COMPARATORSCOMPARATORS The The general principlegeneral principle of comparator is to of comparator is to

indicate the differences in size between the indicate the differences in size between the standard and the work being measured by means standard and the work being measured by means of some pointer on a scale with sufficient of some pointer on a scale with sufficient magnificationmagnification

All comparators consist of three basic featuresAll comparators consist of three basic features

1)A 1)A sensing devicesensing device which faithfully senses the input which faithfully senses the input signalsignal

2) A 2) A magnifyingmagnifying or or amplifying systemamplifying system to increase to increase the signal to suitable magnitude. Mechanical, the signal to suitable magnitude. Mechanical, Optical, Pneumatic, hydraulic and electronic Optical, Pneumatic, hydraulic and electronic methods are used for this purpose.methods are used for this purpose.

3) A 3) A display system (usually a scale and pointer) display system (usually a scale and pointer) which utilizes the amplified signal to provide which utilizes the amplified signal to provide a suitable readout.a suitable readout.

Need for a comparatorNeed for a comparator A comparator is used in mass production to A comparator is used in mass production to

inspect the components to close tolerances with inspect the components to close tolerances with high degree of precision and speed. high degree of precision and speed. Ex: PistonEx: Piston

Use of line standards such as vernier and Use of line standards such as vernier and micrometer calipers require considerable skillmicrometer calipers require considerable skill

Many dimensions can be checked in a very Many dimensions can be checked in a very short timeshort time

Comparators can be classified as Comparators can be classified as

1.1. MechanicalMechanical

2.2. Optical Optical

3.3. Electrical & ElectronicElectrical & Electronic

4.4. Pneumatic Pneumatic

5.5. Fluid displacement comparatorsFluid displacement comparators

Mechanical ComparatorsMechanical Comparators In Mechanical type, the required In Mechanical type, the required

magnification is obtained by using magnification is obtained by using mechanical linkages, levers, gears mechanical linkages, levers, gears and other mechanical devices.and other mechanical devices.

Examples are ;Examples are ;

1.1. REED COMPARATOR,REED COMPARATOR,

2.2. JOHANSSON MIKROKATORJOHANSSON MIKROKATOR

3.3. SIGMA COMPARATORSIGMA COMPARATOR

4.4. DIAL GAUGEDIAL GAUGE

PointerScale

ReedsFixed link

SpindleSpindle displacement

Movable link

Flexure strips

REED COMPARATOR

REED COMPARATORREED COMPARATOR As shown in fig, the movable link is As shown in fig, the movable link is

constrained by thin metal flexure constrained by thin metal flexure strips to move vertically relative to strips to move vertically relative to fixed link attached to housing.fixed link attached to housing.

The change of orientation of the The change of orientation of the flexure strips due to relative flexure strips due to relative motion, the reeds provide a large motion, the reeds provide a large angular movement of the pointer.angular movement of the pointer.

The scale may be calibrated to The scale may be calibrated to indicate any deviation from an indicate any deviation from an initial setting.initial setting.

Attachment screws

Measuring plungerPlunger tip

Cantilever strip

Twisted stripScaleLight pointer

Slit washer

Spring elbow

JOHANSSON MIKROKATER

JOHANSSON MIKROKATORJOHANSSON MIKROKATOR• It works on the principle of a button spinning

in a loop of string.

• A twisted thin metal strip carries a very light pointer made of thin glass at the center of its length.

• The two halves of the strip from the centre are twisted in opposite directions so that any pull on the strip will cause the centre to rotate.

• One end of the strip is fixed to an adjustable cantilever and the other is anchored to the spring elbow.

• One arm of spring elbow carries the measuring plunger.

JOHANSSON MIKROKATOR JOHANSSON MIKROKATOR (contd…)(contd…)

As the measuring plunger moves either As the measuring plunger moves either upwards or downwards, the elbow causes upwards or downwards, the elbow causes the twisted strip to change its length thus the twisted strip to change its length thus making it to further twist or untwist.making it to further twist or untwist.

