Transcript attachment=29596

ENGG METROLOGY &
INSTRUMENTATION
UNIT- I
METROLOGY
UNITS AND MEASUREMENTS
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.
Units and Standards
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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
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
• 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
• 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
• 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.
• Methods of measurement.
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1. Direct Method
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2. Indirect Method
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3. Comparison Method
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4. Coincidence Method.
• Classification of measuring instruments.
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1. Angle measuring instruments
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2. Length measuring instruments
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3. Instruments for surface finish
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4. Instruments for deviations.
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,
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.
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
Linear measuring instruments
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Straight edge.
Outside caliper.
Inside caliper.
Vernier caliper
Screw gauge
vernier height gauge
vernier depth gauge
Dial gauges
Comparators
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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.
. Classification of measuring
Instruments.
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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.
• 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.
Angular measurements
• Measuring the angle of Taper.
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1. Vernier bevel Protractor
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2. Tool room microscope
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3. Sine bar and dial gauge
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4. Auto Collimator
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5. Taper measuring machine
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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
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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 :
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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)
Methods of measuring surface finish
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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
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)
Surface finish measuring instruments
• Profilometer.
• The Tomlinson Surface Meter
• Taylor-Hobson Talysurf.
UNIT IV
TEMPERATUREMEASUREMENTS
CLASSIFICATION OF
TEMPERATUREMEASURING
EQUIPMENTS
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Classification based on the Nature of Change Produced.
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
1. Primarily electrical or electronic in nature
2. Not primarily electrical or electronic in nature.
Bimetallic Thermometers:
• Principle Involved : These use the principles
of metallic expansion when temperature
changes.
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A bimetallic strip is shown in figure which
is straight initially. When temperature
changes, its shape also changes into an arc.
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%
RESISTANCE THERMOMETERS
• Basic principle of resistance thermometers?
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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.
Thermocouples
• Principles Involved : When heat is applied to
the junction of two dissimilar metals, an e.m.f.
is generated. (Figure)
Thermistors:
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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.
UNIT III
FLOW MEASUREMENT
FLOW METERS
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Flow meter measures the actual flow rate.
TYPES OF FLOWMETERS
VENTURIMETER
PITOT TUBE
FLOW NOZZLE
ORIFICE PLATE
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.
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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a1
Area at the inlet (1-1)
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A2
Area at the section (2-2)
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Pressure head difference
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Cd
Discharge coefficient
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,Q=
Cd  a1  a2 2 g x
a21  a2 2
Orifice METER
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a1 – Area at section I-I
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a0 – Area of orifice
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Cd – Discharge coefficient
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• Then, Flow rate
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.
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.
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.
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P
Ps
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Pressure at inlet (Stagnation pressure)
Static pressure
Density, then
• Velocity v = from which flow rate is determined.
UNIT V
FORCE MEASUREMENT
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.
Force measuring equipments
• Scale and balance
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a. Equal arm balance
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b. Unequal arm balance
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c. Pendulum scale
• 2. Elastic force meter – Proving ring
• 3. Load cell
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a. Strain gauge load cell
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b. Hydraulic load cell
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c. Pneumatic load cell
Torque measuring equipments
– Mechanical torsion meter
– Optical torsion meter
– Electrical torsion meter
– Strain gauge torsion meter
Types of strain gauges.
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Unbonded strain gauge
Bonded strain gauge
Fine wire strain gauge
Metal foil strain gauge
Piezo-resistive strain gauge
PROVING RING
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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.
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.
DYNAMO METERS
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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.
D.C. Dynamometer
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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.
Eddy CURRENT DYNAMOMETER
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Current or Inductor Dynamometers:
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.
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.