Comparator is an indirect precision instrument that compares the dimensions of a component to a standard rather than providing an actual measurement. It indicates how much the component differs from the standard dimension through magnification. Comparators require little skill to use, eliminate human error, and provide quick, consistent results. Some key types are mechanical, optical, mechanical-optical, electrical, pneumatic, and fluid displacement comparators. Pneumatic comparators use air pressure differences rather than physical contact to measure dimensions and have advantages like high magnification and independence from operator skill.

1. Comparator
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2. COMPARATOR
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What is comparator ?
Comparator is an indirect precision instrument, which compares the
dimension of a given components with its working standard.
It does not give the actual dimension, but indicates how much it differs
from basic dimension.
Generally, the indicated difference is small, so it requires suitable
magnification device to measure the difference with consistent accuracy.
It has a sensing device, magnifying or amplifying system and display
system to provide suitable read out.
Need of comparator:
To achieve inter-changeability in mass production, the parts should be
produced to a close dimensional tolerance.
Therefore inspection is more concerned with dimensional variation from
standard or basic dimension.
The use of vernier caliper, micrometer is not suitable because of the skill
involved and the time required to measure the dimension.
The use of comparator requires little or no skill for operator, eliminate
human error in measuring, quick and gives highly consistent results.
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3. 3
Principle of operation
i.e. the comparator is first adjusted to zero on its dial with a
gauge block in position. The work piece to be checked is then
placed in position and the comparator gives the difference in
dimension in relation to gauge block. If the dimension is less
or greater than the standard, then difference will be shown in
magnified dial.
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4. 4
e.g. if a comparator has a magnification of 1000 and the difference
between standard and actual dimension is 0.02 mm, then the pointer
movement is 20mm on the dial or recording device of the
comparator.
Use of comparator :
Laboratory standard:- From which working and inspection gauges are
set and co-related.
Working gauge:- To prevent work spoilage and to maintain required
tolerance at all important stages of manufacturing.
Final inspection gauge:- Final inspection gauge where selective
assembly , of production parts is necessary.
For checking newly purchased gauge:- To check newly purchased
parts.
Characteristic of a good comparator :
Robust design and construction:- So that it can withstand the effect of
ordinary uses without affecting the measuring accuracy.
Linear characteristic of scale:- Read out scale should be linear, uniform and
clear.
High magnification:- So that smallest deviation in size of component can be
easily detected.
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5. 5
Quick in result:- The reading are obtained within least time.
Versatility:- It can be used for wide range of measurement.
Minimum wear at contact point:- The measuring plunger should have harden
steel, contact pressure is low and uniform.
Free from oscillation:- Pointer should come rapidly to rest.
Minimum inertia:-
Adjustable table:-
Compensation from temperature effect:-
Classification of comparator
Based on method used for amplifying and recording:
Mechanical Comparator,
Optical Comparator
Mechanical-Optical Comparator
Electrical and Electronics Comparator
Pneumatic Comparator
Fluid displacement Comparator
Projection Comparator
Multi check Comparator
Automatic gauging machines
Electro-Mechanical Comparator
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6. In these comparators, magnification is obtained by mechanical
linkages and other mechanical devices. The magnification may be of
following type
Rack and Pinion. In it the measuring spindle integral with a rack,
engages a pinion which amplifies the movement of plunger through
a gear train.
Mechanical Comparator
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7. Cam and gear train. In this case the measuring spindle acts on a cam
which transmits the motion to the amplifying gear train.
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8. Lever with toothed sector. In this case a lever with a toothed sector
at its end engages a pinion in the hub of a crown gear sector which
further meshes with a final pinion to produce indication.
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9. Compound Levers. Here levers forming a couple with compound
action are connected through segments and pinion to produce final
pointer movement.
