The document discusses the concepts of measurement and metrology, particularly in engineering, highlighting various methods and characteristics of measuring instruments. It covers definitions, standards, accuracy versus precision, and the importance of measurement in quality control and design validation. Additionally, the document emphasizes the characteristics of measuring instruments, types of errors, and the significance of calibration and interchangeability.

1. Unit 1
Concept of Measurement

2. Definition
• Metrology is the name given to the science of
pure measurement.
• Engineering Metrology is restricted to
measurements of length & angle
• Measurement is defined as the process of
numerical evaluation of a dimension or the
process of comparison with standard
measuring instruments

3. Why measure things?
• Check quality?
• Check tolerances?

4. Need of Measurement
• Establish standard
• Interchangeability
• Customer Satisfaction
• Validate the design
• Physical parameter into meaningful number
• True dimension
• Evaluate the Performance

5. Methods of Measurement
• Direct method
• Indirect method
• Comparative method
• Coincidence method
• Contact method
• Deflection method
• Complementary method

6. Direct method
• Measurements are directly obtained
– Ex: Vernier Caliper, Scales

7. Indirect method
• Obtained by measuring other quantities
– Ex : Weight = Length x Breadth x Height x Density

8. Comparative Method
• It’s compared with other known value
– Ex: Comparators

9. Coincidence method
• Measurements coincide with certain lines and
signals
Fundamental method
• Measuring a quantity directly in related with
the definition of that quantity
Contact method
• Sensor/Measuring tip touch the surface area

10. Complementary method
• The value of quantity to be measured is
combined with known value of the same
quantity
– Ex:Volume determination by liquid displacement

11. Deflection method
• The value to be measured is directly indicated
by a deflection of pointer
– Ex: Pressure Measurement

12. Units and standards

13. SI: fundamental Units
Physical Quantity Unit Name Symbol
length meter m
mass kilogram kg
time second s
electric current ampere A
temperature Kelvin K
amount of substance mole mol
luminous intensity candela cd

14. SI: Derived Units
Physical Quantity Unit Name Symbol
area square meter m2
volume cubic meter m3
speed
meter per
second
m/s
acceleration
meter per
second squared
m/s2
weight, force newton N
pressure pascal Pa
energy, work joule J

15. Supplementary units
Physical Quantity Unit Name Symbol
Plane angle Radian rad
Solid angle Steradian sr

16. Standards
• International standards
• Primary standards
• Secondary standards
• Working standards

17. International
International Organization of Legal Metrology, Paris
International Bureau of Weights and Measures at
Sevres, France
India
National Physical Laboratory
Dr. K.S. Krishnan Marg
New Delhi - 110012
India
Phone: 91-11-45609212
Fax: 91-11-45609310
Email: root@nplindia.org or root@nplindia.ernet.in

18. Measuring Instruments
• Deflection and null type instruments
• Analog and digital instruments
• Active and passive instruments
• Automatic and manually operated
instruments
• Contacting and non contacting instruments
• Absolute and secondary instruments
• Intelligent instruments.

19. DEFLECTION AND NULL TYPE
• Physical effect generated by the measuring
quantity
• Equivalent opposing effect to nullify the physical
effect caused by the quantity

20. ANALOG AND DIGITAL
INSTRUMENTS
• Physical variables of interest in the form of
continuous or stepless variations
• Physical variables are represented by digital
quantities

21. ACTIVE AND PASSIVE INSTRUMENTS
• Instruments are those that require some source
of auxiliary power
• The energy requirements of the instruments are
met entirely from the input signal

22. Automatic and manually operated
• Manually operated – requires the service of
human operator
• Automated – doesn't requires human
operator

23. Contacting And Non Contacting Instruments
• A contacting with measuring medium
• Measure the desired input even though they
are not in close contact with the measuring
medium

24. Absolute and Secondary Instruments
• These instruments give the value of the electrical
quantity in terms of absolute quantities
• Deflection of the instruments can read directly

25. Intelligent instruments
• Microprocessors are incorporated with
measuring instruments

26. Characteristics of Measuring Instrument
• Sensitivity
• Readability
• Range of accuracy
• Precision

27. Definition
• Sensitivity- Sensitivity is defined as the ratio of
the magnitude of response (output signal) to the
magnitude of the quantity being measured
(input signal)
• Readability- Readability is defined as the
closeness with which the scale of the analog
instrument can be read

28. Definition
• Range of accuracy- Accuracy of a measuring
system is defined as the closeness of the
instrument output to the true value of the
measured quantity
• Precision- Precision is defined as the ability of the
instrument to reproduce a certain set of readings
within a given accuracy

29. Sensitivity
• If the calibration curve is liner, as shown, the sensitivity
of the instrument is the slope of the calibration curve.
• If the calibration curve is not linear as shown, then the
sensitivity varies with the input.

