introduction to metrology

1. INTRODUCTION TO METROLOGY
Yogesh Kumar Sharma
Assistant Professor
GNA University, Phagwara

2. DEFINITIONS
Metrology is the study of measurements
Measurements are quantitative
observations; numerical descriptions

3. WE WANT TO MAKE GOOD MEASUREMENTS
Making measurements is woven throughout
daily life in a lab.
Often take measurements for granted, but
measurements must be “good”.
What is a “good” measurement?

4. EXAMPLE
A man weighs himself in the morning on his
bathroom scale, 72 Kg.
Later, he weighs himself at the gym, 73 Kg.

5. QUESTIONS???
How much does he really weigh?

6. Do you trust one or other scale? Which
one? Could both be wrong? Do you
think he actually gained a pound?

7. Are these “good measurements”?

8. NOT SURE!! Hmmm
We are not exactly certain of the man’s true
weight because:
❑ Maybe his weight really did change – always
sample issues
❑ Maybe one or both scales are wrong – always
instrument issues

9. DO WE REALLY CARE?
Do you care if he really gained weight?
How many think “give or take” weight is OK?

10. ANOTHER EXAMPLE
Suppose a premature baby is weighed.
The weight is recorded as 5 pounds 8
ounces and the baby is sent home.
Do we care if the scale is off by a pound?

11. MEASUREMENT
Act, or result of a quantitative comparison between
a predetermined standard and an unknown
magnitude.
“whatever exists, exists in some amount”
Scope
Mechanical
M,L,T,
Pressure,
Temp,
Electrical/
Electronics
Transducing
into analogous
electrical qty.

12. “GOOD” MEASUREMENTS
• Standard employed for comparison
accurately defined and commonly accepted
• The apparatus used and method adopted for
comparison must be provable
• Standard must be prescribed, and described
by legal or recognized authority (IBS, ISO)
Signal
Analog
Digital
Low/no noise
problem
Simple data
transmission
Direct display

13. INSTRUMENTATION
Technology of using instruments to measure/control
physical/ chemical properties
STANDARDS ARE:
Physical objects, the properties of which are
known with sufficient accuracy to be used to
evaluate other items.

14. STANDARS OF MEASUREMENTS
1. Line Standard: When length is measured as
the distance between center of two engraved
lines. eg: Yard, meter
a) Imperial Standard Yard: distance between two
center transverse lines on the plugs when temp 62° F

15. STANDARS OF MEASUREMENTS
b) International Standard meter: International
Bureau of weight and measurement. 102 cm length.
Distance between center portion two engraved lines 0°C
Conversion Factors:
1 meter = 39.370113 “
1 Yard = 0.9144m =3 feet
1 inch = 25.399978 mm

16. Easy to Use over wide range
Rapid results
Not accurate due to thickness of
engraved lines
Not convenient for close tolerance
Problem of alignment due to absence
of built in datum
CHARACTERISTICS OF LINE STANDARD

17. Working Standards
Secondary Standards
Primary Standards
HIERARCHY OF MEASUREMENTS or
SUB-DIVISION OF LINE STANDARD
Instruments Used in
Lab/Industry

18. Primary
Standard
Highest Standard
Copies of international
prototypes kept
throughout the world
national std
labs/institutes
Verification and
calibration of
secondary standard
Secondary
Standard
Reference calibrated
standard designed &
calibrated from
primary standard
Kept by measurement
labs/ institutes to
check and calibrate
the general tool.
Working
Standard
Low Accuracy then
secondary
Used by worker who
carry out the
measurements

19. END STANDARD
Length is expressed as distance between two flat parallel
faces. Eg. Varnier calliper, micrometer, slip gauges.
Very accurate, used for precise measurement
Tolerance up-to 0.0005 mm
Not subjected to parallax error
Subjected to wear on their measuring faces, must carefully handling
Time consuming

21. WAVELENGTH STANDARD
Sir Jacques Babinet 1829 suggested wavelength of monochromatic can be
used natural and invariable unit of length.
1960 orange radiation of isotope Kr-86 chosen for define length . 1 meter
length equal to 1650763.73 wavelength of red orange radiation of Kr-86
1983, meter may be defined as path travel by light in vacuum in 0.9144/299792458
sec i.e 3x10-9 sec.
Not changed with variation of environmental condition
Reproducible easily, available with all time
No need to preserve, no fear of destruction
Easily available
Error of reproduction is very less

22. 22
Units of Measurement
SI Units published by BIPM(Bureau of Weights and Measures)
Base Units…
Quantity Unit Symbol
Length metre m
Mass kilogram kg
Time second s
Temperature kelvin K
Electric current ampere A
Luminous intensity candela cd
Amount of substance mole mol

23. STANDARDS ARE AFFECTED BY THE
ENVIRONMENT
Units are unaffected by the
environment, but standards are
❑ Example, Pharaoh’s arm length might
change
❑ Example, a ruler is a physical
embodiment of centimeters
◼ Can change with temperature
◼ But cm doesn’t change

24. Methods of Measurement
Direct
Comparison
Method
In-direct
Comparison
Method
Modes of Measurement
Primary
Measurement
Secondary
Measurement
Tertiary
Measurement

34. Physical System Sensor Transducer
Manipulator
Data Presentation
• Controller
• Indicator
• Recorder
Measurement System

