Intze Overhead Water Tank Design by Working Stress - IS Method.pdf
Classification of Methods of Measurement .pdf
1. 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
2. Need of Measurement
• Establish standard
• Interchangeability
• Customer Satisfaction
• Validate the design
• Physical parameter into meaningful
number
• True dimension
• Evaluate the Performance
7. 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
8. Complementary method
• The value of quantity to be measured
is combined with known value of the
same quantity
– Ex:Volume determination by liquid
displacement
9. Deflection method
• The value to be measured is directly
indicated by a deflection of pointer
– Ex: Pressure Measurement
10. • Primary sensing element
• Variable conversion element
• Variable manipulation element
• Data transmission element
• Data processing element
• Data presentation element
Common elements of
Generalized measuring
system
12. Units and standards
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
13. 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
16. 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.
17. DEFLECTION AND NULL TYPE
• Physical effect generated by the
measuring quantity
• Equivalent opposing effect to nullify the
physical effect caused by the quantity
18. ANALOG AND DIGITAL
INSTRUMENTS
• Physical variables of interest in the
form of continuous or stepless
variations
• Physical variables are represented by
digital quantities
19. 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
20. Automatic and manually
operated
• Manually operated – requires the
service of human operator
• Automated – doesn't requires human
operator
21. 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
22. 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. 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
26. 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
27. 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.
28. 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.
29. 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
30. 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
31. Precision
• The precision of a measurement depends on
the instrument used to measure it.
• For example, how long is this block?
33. Uncertainty
• The word uncertainty casts a doubt
about the exactness of the
measurement results
• True value = Estimated value +
Uncertainty
34. 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
35. 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’.
36. 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
37. Resolution
This is defined as the smallest input
increment change that gives some small but
definite numerical change in the output.
38. Threshold
This minimum value of input below which no
output can be appeared is known as threshold
of the instrument.
input
Output
39. 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.
40. • 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
41. Range
• The ‘Range’ is the total range of
values which an instrument is
capable of measuring.
42. 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.
43. Zero stability
The ability of the instrument to return to
zero reading after the measured has returned to
zero
44. Dead band
This is the range of different input values over
which there is no change in output value.
45. Linearity- The ability to reproduce the input
characteristics symmetrically and linearly
46. • 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
47. Dynamic Characteristics
• The set of criteria defined for the
instruments, which changes rapidly with
time, is called ‘dynamic characteristics’.
49. • 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
50. • 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
51. Errors in Instruments
• Error = True value – Measured value
or
• Error = Measured value - True value
52. Types of Errors
• Error of Measurement
• Instrumental error
• Error of observation
• Based on nature of errors
• Based on control
53. Error of Measurement
• Systematic error -Predictable way in
accordance due to conditions change
• Random error - Unpredictable manner
• Parasitic error - Incorrect execution of
measurement
54. 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
57. Based on control
• Controllable errors
– Calibration errors
– Environmental (Ambient /Atmospheric
Condition) Errors
– Stylus pressure errors
– Avoidable errors
• Non - Controllable errors
58. 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
59. Calibration
• Calibration is the process of
determining and adjusting an
instruments accuracy to make sure
its accuracy is with in manufacturing
specifications.
60. 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.