Thus the pointer at the center of the Thus the pointer at the center of the twisted strip rotates by an amount twisted strip rotates by an amount proportional to the change in length of the proportional to the change in length of the strip and hence to the plunger movement.strip and hence to the plunger movement.

The strip is perforated along the center The strip is perforated along the center line to prevent excessive stress on the line to prevent excessive stress on the center portion.center portion.

A slit washer is used for lower mounting A slit washer is used for lower mounting of the plunger.of the plunger.

JOHANSSON MIKROKATOR JOHANSSON MIKROKATOR (contd…)(contd…)

The amplification of this comparator is given by The amplification of this comparator is given by

turnsofnumber theisn

axis neutral its along measured strip twisted theoflength theis L

ends, therespect towith

strip theofpoint mid at the twist theis where2

n

L

dL

d

The amplification increases with the number of turns. The magnification is of the order 5000

The Mikrokator is an instrument for measuring differences in length. The Mikrokator principle greatly magnifies any deviation in size so that even small deviations produce large deflections of the pointer over the scale.

The transference of movement from the measuring tip to the pointer is mechanical, completely frictionless and free from backlash.

The CE Johansson Mikrokator was introduced in 1938 and is still today the finest mechanical comparator in the world. A wide range of models are available, which gives graduations varying between .0001” (.005 mm) and .00002” (.0001 mm). The unique “Twisted Strip” principle on which all Mikrokators are based, provides many features which are not available on any other measure instrument.

SIGMA COMPARATORSIGMA COMPARATOR It has magnifications in the range of It has magnifications in the range of

300 to 5000. 300 to 5000. It has a plunger attached to a It has a plunger attached to a

rectangular bar which is supported at rectangular bar which is supported at its upper and lower ends by flexure its upper and lower ends by flexure plates. A knife edge is fixed to the plates. A knife edge is fixed to the side of rectangular bar which bears side of rectangular bar which bears on a moving block.on a moving block.

The moving block and the fixed block The moving block and the fixed block are connected by flexible strips at are connected by flexible strips at right angles to each other.right angles to each other.

SIGMA COMPARATORSIGMA COMPARATORScale

Drum

Bronze band

Forked arm

Plunger

Flexure plate

Fixed block

Flexible strips

Flexure plate

Pointer

Y X

Knife edge

Moving block

SIGMA COMPARATOR (contd…)SIGMA COMPARATOR (contd…) If an external force is applied to the moving If an external force is applied to the moving

block, it would pivot about the hinge .block, it would pivot about the hinge . A forked arm or Y-arm attached to the moving A forked arm or Y-arm attached to the moving

block transmits the rotary motion to the block transmits the rotary motion to the indicator driving drum through a bronze band indicator driving drum through a bronze band wrapped around the drum.wrapped around the drum.

Magnification: Magnification: If If YY is the length of forked arm is the length of forked arm and and XX is the distance from the knife edge to is the distance from the knife edge to hinge, then first stage magnification is hinge, then first stage magnification is Y/XY/X

If the pointer length is If the pointer length is RR and the radius of the and the radius of the drum is drum is rr the second stage magnification is the second stage magnification is R/r R/r such that the total magnification is (such that the total magnification is (Y/X)* (R/r)Y/X)* (R/r)

DIAL INDICATORDIAL INDICATOR

DIAL INDICATORDIAL INDICATOR

Plunger

PinionRack

Dial Scale

Pointer

Pillar

Base

Plunger

Cylindrical

component

V block

DIAL INDICATOR (contd..)DIAL INDICATOR (contd..) It consists of a robust base whose surface It consists of a robust base whose surface

is perfectly flat and a pillar carrying a is perfectly flat and a pillar carrying a bracket.bracket.

The bracket incorporates a spindle fitted The bracket incorporates a spindle fitted with a pinion & dial scale.with a pinion & dial scale.

The linear movement of the plunger is The linear movement of the plunger is magnified by means of a rack & pinion magnified by means of a rack & pinion arrangement into sizeable rotation of the arrangement into sizeable rotation of the pointer on the dial scale. pointer on the dial scale.

