U5MEA08-ENGG METROLOGY

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U5MEA08-ENGG METROLOGY INSTRUMENTATION
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• UNIT- I
• METROLOGY
• UNITS AND MEASUREMENTS

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Metrology. Metrology defines as the
Science of pure measurement. But in engineering
purposes, it in restricted to measurements of
length and angles and other qualities which are
expressed in linear or angular terms. 
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Units and Standards

• Units of Measurement
• C.G.S. System of Units
• Centimeter Gram Second system of unit
• M.K.S. System of Units
• Meter kilogram second system of units
• International System (SI) of Units
• the meter (m), kilogram (k), second (s), and
  ampere (A) of the MKSA system and, in addition,
  the Kelvin (K) and the candela (cd) as the units
  of temperature and luminous

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Terminology in instrumentation 

• Precision ? Degree of repetitiveness. If an
  instrument is not precise it will give different
  results for the same dimension for the repeated
  readings.
• Accuracy ? The maximum amount by which the result
  differ from true value(ie) Closeness to true value

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• Calibration
• is the process of establishing the relationship
  between a measuring device and the units of
  measure. This is done by comparing a devise or
  the output of an instrument to a standard having
  known measurement characteristics.
• Sensitivity
• It is ratio between output signal to input signal

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• Readability is a measure of an instrument's
  ability to display incremental changes in its
  output value.
• True size ? Theoretical size of a dimension which
  is free from errors.
• Actual size ? size obtained through measurement
  with permissible error

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• Repeatability is the variation in measurements
  taken by a single person or instrument on the
  same item and under the same conditions. A
  measurement may be said to be repeatable when
  this variation is smaller than some agreed limit.
• Reproducibility is one of the main principles of
  the scientific method, and refers to the ability
  of a test or experiment to be accurately
  reproduced, or replicated, by someone else
  working independently.

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• Methods of measurement.
•   1. Direct Method
• 2. Indirect Method
• 3. Comparison Method
• 4. Coincidence Method.
• Classification of measuring instruments.
•   1. Angle measuring instruments
• 2. Length measuring instruments
• 3. Instruments for surface finish
• 4. Instruments for deviations.

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Sources of error 

• Controllable Errors-
• Calibration Errors ,ambient Conditions , Stylus
  pressure, avoidable errors
• Random Errors
• These occur randomly and the specific causes of
  such errors cannot be determined, but likely
  sources of this type of error are small
  variations in the position of setting standards
  and workpiece, slight displacement of lever
  joints in the measuring joints in the measuring
  instrument,

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• Parallax Error
• On most dials the indicating finger or pointer
  lies in a plane parallel to the scale but
  displaced a small distance away to allow free
  movement of the pointer. It is then essential to
  observe the pointer along a line normal to the
  scale otherwise a reading error will occur.

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Line and End standard measurements

• Line standard
• Length is expressed as the distance between two
  lines.
• End standard
• Length is expressed as the distance between two
  flat parallel faces

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Linear measuring instruments  

• Straight edge.
• Outside caliper.
• Inside caliper.
• Vernier caliper
• Screw gauge
• vernier height gauge
• vernier depth gauge
• Dial gauges

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Comparators

• Classification of comparators
• Mechanical
• Electrical and Electronics comparators
• Optical comparators
• Pneumatic comparators
• Fluid displacement comparators
• Projection comparators.
• Multi check comparators
• Automatic Gauging Machines
• Electro-Mech. Comparators.

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. Classification of measuring Instruments.

• According to the functions
•  Length measuring instrument
• Angle measuring instrument
• Instrument for checking deviation from
  geometrical forms
• Instrument for determining the quality of surface
  finish.

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• According to the accuracy.
•  1. Most accurate instruments
• Example - light interference instrument
• 2. Less accurate instrument
• Example - Tool room Microscope, Comparators,
  Optimizer
• 3. Still less accurate instrument
• Example - Dial indicator, vernier caliper.

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Angular measurements

• Measuring the angle of Taper.
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• 1. Vernier bevel Protractor
• 2. Tool room microscope
• 3. Sine bar and dial gauge
• 4. Auto Collimator
• 5. Taper measuring machine
• 6. Roller, Slip gauge, and micrometer.

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• Angle measurement
• Sine bar
• Sine Centre
• Sine Table
• Taper Measurement
• Using Precisions Balls and Rollers-

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• Slip Gauges
• Direct precise measurement, where the accuracy of
  the work piece demands it.
• For checking accuracy of venire calipers, micro
  metes, and such other measuring instruments.
• Setting up a comparator to specific dimension.
• For measuring angle of work piece and also for
  angular setting in conjunction with a sine bar.
• The distances of plugs, spigots, etc. on fixture
  are often best measured with the slip gauges or
  end bars for large dimensions.
• To check gap between parallel locations such as
  in gap gauges or between two mating parts.
• Slip gauges are rectangular blocks of high grade
  steel with exceptionally close tolerances. These
  blocks are suitably hardened though out to ensure
  maximum resistance to wear. They are then
  stabilized by heating and cooling successively in
  stages so that hardening stresses are removed.