30. Sensitivity
This is the relationship between a change in the
output reading for a given change of the input.
(This relationship may be linear or non-linear.)
Sensitivity is often known as scale factor or
instrument magnification and an instrument with a
large sensitivity (scale factor) will indicate a large
movement of the indicator for a small input
change.

31. Load Cell
Force, F
Output, Vo
Output, Vo (V)
Input, Fi (kN)
Slope = 5 V/kN
K
Input, F (kN) Output, Vo (V)
Sensitivity, K = 5 V/kN
Block Diagram:

32. Example
(1) A 0.01 Ω/A meter with 5 A fsd,
Rm = Ω/A x A
= 0.01 x 5 = 0.05 Ω
Vmax across the Meter will be
= 5 A x 0.05 Ω
= 0.25 V for fsd.
(2) A 0.1 Ω/A meter with 5 A fsd,will drop 2.5 V
(i.e., it is 10 times less sensitive), which may bias
the results

33. Readability
• Readability is defined as the ease with which
readings may be taken with an instrument.
• Readability difficulties may often occur due to
parallax errors when an observer is noting the
position of a pointer on a calibrated scale

34. Readability
What is the value ?
What is the value ?
What is the value ?

35. Accuracy
• Accuracy = the extent to which a measured
value agrees with a true value
• The difference between the measured value &
the true value is known as ‘Error of
measurement’
• Accuracy is the quality of conformity

36. Example: Accuracy
• Who is more accurate when measuring a book that
has a true length of 17.0 cm?
A :
17.0 cm, 16.0 cm, 18.0 cm, 15.0 cm
B ::
15.5 cm, 15.0 cm, 15.2 cm, 15.3 cm

37. Precision
• The precision of a measurement depends on the
instrument used to measure it.
• For example, how long is this block?

38. How big is the beetle?
Measure between the head
and the tail!
Between 1.5 and 1.6 in
Measured length: 1.54 in
The 1 and 5 are known with
certainty
The last digit (4) is estimated
between the two nearest fine
division marks.

39. Example: Precision
Who is more precise when measuring the same 17.0
cm book?
A:
17.0 cm, 16.0 cm, 18.0 cm, 15.0 cm
B ::
15.5 cm, 15.0 cm, 15.2 cm, 15.3 cm

40. Accuracy vs. Precision
High Accuracy
High Precision
High Precision
Low Accuracy

41. Three targets
with three
arrows each to
shoot.
The person hit the bull's-eye?
Both
accurate
and precise
Precise but
not
accurate
Neither
accurate
nor precise
How do they
compare?
Can you define accuracy vs. precision?Can you define accuracy vs. precision?

42. Uncertainty
• The word uncertainty casts a doubt about the
exactness of the measurement results
• True value = Estimated value + Uncertainty

43. Why Is There Uncertainty?
• Measurements are performed with instruments,
and no instrument can read to an infinite number of
decimal places
•Which of the instruments below has the greatest
uncertainty in measurement?

44. Reading a Meterstick
. l2
. . . . I . . . . I3
. . . .I . . . . I4
. . cm
First digit (known) = 2 2.?? cm
Second digit (known) = 0.7 2.7? cm
Third digit (estimated) between 0.05- 0.08 cm
Length reported = 2.77 cm
or 2.76 cm
or 2.78 cm

45. Known + Estimated Digits
In 2.77 cm…
• Known digitsKnown digits 22 andand 77 are 100% certainare 100% certain
• The third digitThe third digit 77 is estimated (uncertain)is estimated (uncertain)
• In the reported length, allIn the reported length, all threethree digitsdigits
(2.77 cm) are significant including the(2.77 cm) are significant including the
estimated oneestimated one

46. Performance of Instruments
• All instrumentation systems are characterized
by the system characteristics or system
response
• There are two basic characteristics of
Measuring instruments, they are
– Static character
– Dynamic character

47. Static Characteristics
• The instruments, which are used to measure
the quantities which are slowly varying with
time or mostly constant, i.e., do not vary with
time, is called ‘static characteristics’.