The dial scale is set to zero by use of slip The dial scale is set to zero by use of slip gauges representing the basic size of the gauges representing the basic size of the part. part.

Advantages of Mechanical Advantages of Mechanical ComparatorsComparators

They are cheaper compared to other They are cheaper compared to other amplifying devicesamplifying devices

Do not require electricity or air and Do not require electricity or air and such the variations in the outside such the variations in the outside sources do not affect the accuracy.sources do not affect the accuracy.

They have a linear scale, robust and They have a linear scale, robust and easy to handle.easy to handle.

Disadvantages of Mechanical Disadvantages of Mechanical ComparatorsComparators

They have more linkages due to which They have more linkages due to which friction is more and accuracy is less.friction is more and accuracy is less.

Mechanisms used have more inertia and Mechanisms used have more inertia and hence they become sensitive to hence they become sensitive to vibrations.vibrations.

Any wear, backlash, play or dimensional Any wear, backlash, play or dimensional faults in the mechanical devices used faults in the mechanical devices used will also be magnified.will also be magnified.

The range of the instruments is limited The range of the instruments is limited as the pointer moves over a fixed scale.as the pointer moves over a fixed scale.

It is also difficult to incorporate It is also difficult to incorporate arrangement for adjusting magnification.arrangement for adjusting magnification.

O

d

C

AScreenLamp

Pivot

Mirror

Plunger

Normal

d

2

h angleof tilt

PRINCIPLE OF OPTICAL LEVER

OPTICAL COMPARATORS

OPTICAL COMPARATORS An optical comparator works on one of the An optical comparator works on one of the

following two main principles:following two main principles:1)Use of optical lever 2) Use of enlarged image1)Use of optical lever 2) Use of enlarged image

If a beam of light AC is directed on to a mirror as If a beam of light AC is directed on to a mirror as shownshown

in fig, it will be reflected onto the screen at O as a in fig, it will be reflected onto the screen at O as a dot.dot.

The angle of incidence = angle of reflectionThe angle of incidence = angle of reflection When the plunger moves up, the mirror tilts by When the plunger moves up, the mirror tilts by

Then the reflected beam moves through 2 Then the reflected beam moves through 2 i.e. the illuminated dot moves to B.i.e. the illuminated dot moves to B.

Also the increase in distance CO of the screen Also the increase in distance CO of the screen from the tilting mirror will increase the from the tilting mirror will increase the magnification.magnification.

L1 L2 L3

PivotMeasuringPlunger

Mecahnicallever

Screen with scale

Light source

Mirror pivot

Mirror

Projection lensGlass plate carrying indexline

Collimating lens

D

Mechanical-Optical Comparator

Mechanical Optical ComparatorMechanical Optical Comparator

In this comparator, small displacements of In this comparator, small displacements of the measuring plunger are first amplified the measuring plunger are first amplified by a mechanical lever. It is equal to (Lby a mechanical lever. It is equal to (L22/L/L11))

The amplified mechanical movement is The amplified mechanical movement is further amplified by optical system due to further amplified by optical system due to tilting of mirror by tilting of mirror by ..

The reflected ray D (image of index line) The reflected ray D (image of index line) will be turned through an angle 2will be turned through an angle 2and and hence optical amplification =2(Lhence optical amplification =2(L44/L/L33) )

The overall magnification = (LThe overall magnification = (L22/L/L11)* )* 2(L2(L44/L/L33) )

Zeiss Ultra OptimeterZeiss Ultra Optimeter

Fixed mirrorM2

MovableMirror M1

Plunger

Work piece

Index lens

Condenser

Green Filter

Lamp

Eye piece

Objective lens

Zeiss Zeiss UltraUltra Optimeter Optimeter The optical system of this instrument The optical system of this instrument

involves double reflection of light and involves double reflection of light and hence gives higher degree of magnification.hence gives higher degree of magnification.

The green filter filters all rays except green The green filter filters all rays except green (to reduce fatigue to eye) and green light (to reduce fatigue to eye) and green light passes through a condenser.passes through a condenser.

The light then passes through an index lens The light then passes through an index lens and the index mark is projected on to a and the index mark is projected on to a movable mirror Mmovable mirror M1.1.