35. Instrumentation
Absolute and secondary
Analog and digital
Mechanical/
Electrical/Electronics
Manual/ Automatic
Self contained/remote
indicating
Self operated/ power operated
Deflection/null output
Classification of Instrumentation

36. FACTORS FOR SELECTION OF INSTRUMENTS
▪ Accuracy
▪ When final data required (time taken)
▪ Cost
▪ In what form data displayed (Indicating,
Recording)
▪ Whether quantity constant/ Time variant
“Never demand an accuracy of measurement
higher than that which is needed, and never forget
that each degree of accuracy is likely to have
disproportionate effect on complexity and cost of
apparatus”

37. FUNCTION OF INSTRUMENTS
• Indicating Function
• Recording Function
• Controlling Function
APPLICATION OF MEASUREMENT
• Monitoring of Process/Operation
• Control of Process/Operation
• Experimental engineering analysis

38. Definition related to measuring instruments
• True or Actual Value
• Indicated value
• Range
• Sensitivity
• Repeatability
• Hysteresis
• Response Time
• Calibration
• Uncertainty of measurement
• Interchangeability a) Universal b) Local
• Magnification:

39. TOLERANCE IS:
Amount of error that is allowed in the
calibration of a particular item. National and
international standards specify tolerances.
Standards for balance calibration can have slight
variation from “true” value
❑ Highest quality 100 g standards have a tolerance
of + 2.5 mg
❑ 99.99975-100.00025 g
❑ Leads to uncertainty in all weight measurements

40. ACCURACY AND PRECISION
Accuracy is how close an individual value is to the true or
accepted value, refer to single measurement.
Precision is the consistency of a series of measurements

45. ERROR IS:
Error is responsible for the difference between a
measured value and the “true” value
% error or Relative error = True value – measured value X 100%
True value
Absolute error = True value – measured value

46. CATEGORIES
OF ERRORS
Gross Errors
Systematic
Errors
Instrumental
Environmental
Observational
Random
Errors

47. GROSS ERROR
Blunders

48. SYSTEMATIC ERROR
• Are repeated consistently with repetition of
experiment, Controllable in Nature
• Many causes, contaminated solutions, malfunctioning
instruments, temperature fluctuations, etc.
Instrumental Error
• Shortcoming in instrument
• Misuse
• Loading effect
Environmental Error
Observational
Error
• Parallax
• Wrong scale reading
• Inaccurate estimate of average reading
• Tendency to read high or low

49. SYSTEMATIC ERROR
Technician controls sources of
systematic error and should try to
eliminate them, if possible
❑ Temperature effects
❑ Humidity effects
❑ Calibration of instruments
❑ Etc.

51. RANDOM ERROR
In U.S., weigh particular 10 g standard
every day. They see:
❑ 9.999590 g, 9.999601 g, 9.999592 g
….
What do you think about this?

52. RANDOM ERROR
Variability
No one knows why
They correct for humidity, barometric pressure,
temperature
Error that cannot be eliminated. Called “random
error”
Do you think that repeating the measurement over
and over would allow us to be more certain of the
“true” weight of this standard?

53. RANDOM ERROR
Yes, because in the presence of only
random error, the mean is more likely to be
correct if repeat the measurement many
times

54. THERE IS ALWAYS
RANDOM ERROR
If can’t see it, system isn’t sensitive
enough
Less sensitive balance: 10.00 g,
10.00 g, 10.00 g
Versus 9.999600 g…

55. SO…
Can we ever be positive of true weight
of that standard?
No
There is uncertainty in every weight
measurement

56. SOURCES OF ERRORS
• NOISE
• RESPONSE TIME
• DESIGN LIMITATION
• TRANSMISSION
• DETERIORATION OF MEASURING
SYSTEM
• AMBIENT INFLUENCES
• ERROR IN OBSERVATION
• METHOD OF LOCATION OF INSTRUMENT

60. • SINGLE SAMPLE TEST
• MULTI SAMPLE TEST
STATISTICAL
ANALYSIS OF DATA
• ARITHMETIC MEAN
• GEOMETRIC MEAN
• MEDIAN
• MODE
STATISTICAL
AVERAGE
• DEVIATION: Departure of observed
reading from AM of group of reading
• AVERAGE DEVIATION: Sum of absolute
values of deviation divided by no. of
reading
• STANDARD DEVIATION: square root of
the sum of individual deviation squared
divided by no. of reading
• VARIENCE: square of Standard Deviation
DISPERSION
FROM MEAN

65. Mean
Median
Mode

66. NORMAL OR GAUSSIAN CURVE OF ERROR

67. EXPRESS PRECISION
Standard deviation
❑ Expression of variability
❑ Take the mean (average)
❑ Calculate how much each
measurement deviates from mean
❑ Take an average of the deviation, so it
is the average deviation from the
mean

68. NORMAL DISTRIBUTION CURVE

69. Match these descriptions with the 4 distributions in
the figure:
Good precision, poor accuracy
Good accuracy, poor precision
Good accuracy, good precision
Poor accuracy, poor precision

70. METROLOGISTS
Metrologists try to figure out all the possible
sources of uncertainty and estimate their
magnitude
One or another factor may be more
significant. For example, when measuring
very short lengths with micrometers, care a
lot about repeatability. But, with
measurements of longer lengths,
temperature effects are far more important

71. ROUNDING
A Biotechnology company specifies that
the level of RNA impurities in a certain
product must be less than or equal to
0.02%. If the level of RNA in a particular
lot is 0.024%, does that lot meet the
specifications?