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10. Twisted Taut Strip. The movement of measuring spindle tilts the
knee causing straining which further causes the twisted taut band to
rotate proportionally. The motion of strip is displayed by the
attached pointer.
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11. Lever combined with band wound around drum. In this case, the
movement of the measuring spindle tilts the hinged block, causing
swing of the fork which induces rotation of the drum.
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12. The usual magnification obtained by these comparators is about 250
to 1000. Some mechanical comparators are :-
1. Dial Indicator
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13. 2. The Johansson Mikrokator
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14. 3. Reed type mechanical comparator
A  Fixed block
B  Floating block
C Horizontal reeds
D Vertical reeds
REED :- One of the thin pieces of metal, the
vibration of which produce the tone.
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Sensitivity order
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15. 4. Sigma comparator
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16. 16
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17. 17
Magnification range: 300 to 5000
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18. 18
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19. Pneumatic Comparator
Air gauging important characteristics:
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(b) As no physical contact is made either with the setting gauge or the part
being measured, there is no loss of accuracy because of gauge wear.
(c) Internal dimensions can be readily measured not only with respect to
tolerance boundaries but also geometric form.
(d) High pressure air gauging can be done with cleansing of the parts
which helps to eliminate errors due to dirt and foreign matter.
(e) Gauging pressures can be kept sufficiently low to prevent part
deflection. (In general, high pressure gauges are suitable for those parts
in which tolerances are relatively large and low pressure air gauges are
preferable for highly precise work.)
(f) It is independent of operator skill
(a) Very high amplifications are possible. It can be used to measure
diameters, length, squareness, parallelism, concentricity, taper, center
distance between holes and other geometric conditions.
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20. Generally there are two types of pneumatic gauge :-
1. Flow or velocity type
Flow or velocity type pneumatic gauges operate by sensing
and indicating the momentary rate of air flow. Flow could be
sensed by a glass tube with tapered bore, mounted over a
graduated scale. Inside the bore a float is lifted by the air flow.
2. Back pressure type
The basic principle and the theory of pneumatic gauging in the
back pressure gauges is described by the figure below.
p=P when i) Om is blocked
ii) Om is infinitely large
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22. Pneumatic sensitivity
P= Source pressure or
Pressure at Oc
p=Pressure between
two orifice
C= geometric area of Oc
M= geometric area of Om
p=P when i) Om is blocked
ii) Om is infinitely large
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23. The characteristic of p/P and M/C is plotted on experimental
results. Here p & C is kept constant M varied and P varied
(0.13-5 kg/cm2), then the curve will be as shown below.
It is observed that 0.6 p/P to 0.8 p/P the curve is approximately
straight-line. So it can be written as
p/P = (A-b)(M/C) (1)
Where A=1.1, b=0.6 when P=0.13kg/cm2
=0.4 when P=5.00 kg/cm2
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24. Differentiating eq. 1 w.r.t M
(Sensitivity )
Let at mid point i.e. p/P=0.7 & M=Ma ; putting in eq. (1)
0.7=1.1-b(Ma/C) =>b(Ma/C)=0.4 =>b/C= 0.4/Ma
Now putting b/C value in equation (2)
So the sensitivity is directly depends upon operating pressure P
and inversely varies with Ma.
(2)
P
c
b
-
)P
C
M
b
(A
M
M
p







P
M
0.4
-
P
c
b
-
M
p
a




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25. Principle
The pneumatic gauging is based on Bornoulli’s theory. The
basic principle used in gauging is to convert change in length of
surface displacement into change of pressure of air.
Solex pneumatic gauge.
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Free Flow Air Gauges (Flow or velocity type):
In this compressed air after the filtering and pressure reducing unit flows
through a tapered glass tube containing a small metal float and then through a
plastic tube to the gauge head having two diametrically opposed orifices for air
escapement into atmosphere as showm in the Fig.The position of the tube is
dependent upon the amount of air flowing through the gauge head, which in
turn is dependent upon the clearance between the bore to be measured and the
gauge head. Fig. shows a curve between the air flow and the clearance
between the part and the orifice in gauge head.
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