It is then reflected on to another fixed It is then reflected on to another fixed mirror Mmirror M22 and then back to the first and then back to the first movable mirror.movable mirror.

Zeiss Zeiss UltraUltra Optimeter (contd…) Optimeter (contd…) The objective lens brings the reflected beam The objective lens brings the reflected beam

from the movable mirror to focus at a from the movable mirror to focus at a transparent graticule.transparent graticule.

The graticule contains a precise scale which is The graticule contains a precise scale which is viewed by an eye piece.viewed by an eye piece.

The projected image of the index line on the The projected image of the index line on the graticule can be adjusted to set initial zero graticule can be adjusted to set initial zero reading by a screw.reading by a screw.

Any movement of the plunger will tilt the Any movement of the plunger will tilt the movable mirror, which causes a shift in the movable mirror, which causes a shift in the reflected index line on the eye piece graticule reflected index line on the eye piece graticule scale.scale.

This in turn measures the displacement of the This in turn measures the displacement of the plungerplunger

Advantages & Disadvantages of Optical Advantages & Disadvantages of Optical comparatorscomparators

Advantages:Advantages:

1) Few moving linkages and hence no friction & 1) Few moving linkages and hence no friction & wear.wear.

2) High range of measurements and no parallax error2) High range of measurements and no parallax error

3) Magnification is usually high.3) Magnification is usually high. Disadvantages:Disadvantages:

1) Heat from the source of light, transformers etc., 1) Heat from the source of light, transformers etc., may cause the setting to drift.may cause the setting to drift.

2) An electric supply is required to operate these 2) An electric supply is required to operate these comparators.comparators.

3) The size is large and costly.3) The size is large and costly.

4) Use of eyepiece to view is inconvenient for 4) Use of eyepiece to view is inconvenient for prolonged use. Also a dark room is essential to take prolonged use. Also a dark room is essential to take readings.readings.

Electrical& Electronic Electrical& Electronic ComparatorsComparators

R1 R2

R3 R4

Battery

Coils

Arm

MeasuringPlunger

Coils

IronArmature

Electrical& Electronic Electrical& Electronic Comparators (Contd…)Comparators (Contd…)

These comparators depend on the principle These comparators depend on the principle of balancing the Wheatstone bridge, of balancing the Wheatstone bridge,

(R(R11/R/R22) = (R) = (R33/R/R44) applicable for only to ) applicable for only to direct current obtained from a battery.direct current obtained from a battery.

In actual instruments, one pair of In actual instruments, one pair of inductances takes the form of a pair of coils inductances takes the form of a pair of coils in the measuring head of the instrument.in the measuring head of the instrument.

An iron armature inside these coils moves An iron armature inside these coils moves along with the measuring plunger, and along with the measuring plunger, and upsets the balance of the circuit and causes upsets the balance of the circuit and causes the deflection of the pointer meter which is the deflection of the pointer meter which is calibrated directly in linear units.calibrated directly in linear units.

Magnifications of the order of 30,000 are Magnifications of the order of 30,000 are possible.possible.

Electrolimit gaugeElectrolimit gauge

Electromagneticcoils

A

B

Armature

SpringThin steel strips

Spring

Plunger

Electrolimit gauge (contd…)Electrolimit gauge (contd…) Fig shows the principle of Electrolimit Fig shows the principle of Electrolimit

gauge or measuring head.gauge or measuring head. Vertical movements of the plunger are Vertical movements of the plunger are

transmitted to an armature, which in transmitted to an armature, which in turn is suspended by thin metal strips.turn is suspended by thin metal strips.

At the left end, the armature between At the left end, the armature between two electromagnetic coils form the two two electromagnetic coils form the two arms of an AC bridge circuit.arms of an AC bridge circuit.

Any movement of the armature sets up Any movement of the armature sets up out of balance effects which are out of balance effects which are recorded by a micrometer from which recorded by a micrometer from which direct reading of plunger movements direct reading of plunger movements can be obtained.can be obtained.

