Unit 1 Concept of Measurement Syllabus General concept – Generalized measurement system-Units and...
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Unit 1 Concept of Measurement
Unit 1 Concept of Measurement Syllabus General concept – Generalized measurement system-Units and standards-measuring instruments- sensitivity, readability,
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
Slide 4
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
Slide 5
Why measure things? Check quality? Check tolerances? Allow
statistical process control (SPC)?
Slide 6
Need of Measurement Establish standard Interchange ability
Customer Satisfaction Validate the design Physical parameter into
meaningful number True dimension Evaluate the Performance
Slide 7
Methods of Measurement Direct method Indirect method
Comparative method Coincidence method Contact method Deflection
method Complementary method
Slide 8
Direct method Measurements are directly obtained Ex: Vernier
Caliper, Scales
Slide 9
Indirect method Obtained by measuring other quantities Ex :
Weight = Length x Breadth x Height x Density
Slide 10
Comparative Method Its compared with other known value Ex:
Comparators
Slide 11
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
Slide 12
Complementary method The value of quantity to be measured is
combined with known value of the same quantity Ex:Volume
determination by liquid displacement
Slide 13
Deflection method The value to be measured is directly
indicated by a deflection of pointer Ex: Pressure Measurement
Slide 14
GENERALIZED MEASURING SYSTEM
Slide 15
Common elements of system Primary sensing element Variable
conversion element Variable manipulation element Data transmission
element Data processing element Data presentation element
Slide 16
Primary sensing element Variable conversion element Variable
manipulation element Data transmission element Data processing
element Data presentation element Temperature Observer
Slide 17
Units and standards
Slide 18
SI: fundamental Units Physical QuantityUnit NameSymbol
lengthmeterm masskilogramkg timeseconds electric currentampereA
temperatureKelvinK amount of substancemolemol luminous
intensitycandelacd
Slide 19
SI: Derived Units Physical QuantityUnit NameSymbol areasquare
meterm2m2 volumecubic meterm3m3 speed meter per second m/s
acceleration meter per second squared m/s 2 weight, forcenewtonN
pressurepascalPa energy, workjouleJ
Slide 20
Supplementary units Physical QuantityUnit NameSymbol Plane
angleRadian rad Solid angleSteradian sr
Slide 21
Standards International standards Primary standards Secondary
standards Working standards
Slide 22
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: [email protected] or
[email protected]@[email protected]
Slide 23
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.
Slide 24
DEFLECTION AND NULL TYPE Physical effect generated by the
measuring quantity Equivalent opposing effect to nullify the
physical effect caused by the quantity
Slide 25
ANALOG AND DIGITAL INSTRUMENTS Physical variables of interest
in the form of continuous or stepless variations Physical variables
are represented by digital quantities
Slide 26
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
Slide 27
Automatic and manually operated Manually operated requires the
service of human operator Automated doesn't requires human
operator
Slide 28
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
Slide 29
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
Slide 30
Intelligent instruments Microprocessors are incorporated with
measuring instruments
Slide 31
Help topics http://www.tresnainstrument.com/education. html
http://www.tresnainstrument.com/education. html
Slide 32
Characteristics of Measuring Instrument Sensitivity Readability
Range of accuracy Precision
Slide 33
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
Slide 34
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
Slide 35
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.
Slide 36
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.
Slide 37
Load Cell Force, F Output, V o Output, V o (V) Input, F i (kN)
Slope = 5 V/kN K Input, F (kN) Output, V o (V) Sensitivity, K = 5
V/kN Block Diagram:
Slide 38
Example (1) A 0.01 /A meter with 5 A fsd, R m = /A x A = 0.01 x
5 = 0.05 V max across the Meter will be = 5 A x 0.05 = 0.25 V for
fsd. (2) A 0.1 /A meter with 5 A fsd,will drop 2.5 V (i.e., it is
10 times less sensitive), which may bias the results
Slide 39
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
Slide 40
Readability What is the value ?
Slide 41
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
Slide 42
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
Slide 43
Precision The precision of a measurement depends on the
instrument used to measure it. For example, how long is this
block?
Slide 44
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.
Slide 45
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
Slide 46
Accuracy vs. Precision High Accuracy High Precision Low
Accuracy
Slide 47
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?
Slide 48
Uncertainty The word uncertainty casts a doubt about the
exactness of the measurement results True value = Estimated value +
Uncertainty
Slide 49
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?
Slide 50
Reading a Meterstick. l 2.... I.... I 3....I.... I 4.. 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
or2.76 cm or2.78 cm
Slide 51
Known + Estimated Digits In 2.77 cm Known digits 2 and 7 are
100% certain Known digits 2 and 7 are 100% certain The third digit
7 is estimated (uncertain) The third digit 7 is estimated
(uncertain) In the reported length, all three digits (2.77 cm) are
significant including the estimated one In the reported length, all
three digits (2.77 cm) are significant including the estimated
one
Slide 52
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
Slide 53
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.
Slide 54
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
Slide 55
Resolution This is defined as the smallest input increment
change that gives some small but definite numerical change in the
output.
Slide 56
Threshold This minimum value of input below which no output can
be appeared is known as threshold of the instrument. input
Output
Slide 57
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
instruments sensitivity varies as ambient conditions change.
Slide 58
input Output zero drift input Output sensitivity drift input
Output sensitivity drift zero drift
Slide 59
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
Slide 60
Range The Range is the total range of values which an
instrument is capable of measuring.
Slide 61
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.
Slide 62
Zero stability The ability of the instrument to return to zero
reading after the measured has returned to zero
Slide 63
Dead band This is the range of different input values over
which there is no change in output value.
Slide 64
Linearity- The ability to reproduce the input characteristics
symmetrically and linearly
Slide 65
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
Slide 66
Dynamic Characteristics The set of criteria defined for the
instruments, which are changes rapidly with time, is called dynamic
characteristics.
Slide 67
Dynamic Characteristics Steady state periodic Transient Speed
of response Measuring lag Fidelity Dynamic error
Slide 68
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
Slide 69
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
Slide 70
Errors in Instruments Error = True value Measured value or
Error = Measured value - True value
Slide 71
Types of Errors Error of Measurement Instrumental error Error
of observation Based on nature of errors Based on control
Slide 72
Error of Measurement Systematic error -Predictable way in
accordance due to conditions change Random error - Unpredictable
manner Parasitic error - Incorrect execution of measurement
Slide 73
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
Slide 74
Error of observation Reading error Parallax error Interpolation
error
Slide 75
Nature of Errors Systematic error Random error
Slide 76
Based on control Controllable errors Calibration errors
Environmental (Ambient /Atmospheric Condition) Errors Stylus
pressure errors Avoidable errors Non - Controllable errors
Slide 77
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
Slide 78
Calibration Calibration is the process of determining and
adjusting an instruments accuracy to make sure its accuracy is with
in manufacturing specifications.
Slide 79
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.
Slide 80
Compiled byN.SRILAKSHMANAN Assistant Professor Department of
Mechanical Engineering SNSCE