1. Unit 1 Concept of Measurement

2. Syllabus
General concept – Generalized measurement system-Units and standards-measuring instruments- sensitivity, readability, range of accuracy, precision-static and dynamic response repeatability- systematic and random errors-correction, calibration, interchangeability

3. 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

4. Why measure things? QUALITY CAN NOT BE MEASURED INTO A PRODUCT!!!!
Check quality?
Check tolerances?
Allow statistical process control (SPC)?
QUALITY CAN NOT BE MEASURED
INTO A PRODUCT!!!!

5. Need of Measurement Establish standard Interchange ability
Customer Satisfaction
Validate the design
Physical parameter into meaningful number
True dimension
Evaluate the Performance

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

7. Direct method Measurements are directly obtained
 Ex: Vernier Caliper, Scales

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

9. Comparative Method It’s compared with other known value
 Ex: Comparators

10. 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

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

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

13. Generalized measuring system

14. Common elements of system
Primary sensing element
Variable conversion element
Variable manipulation element
Data transmission element
Data processing element
Data presentation element

15. Primary sensing element Variable conversion element
Variable manipulation element
Temperature
Data transmission element
Observer
Data presentation element
Data processing element

16. Units and standards

17. SI: fundamental Units length meter m mass kilogram kg time second s
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

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

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

20. Standards International standards Primary standards
Secondary standards
Working standards

21. 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 India Phone:  Fax: 

22. 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.

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

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

25. 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

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

27. 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

28. 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
A galvanometer is a type of ammeter: an instrument for detecting and measuring electric current. It is an analog electromechanical transducer that produces a rotary deflection of some type of pointer in response to electric current flowing through its coil in a magnetic field.

29. Intelligent instruments
Microprocessors are incorporated with measuring instruments

30. Help topics

31. Characteristics of Measuring Instrument
Sensitivity
Readability
Range of accuracy
Precision

32. 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

33. 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

34. 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.

35. 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.

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

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

38. 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

39. Readability
What is the value ?

40. 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

41. 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

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

43. 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.

44. 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

45. Accuracy vs. Precision High Accuracy High Precision High Precision
Low Accuracy

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

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

48. 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?

49. Reading a Meterstick
. l I I I I cm
First digit (known) = ?? cm
Second digit (known) = ? cm
Third digit (estimated) between cm
Length reported = cm
or cm
or cm

50. Known + Estimated Digits
In 2.77 cm…
Known digits 2 and 7 are 100% certain
The third digit 7 is estimated (uncertain)
In the reported length, all three digits (2.77 cm) are significant including the estimated one

51. 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

52. 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’.

53. 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

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

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

56. 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.

57. Output
Output
sensitivity drift
zero drift
input
input
sensitivity drift
Output
zero drift
input

58. 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

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

60. 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.

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

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

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

64. 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

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

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

67. 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

68. 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

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

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

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

72. 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

73. Error of observation
Reading error
Parallax error
Interpolation error

74. Nature of Errors
Systematic error
Random error

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

76. 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

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

78. 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.

79. Thank you N.SRILAKSHMANAN Assistant Professor
Compiled by
N.SRILAKSHMANAN
Assistant Professor
Department of Mechanical Engineering
SNSCE