Linear Variable Differential Linear Variable Differential TransformerTransformer

LVDT is the most popular electro LVDT is the most popular electro mechanical device used to convert mechanical device used to convert mechanical displacement into electrical mechanical displacement into electrical signal.signal.

It is a differential transformer which It is a differential transformer which provides an A.C voltage output proportional provides an A.C voltage output proportional to displacement of a core passing through to displacement of a core passing through the windings.the windings.

Of the three symmetrically placed coils, the Of the three symmetrically placed coils, the center coil is energized from an A.C source center coil is energized from an A.C source and the two end coils connected together in and the two end coils connected together in phase opposition are used as pick up coils.phase opposition are used as pick up coils.

Linear Variable Differential Linear Variable Differential TransformerTransformer

Input displacement

Fig.b

InputVoltage

OutputVoltage

displacement

Fig.a

Linear Variable Differential Linear Variable Differential Transformer (contd…)Transformer (contd…)

Based on the position of the core, a voltage is Based on the position of the core, a voltage is induced in each of the pick up coils.induced in each of the pick up coils.

A position of the core at which the voltages A position of the core at which the voltages induced in the pick up coils are same, but induced in the pick up coils are same, but cancel each other due to phase opposition, is cancel each other due to phase opposition, is called the “null position”.called the “null position”.

The fig shows the typical differential The fig shows the typical differential transformer characteristics obtained by transformer characteristics obtained by plotting output voltages vs. core plotting output voltages vs. core displacements.displacements.

All commercial devices are designed to All commercial devices are designed to operate only in the linear range and hence operate only in the linear range and hence the name LVDT.the name LVDT.

Supply requirements 6.3 V at 60 Hz, Range of Supply requirements 6.3 V at 60 Hz, Range of displacement-25 mm. displacement-25 mm.

Linear Variable Differential Linear Variable Differential Transformer (contd…)Transformer (contd…)

Fig c

Linear range

CoreDisplacement

Output Voltage

Null position

Commercial LVDTs

Sectional View of LVDT

Advantages of LVDTAdvantages of LVDT

It can be used as a primary detector It can be used as a primary detector transducer, as it does not require transducer, as it does not require assistance of another element such as a assistance of another element such as a strain gauge to convert mechanical strain gauge to convert mechanical displacement into electrical voltage.displacement into electrical voltage.

The core is completely separable from the The core is completely separable from the rest of the equipment and hence no rest of the equipment and hence no mechanical losses.mechanical losses.

It is insensitive to temperature changes.It is insensitive to temperature changes. Provides a comparatively high output and Provides a comparatively high output and

does not require amplification.does not require amplification. It is of reasonable cost.It is of reasonable cost.

Disadvantages of LVDTDisadvantages of LVDT

1) It cannot be used for dynamic 1) It cannot be used for dynamic measurements due to its appreciable measurements due to its appreciable mass compared to strain gauge.mass compared to strain gauge.

2) Requirement of 60 cps supply 2) Requirement of 60 cps supply voltage becomes a limiting factor for voltage becomes a limiting factor for dynamic measurements.dynamic measurements.

3) The direction from the null point 3) The direction from the null point is to be indicated which makes the is to be indicated which makes the circuit arrangement complex.circuit arrangement complex.

Pneumatic comparatorsPneumatic comparators In Pneumatic comparators air is used as a In Pneumatic comparators air is used as a

means of magnification and hence they use means of magnification and hence they use principle of air jet.principle of air jet.

A chamber is fitted with control orifice C and A chamber is fitted with control orifice C and a gauging orifice G through which air flows a gauging orifice G through which air flows from a supply at a constant pressure Pfrom a supply at a constant pressure P11..

If the size of the control orifice C remains If the size of the control orifice C remains constant, any variation in size of G will cause constant, any variation in size of G will cause alteration of pressure Palteration of pressure P22 in the chamber. in the chamber.

This variation is measured by a suitable This variation is measured by a suitable pressure gaugepressure gauge

graduated to read in linear units.graduated to read in linear units.

