4. • Primary Sensing Element: first element to
receive energy from measurand and produces
an output which is converted into analogous
electrical signal by transducer.
• Variable Conversion Element: Converts output
of primary (may be V, Freq.,..) into suitable form
without changing information content. This is
optional.
• Variable manipulation Element: To manipulate
the signal, preserving its original nature.
Amplifies input signal to required magnification.
This element may also precede VCE.
5. • Data Transmission Element: Transmits data from one
element to other. Eg. shaft & gear.
• Data Processing Element: To modify the data before
displayed. Used for-
1. converting data into useful form.
2. separating the signal hidden in noise.
3. provides corrections to measurand.
• Data Presentation Element: Final element to
communicate with observer for monitoring, control,
analysis,.. Value of measured variable indicated by-
1. analog indicator
2. digital indicator
3. recorder
8. • Primary sensing element & VCE –
Temperature Bulb – senses the temperature (i/p)
and converts into pressure.
• Pressure is send to DTE (Data Transmission) –
Capillary tube.
• Bourdon gauge – VCE (converts pressure into
displacement).
• VME-linkage and gear – displacement is
manipulated to get larger pointer deflection.
• DPE (Data Presentation) – Pointer and scale –
to present the output to observer.
10. UNITS
• To specify and perform calculation with physical quantity,
the physical quantities must be defined both in kind and
magnitude.
• UNIT is the std. measuring of each kind of physical
quantity.
• Eg. CGS & MKS systems are different systems of units.
• SI - International System of units was established by
General Conference of Weights and Measures - CGPM
(Conference General des Poids et Measures)
• SI - System International Units- is the generally
accepted one – based on m,kg,s,A,K,Cd.
11. • SI is based on decimal Arithmetic.
• Hence this is of great advantage and is
simple.
• Three types are:
1. Fundamental units
2. Supplementary units
3. Derived Units
12. 1. FUNDAMENTAL UNITS (BASE UNIT):
• Independently chosen, not dependent on other units.
• Length (m), Mass (kg), Time (s), Temp. (K), Electric current (A),
Luminous intensity (Cd)
• metre – It is the length equal to 1 650 763 73 wavelength of light
emitted in vacuum by the atom of Krypton-86 on its transition
between the levels 2p10 and 5d5.
• In 1982, the definition of the meter was changed to the distance
light travels in 1/299,792,458ths of a second. For the
measurement, light from a helium-neon laser illuminates iodine
which fluoresences at a highly stable frequency.
• kilogram - The standard kilogram is defined in terms of platinum-
iridium mass maintained at very accurate conditions at the
International Bureau of Weights and Measures in Sevres, France.
13. • second - The second is the duration of 9 192
631 770 periods of the radiation corresponding
to the transition between the two hyperfine levels
of the ground state of the cesium-133 atom.
• ampere - The ampere is that constant current
which, if maintained in two straight parallel
conductors of infinite length, of negligible cross
section, and placed 1 metre apart in a vacuum,
would produce between these conductors a
force equal to 2 x 10-7 newton per metre of
length.
14. • kelvin - The kelvin is the fraction 1/273.16
of the thermodynamic temperature of the
triple point of water.
• candela - The candela is the luminous
intensity in the perpendicular direction, of
a surface 1/600 000m2 of a black body at
the temperature of freezing platinum under
a pressure of 101 325 N.
15. 2. SUPPLEMENTARY UNITS
• Plane angle – radian –rad
• Solid angle – steradian –sr
• radian - The radian is the plane angle
subtended at the centre of an arc of unit length
at unit radius.
• steradian - The steradian is the solid angle
subtended at the centre by unit area of spherical
surface at unit radius.
16. 3. DERIVED UNITS:
• Expressed in terms of fundamental and supplementary
units by defining units.
• There are 3 categories. They are:
1. Mechanical units – units for force, pressure, stress,
weight, torque, acceleration, velocity, density.
2. Electrical & Magnetic units- units for power, energy,
resistance, electric field strength, capacitance,
magnetic field strength, magnetic flux density.
3. Thermal Units- units for specific heat capacity, latent
heat, sensible heat.
17. STANDARDS
• Physical representation of unit of measurement.
• Known accurate measure of physical quantity.
• Standards are used to determine the values of
physical quantity by comparison method.
• Depending on functions and applications, different
types of standards are:
1. International
2. Primary
3. Secondary
4. Working
18. INTERNATIONAL STANDARDS
• Defined by international agreement.
• Periodically evaluated & checked.
• Represents units to closest possible accuracy.
• Not available for ordinary usage like measurement.
• PRIMARY STANDARDS
• Main function is calibration & verification of secondary standards.
• Primary standard is maintained at National Standard Laboratory.
• For India, National Physical Lab is at Delhi.
• This standard is ultimate reference standard with more accuracy.
