chapter 1 comparator

1. Chapter 1 Metrology Basics
 C505.1
Understand basics of metrology, standards and calculate
least count of measuring instruments.

2. Selection of instrument
 Range of instrument
 Scale spacing
 Sensitivity
 Scale division value
 Accuracy of instrument
 Repeatability
 Effect of environmental condition
 Errors generated during measurement
 Cost of instrument
 Life of instrument & need of calibration

3. Precautions & care while handling
instrument
 Avoid mishandling
 Surfaces not be touched by hand
 Wash hand properly apply petroleum jelly before
handling instrument

5. Line Standards
 Line standard is the standard in which distance is
measured between two lines.
 Characteristics-
It is Quick and easy
Parallax error can be generated
They are not much accurate
e.g. steel rule.

6. End Standard
 End standard- When distance is measured between
two surfaces
 Characteristics
Accurate up to 0.005mm
They are costly, difficult to use.
The surfaces are to be protected.
e.g. Slip gauges, end bars.

7. Wavelength Standard
 Material standards are subjected to destroy day by day
Concepts leads to generate wavelength standard
 Simple light rays are used as source of wavelength
standard.
 Cadmium 11,krypton-86 and mercury 198 can be
sources of light wavelengths
 It was decided that krypton-86 in a hot cathode
discharge lamp at 68 ̊ K,generates orange radiations
and they can be used as ultimate wavelength standard.
1 meter=1650763.73×wavelength Kr-86

8. Difference between line,end
and wavelength standard

9. Slip Gauges
Johnasson gauges
Reactangular blocks of steel .
Reference standards

10. Slip Gauge Accessories
 Measuring Jaws
 Scribing and center
point
 Holders and base

11. Wringing
 Wringing –the property of measuring faces of a gauge
block of adhering by sliding or pressing hr gauge
against the measuring faces of other gauge block or
the reference faces of datum surfaces,
 It occurs due to molecular adhesion between a liquid
film and mating surfaces

12. Wringing
 First keep two slip on
one another and slide
them
 Press the faces into
contact ,perpendicular to
each other and give a
small twisting motion.

13. Sets of slip gauges
M45

14. M 87

15. Comparators
 Works on relative measurement
 Definition-Is a device
Pick up small variation in dimension
Magnifies it
Displays it by indicating devices

16. Requirements of good
comparator
 Robust design and construction
 Linear scale
 High magnification
 Versatility
 Indicator should be constant in it return to zero
 No effect of environmental conditions
 Measuring pressure-low & constant
 Sensitive As per requirement
 Reliable
 Easy to handle & compact
 Cheaper and easily available in market

17. Use of comparators
 In mass production
 As laboratory standard
 For inspection of newly purchased gauges
 In selective assembly of parts
 As a working gauge & inspection

18. Classification of comparators
 Mechanical Comparator
 Pneumatic comparator
 Optical comparator
 Electrical Comparator
 Electronics Comparator
 Combined comparator

19. Dial Gauge Indicator
Working principle-
Conversion of linear movement
into angular movement using
rack and pinion.
Gear train for magnification and
scale pointer as indicating device

20. Construction and working of dial
gauge indicator
R=Rack,A=Pinion
S= spring
A,B,C,D,E,F=Gears
P=plunger
G=Guide way
Po= pointer
Sc=Scale
H=Support with hair coil
spring

21. Construction and working
 When plunger P moves up with rack R,gear A rotates
as shown .Linear movement is converted into angular
 As A rotates, B also rotates which is in mesh with C, C
is on the same shaft as that of D.
 When D rotates pinion E rotates To the center of E Po
is attached.
 Pointer Po shows reading on the scale.

22. Sigma Comparator
 Principle-
 Two rectangular blocks
out of which one is fixed
and other movable are
assembled as shown
 The force is applied from
the top side it will give
angular deflection of θ

24.  Plunger is mounted on a
slit washer to move up and
down for frictionless
movements
 A knief edge pivots on the
groove, whose lower edge
is on the moving block
 Y-arm is attached to the
movable block as shown Y
arm is wounded on the
small drum of radius R
with the phoshorous
bronze taps.

25. Working of sigma comparator
 When plunger moves in downward direction knief
edge also moves in the same
 Knief edge will give force on movable block this will
tend to deflect it by a small angle θ
 Y arm causes change in position by rotating drum of
small radius ‘r’ The pointer is attached to the centre of
drum which will rotate and shows some reading on the
scale
 The magnification of a comparator depends upon L,l,x
and r
 Magnification=L*l/x*r

26. Pneumatic comparator
Working principle-
Back pressure gauge
 Air flows with a constant
pressure through two orifices
o1 and o2
 P1 is upstream pressure of the
first orifice and P2 is the
pressure between two orifice.
 P1 +P2 when O2 is blocked.
Also it tends to zero as O2 is
blocked
 Also it tends to zero as O2 is
increased upto ∞
 P2 depends upon L

27. Pneumatic comparator

28. Construction & working of
Pneumatic comparator
 Constuction
 Uses high pressure air from
compressor
 High pressure air pipe is
connected to gauging head
 High pressure air is passing
through air filter
 Manometer is used to measure
backpressure generated in the
system
 Orifice is used in pipeline
between dip tube and
manometer for avoiding
backpressure to be discharged in
compressor line.

29. Working
 Water is filled in tank and
dip tube is inserted upto
level H
 High pressure and excess air
may bubble out in water
tank
 Air flows through control
orifice to gauging head
 Because of restrictions given
by gauging head back
pressure is exerted on the
air

30.  Back pressure
generated is shown by
manometer i.e.by
height difference ∂h
 The manometer height
can be calibrated by
using standard
gauging heads and
jobs

31. Electrical Comparator
 Principle-
 It works on mutual inductance
 When core is inserted in a magnetic fields of primary
windings, the voltage induced in secondary is
proportional to the core position.

32. LVDT

33. Construction And working
 Electrical principle and
mechanical movements of
plunger which is joined to
core
 Transformer consisting of
three coils wounded on
insulated bobin
Working
Itf the core is centralized in
the middle position of two
secondary windings, the
voltage induced in
secondary winding gives
net output 0

34. Relative advantages and disadvantages
Comparator Advantages Disadvantages
Mechanical 1. Cheaper
2. No need of electricity
3. Linear scale
1. More moving parts
2. More friction
3. Less accuracy
4.Robustn and compact 4. Sensitive to vibration
5. Easy to handle 5. Range is limited
6.portable 6. Parallax error is possible
Electrical 1.Small moving parts 1.Needs Electric supply
2. Very high magnification
3.High range
4. Not sensitive to vibration
2. Variation in supply affects accuracy
3. Heating coil may cause errors
4. expensive
Pneumatic 1. No contact with job
2. Small moving parts
3. More accuracy
4. High magnification
5. Instrument can be used
placed at remote from job
1. Auxiliary equipment needed
2. Scale not uniform
3. Not portable
4. Different gauging head required
6. Used to check ovality and