To pressure gaugePressure P2

Workpiece

G (Gauging orifice)

Chamber

Air @ constant Pr P1

C (Control orifice)

Principle of Pneumatic comparator

Systems of Systems of PneumaticPneumatic comparatorscomparators

Based on the physical phenomenon, Based on the physical phenomenon, Pneumatic comparators are classified as;Pneumatic comparators are classified as;

(a) Flow or velocity type (b) Back pressure (a) Flow or velocity type (b) Back pressure typetype

Flow types operate by sensing & indicating Flow types operate by sensing & indicating the momentary rate of flow.the momentary rate of flow.

Compressed air after filtering & pressure Compressed air after filtering & pressure regulation flows through a glass tube with a regulation flows through a glass tube with a small metal float.small metal float.

The air then passes through a plastic tube to The air then passes through a plastic tube to the gauge head with two diametrically the gauge head with two diametrically opposite orifices for the air to escape.opposite orifices for the air to escape.

Pneumatic comparators Pneumatic comparators (contd…)(contd…)

Air supply

Filter

Pr regulator

Scale

Float

Plastic tube Bore to bemeasured

Gauging head

Flow or Velocity type Comparator

FlowFlow or velocity type (contd…)or velocity type (contd…) The position of the float depends The position of the float depends

upon the amount of air flowing upon the amount of air flowing through the gauging head, which in through the gauging head, which in turn depends upon the clearance turn depends upon the clearance between the bore to measured and between the bore to measured and the gauging head.the gauging head.

These types when assembled side by These types when assembled side by side, can measure multiple inter side, can measure multiple inter related dimensions with great ease, related dimensions with great ease, accuracy & speed.accuracy & speed.

Back pressure type Pneumatic Back pressure type Pneumatic ComparatorsComparators

Constantpressuresource

P1 P2To atmosphere

Oc mO

Principle of back pressure type Pneumatic comparator

Back Pressure Circuit

Air supply

FilterRegulator

Bourdon tube

Scale

Work piece

Measuring head

The principle of back pressure gauges is that The principle of back pressure gauges is that when the orifice Owhen the orifice Omm is blocked, the upstream is blocked, the upstream pressure Ppressure P11 becomes equal to pressure P becomes equal to pressure P22 between the two orifices.between the two orifices.

When the orifice opening OWhen the orifice opening Om m is increased is increased indefinitely, the pressure Pindefinitely, the pressure P22 tends to become tends to become zero.zero.

In the basic back pressure unit shown in fig, In the basic back pressure unit shown in fig, a bourdon tube deflects according to back a bourdon tube deflects according to back pressure changes built up in the circuit when pressure changes built up in the circuit when work piece is placed over the measuring work piece is placed over the measuring head.head.

The deflection is amplified by gear & lever The deflection is amplified by gear & lever and indicated on a dial.and indicated on a dial.

Back pressure type Pneumatic Back pressure type Pneumatic ComparatorsComparators

Solex Pneumatic gaugeSolex Pneumatic gauge Solex type of gauge employs a water Solex type of gauge employs a water

manometer for the indication of back pressure.manometer for the indication of back pressure. It consists of a water tank filled up to a certain It consists of a water tank filled up to a certain

level and a dip tube immersed into it upto level and a dip tube immersed into it upto required depth.required depth.

Air, sent at higher pressure than required, Air, sent at higher pressure than required, bubbles to the top of the water tank.bubbles to the top of the water tank.

The air at the desired constant pressure then The air at the desired constant pressure then passes through the control orifice and escapes passes through the control orifice and escapes through the measuring jets.through the measuring jets.

The back pressure in the circuit is indicated by The back pressure in the circuit is indicated by the head of water displaced in the manometer the head of water displaced in the manometer tube. tube.

Solex Pneumatic gaugeSolex Pneumatic gauge

Air

Air filter Control orifice

Scale Plug withmeasuring jaws

workpieceManometer

Water tank

Dip tube

Excess airbubbling at top

Solex Pneumatic gauge

Solex Pneumatic gauge (contd…)Solex Pneumatic gauge (contd…) The pressure in the manometer is regulated by The pressure in the manometer is regulated by

the relative rates of escape of air through the the relative rates of escape of air through the control orifice and the measuring jets.control orifice and the measuring jets.