19. • SECONDARY STANDARDS
• Basic reference standard maintained by particular
industry.
• Each lab sends its own secondary standards to national
lab for calibration & comparison against primary
standard.
• WORKING STANDARDS
• Main tools of a lab.
• Used to check & calibrate lab instruments for accuracy &
performance.
• Eg. plug gauge is used to check bore diameter of
bearings.
21. MEASURING INSTRUMENTS
• There are a large no. of instruments.
• Eg. time constant measurement – weight measurement
• Time varying measurement – pressure measurement.
• Output of time varying variables cannot be read on scale
& pointer.
• Thus, there are many instruments with different
principles.
22. CLASSIFICATION OF INSTRUMENTS
BASED ON APPLICATION AND MODE OF
OPERATION
1. Deflection & Null type instrument
2. Analog and Digital instrument
3. Active & passive type instrument
4. Automatic & Manually operated
instrument
5. Absolute & Secondary instrument
6. Contacting & Non-contacting instrument
7. Intelligent instrument
23. 1.DEFLECTION AND NULL TYPE
INSTRUMENT
• In deflection type, measured quantity generates the
effect and hence the deflection of pointer occurs. Eg.
weight measurement by Spring balance.
• In Null type, nullifying effect causes measurement of
weights. Eg. Beam balance.
• Null type is more accurate.
• In observer view, deflection type is more convenient.
• Sensitivity of null type is high as null pointer covers small
range around null point.
25. 2. ANALOG AND DIGITAL
INSTRUMENTS
• Analog instruments - output varies in continuous manner,
takes infinite values in a given range. Eg. pressure gauge
• Digital instruments - output varies in a discrete manner,
takes finite values in a given range.
• ADVANTAGES OF DIGITAL INSTRUMENTS:
Direct & precise readings
Digital signals are noise resistant during transmission.
Digital circuits operate on low voltage and suitable for digital
computer processing.
26. 3. ACTIVE AND PASSIVE TYPE
INSTRUMENTS
• ACTIVE:
Quantity being measured activates the
magnitude of some external power input
source to produce measurement.
Apart from measuring quantity, external
energy input source is present.
Eg. liquid level indicator.
27. • PASSIVE
Output produced entirely by quantity being
measured. Eg. Pressure gauge.
Resolution is less and cannot be increased
easily.
In active, resolution is controlled by adjusting
magnitude of external energy input.
29. ACTIVE:
• Resolution controlled.
• Design is complicated, hence costlier.
PASSIVE:
• No control over resolution.
• Design is easy, hence cheaper.
30. 4. AUTOMATIC AND MANUALLY
OPERATED INSTRUMENT
• In manual, human service is required.
• In automatic, auxiliary devices are
incorporated in instrument to dispense
with human operator. Eg. electronic
weighing scale.
31. 5. ABSOLUTE AND SECONDARY
INSTRUMENTS
• ABSOLUTE
Gives value of measured quantity in terms of constant of
instrument and their deflections only. Eg. Tangent
Galvanometer.
This is rarely used.
• SECONDARY
Pre calibrated by comparison with absolute instruments.
Output is determined by deflection of instruments.
Without calibration of instrument, deflection is
meaningless. This is widely used.
32. 6.CONTACTING AND NON
CONTACTING INSTRUMENTS
• CONTACTING
Instrument is in contact with measuring medium. Eg.
Thermometer
Instruments are placed in electrical circuit.
• NON CONTACTING
Instrument is not in contact. Eg. pyrometer
Measurement at distance.
33. 7. INTELLIGENT INSTRUMENTS
• Incorporates a microprocessor.
• Microprocessor based instrument facilitates programmed
signal processing and application of data manipulation
algorithm to measured variables.
• FACILITIES:
Selection of time constants.
Linearising non-linear output.
Remote can be used to operate instruments
Final output is in desired form and units.
Programming is possible to correct the output.
35. SENSITIVITY
• Sensitivity is the ratio of Infinitesimal change of output
signal to Infinitesimal change of input signal.
• Static sensitivity is represented by the slope of the
calibration curve.
36. • LINEAR:
Sensitivity will be constant for all values of
input.
• NON LINEAR:
Sensitivity depends on value of input
quality.
37. • Sensitivity has no unique unit.
• It has wide range of units depending upon
instrument.
• Sensitivity should be as high as possible,
for this range instrument should not greatly
exceed value to be measured.
38. • When instrument consists of different elements
connected in series with static sensitivities K1, K2, K3,…
overall sensitivity (K) is given as:
40. READABILITY
• It is the closeness with which the scale of an analog
instrument can be read.
• Eg. 30 cm scale span has higher readability than 15cm
scale span in deflection type weighing scale.
• Term used frequently in analog measurements.
• Depends on instrument & observer.
• For better readability, pointer end should be sharp and
pointer should be larger.
• Parallax effect should be minimized.