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Surface finish measurement

• Surface finish refers to the quality finish or
  roughness over the surface.
• Surface texture
• Repetitive or random deviations form the normal
  surface which form the pattern of the surface.
  Surface texture include roughness, waveness, lay
  and flows.
•  . Primary texture This refers to the
  roughness of a surface, as opposed to its
  waviness (secondary texture)

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Methods of measuring surface finish 

• . 1) Surface Inspection (or) comparison
  method
• 2. Direct Instrument
• a) Touch Inspection
• b) Visual Inspection
• c) Scratch Inspection
• d) Microscopic Inspection
• e) Surface photograph
• f) Micro - Interferometer
• g) Wallace surface Dynamometer
• h) Reflected light Intensity

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Roughness measurement

• Maximum Peak to Valley. Height of Roughness.
• Root Mean Square Value (R.M.S. Value)..
• Centre Line Average Method (C.L.A. Value)

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Surface finish measuring instruments

• Profilometer.
• The Tomlinson Surface Meter 
• Taylor-Hobson Talysurf.

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• UNIT IV
• TEMPERATUREMEASUREMENTS

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CLASSIFICATION OF TEMPERATUREMEASURING EQUIPMENTS

• Classification based on the Nature of Change
  Produced.
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• 1. Glass thermometers
• 2. Pressure gauge thermometers
• 3. Differential expansion thermometers
• 4. Electrical resistance thermometers
• 5. Thermo couples
• 6. Optical pyrometers
• 7. Radiation pyrometers
• 8. Fusion pyrometers
• 9. Calorimetric pyrometers  
• Based on Electrical and non-electrical Principles
  
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• 1. Primarily electrical or electronic in nature
• 2. Not primarily electrical or electronic in
  nature.

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Bimetallic Thermometers

•  Principle Involved These use the principles of
  metallic expansion when temperature changes.
•   A bimetallic strip is shown in figure which is
  straight initially. When temperature changes, its
  shape also changes into an arc.

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BIMETALIC THERMOMETER USE

• The displacement of the free end can be converted
  into an electric signal through use of secondary
  transducers like variable resistance, inductance
  and capacitance transducers. Figure shows a strip
  of bimetal in the form of a spiral. The curvature
  of the strip varies with temperature. This causes
  the pointer to deflect. A scale is provided which
  has been calibrated to show the temperature
  directly.
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• This kind of spiral is mostly used in devices
  measuring ambient temperature and
  air-conditioning thermostats.
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• Advantages of Bimetallic Thermometers
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• 1. Simple
• 2. Inexpensive
• 3. Accuracy of ? 0.5 to 2

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RESISTANCE THERMOMETERS

• Basic principle of resistance thermometers?
•   When an electric conductor is subjected to
  temperature change the resistance of the
  conductor changes. This change in resistance of
  the conductor becomes a measure of the change in
  temperature when calibrated.

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Thermocouples

• Principles Involved When heat is applied to the
  junction of two dissimilar metals, an e.m.f. is
  generated. (Figure)

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Thermistors

•   Thermistor is a temperature sensitive variable
  resistor made of a ceramic like semiconducting
  material. They are made of metal oxides and their
  mixtures like oxides of cobalt, copper, nickel,
  etc. Unlike metals, thermistors respond
  negatively to temperature. They behave as
  resistors with a high negative temperature
  coefficient of resistance. Typically, for each 1?
  C rise in temperature, the resistance of a
  thermistor decreases by about 5. This high
  sensitivity to temperature changes makes the
  thermistor useful in precision temperature
  measurements. The resistance of thermistors vary
  from 0.5? to 0.75M ?. Variation of resistivity
  with temperature is shown in figure.

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UNIT III

• FLOW MEASUREMENT

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FLOW METERS

• Flow meter measures the actual flow rate.
•  TYPES OF FLOWMETERS
• VENTURIMETER
• PITOT TUBE
• FLOW NOZZLE
• ORIFICE PLATE

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VENTURIMETER

• USES
• 1. Low head loss about 10 of differential
  pressure head.
• 2. High co-efficient of discharge.
• 3. Capable of measuring high flow rates in pipes
  having very large diameter.
• 4. Characteristics are well established so they
  are extensively used in process and other
  industries.

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VENTURI PRINCIPLE

• This is just like an orifice meter. It has three
  distinct parts, namely convergent cone, throat
  and divergent cone. A manometer measures the
  pressure difference between two sections as shown
  in figure.
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• Let a1 - Area at the inlet (1-1)
• A2 - Area at the section (2-2)
• x - Pressure head difference
• Cd - Discharge coefficient
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, Q
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Orifice METER

• Let a1 Area at section I-I
• a0 Area of orifice
• Cd Discharge coefficient
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• Then, Flow rate

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ROTO METERS

• Rotameter  
• A rotameter is a variable area type flow meter.
  It consists of a vertical tapered tube with a
  float which is free to move within the tube. The
  fluid goes from the bottom to the top. When no
  fluid flows, the float rests at the bottom of the
  tube. The float is made of such a diameter that
  it completely blocks the inlet. When flow starts
  in the pipeline and fluid reaches the float, the
  buoyant effect of fluid makes the float lighter.
  The float passage remains closed until the
  pressure of the flowing material plus the
  buoyance effect exceeds the downward pressure due
  to the float weight. Thus, depending on flow, the
  float assumes a position. Thus the float gives
  the reading of flow rate.