48. STATIC CHARACTERISTICS OF AN INSTRUMENTS
• Accuracy
• Precision
• Sensitivity
• Resolution
• Threshold
• Drift
• Error
• Repeatability
• Reproducibility
• Dead zone
• Backlash
• True value
• Hysteresis
• Linearity
• Range or Span
• Bias
• Tolerance
• Stability

49. Resolution
This is defined as the smallest input increment
change that gives some small but definite
numerical change in the output.

50. Threshold
This minimum value of input below which no
output can be appeared is known as threshold
of the instrument.
input
Output

51. Drift
Drift or Zero drift is variation in the output of
an instrument which is not caused by any
change in the input; it is commonly caused by
internal temperature changes and component
instability.
Sensitivity drift defines the amount by which
instrument’s sensitivity varies as ambient
conditions change.

52. input
Output
zero
drift
input
Output
sensitivity drift
input
Output
sensitivity drift
zero
drift

53. • Error – The deviation of the true value from
the desired value is called Error
• Repeatability – It is the closeness value of
same output for same input under same
operating condition
• Reproducibility - It is the closeness value of
same output for same input under same
operating condition over a period of time

54. Range
• The ‘Range’ is the total range of values which
an instrument is capable of measuring.

55. Hysteresis
This is the algebraic difference between the average
errors at corresponding points of measurement
when approached from opposite directions, i.e.
increasing as opposed to decreasing values of the
input.
Actual/ Input
Value
Measured
Value
Ideal
Hysteresis is
caused by
energy
storage/
dissipation in
the system.

56. Zero stability
The ability of the instrument to return to
zero reading after the measured has returned to
zero

57. Dead band
This is the range of different input values over
which there is no change in output value.

58. Linearity- The ability to reproduce the input
characteristics symmetrically and linearly

59. • Backlash – Lost motion or free play of
mechanical elements are known as backlash
• True value – The errorless value of measured
variable is known as true value
• Bias – The Constant Error
• Tolerance- Maximum Allowable error in
Measurement

60. Dynamic Characteristics
• The set of criteria defined for the
instruments, which are changes rapidly with
time, is called ‘dynamic characteristics’.

61. Dynamic Characteristics
• Steady state periodic
• Transient
• Speed of response
• Measuring lag
• Fidelity
• Dynamic error

62. • Steady state periodic – Magnitude has a
definite repeating time cycle
• Transient – Magnitude whose output does not
have definite repeating time cycle
• Speed of response- System responds to
changes in the measured quantity

63. • Measuring lag
– Retardation type :Begins immediately after the
change in measured quantity
– Time delay lag : Begins after a dead time after the
application of the input
• Fidelity- The degree to which a measurement
system indicates changes in the measured
quantity without error
• Dynamic error- Difference between the true
value of the quantity changing with time & the
value indicated by the measurement system

64. Errors in Instruments
• Error = True value – Measured value
or
• Error = Measured value - True value

65. Types of Errors
• Error of Measurement
• Instrumental error
• Error of observation
• Based on nature of errors
• Based on control

66. Error of Measurement
• Systematic error -Predictable way in
accordance due to conditions change
• Random error - Unpredictable manner
• Parasitic error - Incorrect execution of
measurement

67. Instrumental error
• Error of a physical measure
• Error of a measuring mechanism
• Error of indication of a measuring instrument
• Error due to temperature
• Error due to friction
• Error due to inertia

68. Error of observation
• Reading error
• Parallax error
• Interpolation error

69. Nature of Errors
• Systematic error
• Random error

70. Based on control
• Controllable errors
– Calibration errors
– Environmental (Ambient /Atmospheric Condition)
Errors
– Stylus pressure errors
– Avoidable errors
• Non - Controllable errors

71. Correction
• Correction is defined as a value which is added
algebraically to the uncorrected result of the
measurement to compensate to an assumed
systematic error.
• Ex : Vernier Caliper, Micrometer

72. Calibration
• Calibration is the process of determining and
adjusting an instruments accuracy to make
sure its accuracy is with in manufacturing
specifications.

73. Interchangeability
• A part which can be substituted for the
component manufactured to the small shape
and dimensions is known a interchangeable
part.
• The operation of substituting the part for
similar manufactured components of the
shape and dimensions is known as
interchangeability.

74. Compiled by
V. S. KumawatV. S. Kumawat
Assistant Professor
Department of Mechanical Engineering
Amity University, Rajasthan