42. RANGE OF ACCURACY
• Ability of an instrument to respond to a true value of a
measured variable under reference conditions.
• It shows how closely measured value agrees with a true
value.
• Accuracy of an instrument can be specified in the
following ways:
1. Accuracy As Percentage Of Full Scale Reading
2. Accuracy As Percentage Of True Value
3. Accuracy As Percentage Of Scale Span
47. PRECISION
• Degree of exactness for which instrument is designed
to perform.
• Refers to repeatability/consistency of measurement
when measurements are done in identical conditions at
short time.
• It is the ability of an instrument to reproduce a group of
measurements of same measured quantity under same
conditions
• The two characteristics of precision are:
1. Conformity
2. Significant Figures
48. 1. CONFORMITY
• Eg. resistance of resistor = 2834267 ohms,
which indicates 2.8 M ohms in scale.
• Though no deviations, error by limitation of
scale reading is Precision error.
• Thus conformity is necessary, but this is not
sufficient condition for precision.
• Similarly, precision necessary, but not sufficient
condition for accuracy.
49. 2. SIGNIFICANT FIGURE
• Conveys actual information regarding magnitude
and measurement precision of quantity.
• Precision depends on no. of significant figures in
which reading is expressed.
• Significant figure = no. of digits for measuring
output.
• Eg. 240V should be closer to 239V or 241V.
Thus there are three significant figures.
52. ACCURACY vs PRECISION
• Many times, accuracy & precision used
interchangeably.
• Eg. for known pressure value 100MPa, six
readings are 103,104,102,103,102,104 Mpa.
Average = 103 MPa.
• Maximum deviation = +/- 1Mpa in 100 Mpa.
• Thus scale calibrated to read +/- 1Mpa .
53. • Accuracy =
• Thus precision is +/- 1%, Accuracy = 4%.
• Thus there is high degree of precision, but does not
guarantee accuracy.
56. STATIC RESPONSE
• The static characteristics are used to measure
unvarying process conditions.
• Static characteristics are obtained by Calibration.
• The related definitions are: accuracy, precision,
repeatability, reproducibility, sensitivity, drift,…
1. Accuracy:
The degree of closeness of a measurement
compared to the expected value is known as
accuracy.
61. DYNAMIC RESPONSE
• Dynamic response is the behavior of an
instrument under such time varying input-
output conditions.
• Dynamic analysis is the analysis of
dynamic response.
62. Dynamic quantity
• The two types are:
1. Steady state periodic
2. Transient
Steady state periodic
• Output whose magnitude has definite repeating time
cycle.
Transient
• Output whose magnitude does not repeat with time.
• No. of parameters required to define the dynamic
behavior of any instrument is decided by the group to
which that system belongs.
• Systems may be Zero order, First order, Second order
or Higher order.
64. REPEATABILITY
• Closeness of agreement among number of
consecutive measurements of output for
same value of input under same operating
conditions.
• Specified in terms of units for a given
period of time.
66. REPRODUCIBILITY
• Closeness of agreement among the
repeated measurements of output for
same value of input under same operating
conditions over period of time.
• Both reproducibility & repeatability are
measure of closeness with which given
input may be measured again & again.
67. • Reproducibility is defined by stability and constancy.
• STABILITY
Reproducibility of mean reading of an instrument
repeated on different occasions separated by intervals
of time which are long enough as compared to the time
of taking readings.
• CONSTANCY
Reproducibility of mean reading of an instrument
when constant input is presented continuously and
the conditions of test are allowed to vary within
specified limits.
69. ERRORS IN MEASUREMENT
• Error = Measured value - True value
• Errors are basically classified into two:
1. Bias or systematic errors
2. Precision or Random errors
70.
71.
72.
73.
74. TYPES OF ERROR
ERROR
STATIC LOADING DYNAMIC
CHARACTERISTIC READING ENVIRONMENT
SYSTEMATIC RANDOM
CALIBRATION AMBIENT AVOIDABLE STYLUS PRESSURE
77. CORRECTION
• Value which is added algebraically to the
uncorrected result of measurement to
compensate for assumed systematic error.
• If numerical value is multiplied with
uncorrected result to compensate for error,
it is called Correction Factor.
79. INTERCHANGEABILITY
• Part substituted for manufactured component to
same shape & dimensions – Interchangeable
part.
• Operation of substituting part –
Interchangeability.
• Because of this Interchangeability, production is
increased.
• Nowadays Interchangeable parts are called
Spare parts.
80. ADVANTAGES OF INTERCHANGEABILITY
• Easy replacement of worn out parts
• Repair is easy
• Maintenance cost is less
• Machine shutdown is decreased.
81. TYPES OF INTERCHANGEABILITY
1. UNIVERSAL / FULL INTERCHANGEABILITY
– any component mat with other component
without any alteration.
2. SELECTIVE ASSEMBLY
– selective parts are interchanged.