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Pitot Tube

• Principle Transformation of kinetic energy of a
  liquid into potential energy in the form of a
  static head.
• Figure shows a pitot tube installed in a pipeline
  where it acts like a probe. The tube consists of
  two concentric tubes, the inner tube with its
  open ends faces the liquid.

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Pitot tube principle

• outer tube has a closed end and has four to eight
  holes in its wall. The pressure in the outer tube
  is the static pressure in the line. Total
  pressure is sum of static pressure and the
  pressure due to the impact of fluid.
• If P - Pressure at inlet (Stagnation pressure)
• Ps - Static pressure
• ? - Density, then
• Velocity v from which flow rate is determined.
  

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UNIT V

• FORCE MEASUREMENT

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FORCE MEASUREMENT

• Force.
•  The mechanical quantity which changes or tends
  to change the motion or shape of a body to which
  it is applied is called force.
•  .Force measureing equipments 
• load cells
• Load cells are devices used for force measurement
  through indirect methods.

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Force measuring equipments

• Scale and balance
• a. Equal arm balance
• b. Unequal arm balance
• c. Pendulum scale
• 2. Elastic force meter Proving ring
• 3. Load cell
• a. Strain gauge load cell
• b. Hydraulic load cell
• c. Pneumatic load cell

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Torque measuring equipments

• Mechanical torsion meter
• Optical torsion meter
• Electrical torsion meter
• Strain gauge torsion meter

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Types of strain gauges.

• Unbonded strain gauge
• Bonded strain gauge
• Fine wire strain gauge
• Metal foil strain gauge
• Piezo-resistive strain gauge

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PROVING RING

• Use of proving Rings
•   Proving rings are steel rings used for
  calibration of material testing machines in
  situations where, due to their bulkness, dead
  weight standards cannot be used.
• P ring is a circular ring of rectangular section
  and may support tensile or comprehensive force
  across its diameter.
• ? the change in radius in the direction of force,
  is given by
•  where d is the outer diameter of the ring and
• K is stiffness.
•  Deflection of the ring is measured using a
  precision micrometer. To get precise
  measurements, one edge of the micrometer is
  mounted on a vibrating reed which is plucked to
  obtain a vibratory motion. The micrometer contact
  is then moved forward until a noticeable damping
  of the vibration is observed.

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LOAD CELLS

• Use of Load Cell
•  Force transducers intended for weighing purposes
  are called load cells. Instead of using total
  deflection as a measure of load, strain gauge
  load cells measure load in terms of unit strains.
  A load cell utilizes an elastic member as the
  primary transducer and strain gauges as secondary
  transducer. Figure shows one such load cell
  arrangement.

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DYNAMO METERS

• Mechanical Dynamometer
• These come under the absorption type. An example
  for this kind is prony brake.
•   In Prony brake, mechanical energy is converted
  into heat through dry friction between the wooden
  brake blocks and the flywheel (pulley) of the
  machine. One block carries a lever arm. An
  arrangement is provided to tighten the rope which
  is connected to the arm. Rope is tightened so as
  to increase ht frictional resistance between the
  blocks and the pulley.
•  If F Load applied and
• Power dissipated
• r - Lever arm
• N Speed of flywheel (rpm)
• Torque T F.r
•  The capacity of Prony brake is limited because
•  Due to wear of wooden blocks, friction
  coefficient varies. So, unsuitable for large
  powers when used for long periods.
•  To limit temperature rise, cooling is to be
  ensured.
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D.C. Dynamometer

•   D.C. dynamometer is usable as an absorption as
  well as transmission dynamometer. So, it finds
  its use in I.C. Engines, steam turbines and
  pumps. A d.c. dynamometer is basically a d.c.
  motor with a provision to run it as a d.c.
  generator where the input mechanical energy,
  after conversion to electrical energy, can either
  be dissipated through a resistance grid or
  recovered for use. When used as an absorption
  dynamometer it acts as d.c. generator. (figure)
  Cradling in trunnion bearings permits the
  determination of reaction torque.

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Eddy CURRENT DYNAMOMETER

• Current or Inductor Dynamometers
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• This is an example for absorption type
  dynamometers.
• Principle When a conducting material moves
  through a magnetic flux field, voltage is
  generated, which causes current to flow. If the
  conductor is a wire forming a part of a complete
  circuit will be caused to flow through that
  circuit, and with some form of commutating device
  a form of a.c. or d.c. generator may result.
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•  An eddy current dynamometer is shown in figure.
  It consists of a metal disc or wheel which is
  rotated in the flux of a magnetic field. The
  field if produced by field elements or coils
  excited by an external source and attached to the
  dynamometer housing which is mounted in trunnion
  bearings. As the disc turns, eddy currents are
  generated. Its reaction with the magnetic field
  tends to rotate the complete housing in the
  trunnion bearings. Water cooling is employed.
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