki h l i


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Materials used for making thermocouple wire

Materials RangeC Sensivity mV/CE chromel-constantan

0 980 63

J Iron-constantan -180 760 53

K Chromel-alumel -180 1260 41R Platinum-platinum/rhodium

0 1750 8

T Copper-constantan -180 370 43


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A thermocouple can be used with the referencejunction at a temperature other than 0C junction andhence a correction has to be applied before the tablescan be used. The correction is applied using what is

known as the law of intermediate temperatures:



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Use the relationship between the temperature ofmetal and its electrical resistance.

Wire is wound over a ceramic coated tube to form acoil, coated in ceramic and fixed inside a protectivecasing.

This arrangement forms a temperature probe. Theends of the coil of wire are connected to one arm of a

Wheatstone bridge.


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The probe is placed in the substance whosetemperature is to be measured and after a responsetime of several seconds, the temperature of thesubstance is displayed on the meter.

The leads that connect the coil of wire to theWheatstone bridge also change in resistance withchanges in temperature, which introduces errors intothe measurement process.

To counteract these errors compensating leads areconnected in an adjacent arm of the bridge, as shown.

The imbalance of the bridge is directly proportional to

the change in the probe resistance the indicator can


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Nickel, copper, and some other types of metal havebeen used in electrical resistance thermometersthough nowadays platinum is by far the mostcommonly used.

Despite being expensive, platinum has the advantageof often being the reference material for internationalstandards.

The relationship between temperature and resistanceis linear over a wide range for platinum.

It also has a high melting point and is thus for hightemperature measurement.

Ad t


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Advantages

Available in many forms

Wide range of applications.

They can measure the temperature of gases and

liquids, the surface temperature of most solids and theinternal temperature of some soft solids.

Stable and can withstand harsh environments

Used in chemical industry for measuring thetemperature of corrosive liquids and slurries.

The food industry uses this type of thermometer tomeasure temperature of foodstuffs such as meat.

Di d t


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Disadvantages

Accurate but have a slow response time

Fairly large and fragile

expensive

THERMISTOR


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THERMISTOR

The coil of wire in the metal electrical resistancethermometer has a disadvantage in that the changesin resistance are comparatively small, typically 5millions per degree Celsius.

Thermistor used the same principle as metal electricalresistance thermometers, that is, that resistancechanges with temperature.

Thermistors use semiconductors.

d t


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advantages

Semiconductors provide much larger changes inresistance for the same temperature change.

Made from mixtures of rare earth metal oxides suchas manganese, nickel, chromium and cobalt, mixed

with finely divided copper.

The resistance of these materials is very sensitive totemperature change.

F


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Forms


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The resistance of themistors normally decreases withan increase in temperature.


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The relationship between resistance and temperature isexponential.

The resistance of a thermistor is given by:

Where:

is the resistance of the thermistor at T kelvin

T is absolute temperature in kelvin

is the resistance of the thermistor at reference

Ch 2_type of Measurement in Industrial Application J5800

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