The manometer tube is graduated linearly to The manometer tube is graduated linearly to show changes in pressure resulting from changes show changes in pressure resulting from changes in internal diameter of the work being measured.in internal diameter of the work being measured.

By revolving the workpiece around the By revolving the workpiece around the measuring gauge, the roundness or concentricity measuring gauge, the roundness or concentricity of the bore can be checked.of the bore can be checked.

By moving the workpiece along the length of the By moving the workpiece along the length of the measuring gauge, taper of the bore, if any, can measuring gauge, taper of the bore, if any, can be checked.be checked.

Best suited for checking roundness and taper of Best suited for checking roundness and taper of cylinder bores & gun barrels.cylinder bores & gun barrels.

Advantages & Disadvantages of Advantages & Disadvantages of Pneumatic ComparatorsPneumatic Comparators

Advantages:Advantages:

(1) No physical contact between gauge & work and (1) No physical contact between gauge & work and hence practically no wear take place.hence practically no wear take place.

(2) High magnification & accuracy possible.(2) High magnification & accuracy possible.

(3) Suitable for checking small bores.(3) Suitable for checking small bores.

(4) Measuring pressure is very small and jet of air (4) Measuring pressure is very small and jet of air cleans the dust, if any, from the workpiece.cleans the dust, if any, from the workpiece.

Disadvantages:Disadvantages:

(1) Requires elaborate auxiliary equipment such as (1) Requires elaborate auxiliary equipment such as pressure regulator, compressor, etc.pressure regulator, compressor, etc.

(2) Parallax error in glass tubes which is an (2) Parallax error in glass tubes which is an indicating device.indicating device.

(3) Scale is generally not linear and apparatus in (3) Scale is generally not linear and apparatus in not portable.not portable.

Brooke’s level ComparatorBrooke’s level Comparator

Spirit level

ScaleBubble

Ball feet

Column

Bracket

Spirit level

Rotatable disc

Base

Levelling scews

It is a mechanical comparator which uses It is a mechanical comparator which uses a sensitive spirit level and a rotatable a sensitive spirit level and a rotatable disc.disc.

The rotatable disc is mounted on a The rotatable disc is mounted on a massive base with leveling screws. massive base with leveling screws.

A bracket supports the spirit level and A bracket supports the spirit level and allows it to be raised or lowered along allows it to be raised or lowered along the main column of the instrument.the main column of the instrument.

The upper surface of the rotatable disc is The upper surface of the rotatable disc is a plane lapped surface while the lower a plane lapped surface while the lower face has a lapped annular ring allowing face has a lapped annular ring allowing easy rotation of the base.easy rotation of the base.

Brooke’s level ComparatorBrooke’s level Comparator

GSG S

(c) (d)

Brooke’s level ComparatorBrooke’s level Comparator

In operation, the bubble is first brought to the In operation, the bubble is first brought to the center of the scale by allowing the ball feet to rest center of the scale by allowing the ball feet to rest on the surface of the rotatable disc and adjusting on the surface of the rotatable disc and adjusting the leveling screws of the base.the leveling screws of the base.

The standard S and the gauge to be compared G The standard S and the gauge to be compared G are now wrung to the disc, and the ball feet are are now wrung to the disc, and the ball feet are made to rest on them as shown in fig.made to rest on them as shown in fig.

The position of the bubble is noted, the spirit level is The position of the bubble is noted, the spirit level is then raised, and disc rotated through 180then raised, and disc rotated through 18000..

A second reading is taken in the position shown. A second reading is taken in the position shown. The displacement of the bubble along the scale The displacement of the bubble along the scale

represents twice the difference in height between represents twice the difference in height between the two gauges.the two gauges.

Thus, Thus, H= ½(L) where H= ½(L) where H= difference in height H= difference in height between gauges, L = bubble dispalcement.between gauges, L = bubble dispalcement.

Brooke’s level Comparator Brooke’s level Comparator (Contd…)(Contd…)