DEBRE BERHAN UNIVERSITY
SCHOOL OF MECHANICAL ENGINEERING
INSTRUMENTATION & MEASUREMENT
UNIT – I
CONCEPT OF MEASUREMENT
BY,
V.DINESH KUMAR
Contents:
•
•
•
•
•
General concepts of Measurement
Need for Measurement
Methods of Measurements
UnitsTypes of units
Fundamental Units
Supplementary Units
 Derived Units
• Standards
• Types of standards
 International Standards
 Primary Standards
 Secondary Standards
 Working Standards.
GENERAL CONCEPT OF MEASUREMENT
• Measurement is a comparison of a given
unknown quantity with one of its
predetermined standard values adopted as a
unit.
• Measurement provides is with means of
describing various phenomena in quantitative
terms.
• It plays an important role in all branches of
engineering and science.
IMPORTANT REQUIREMENT OF THE
MEASUREMENT
The standard used for comparison must be
accurate and internationally accepted.
The apparatus or instrument and the
process used for comparison must be
provable.
FUNDAMENTAL MEASURING PROCESS
FUNDAMENTAL MEASURING PROCESS
• The word measurand is used to designate the
particular physical parameter being observed,
unknown quantity which is to measured. It is the
input quantity to the measuring process.
• This unknown quantity is compared with the
available standard quantities such length, mass,
time, and it produces a result.
APPLICATION OF MEASUREMENT:
• Measurement Provides the Fundamentals basis for research and
development activities. In research activity, the experimental part
is based on measurement.
• Measurement is a fundamental element of any automatic control
system. In control system, discrepancy or error between the actual
value and described value of a variable is to be determined.
• Measurement is used to evaluate the performance of any plant or a
process. For eg: in a modern power plant, temperature pressure,
vibrational amplitude, etc, must be constantly monitored by the
measurement to ensure the proper performance of the system.
• Measurement is also the basis for commercial activities such as
production, prizing, sale, and purchase.
• Measurement is used to monitor data in the interest of health and
safety, eg: forecasting wheather and predicting the one set of
earthquakes.
• It establish the validity of design and determines data for new and
improved design.
NEED FOR MEASUREMENT:
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
To determine the true dimensions of part.
To increase our knowledge and understanding of the world.
To ensure public health and human health.
To convert physical parameters into meaningful numbers.
To test whether the element that constitute the system
function as per the design.
To evaluate the performance of a system.
To study some basic law of nature.
To ensure interchangeability with a view for promoting mass
production.
To evaluate the response of the system to a particular point.
To check the limitations of theory in practical situations.
To establish the validity of design and for finding new data
and new designs.
METHODS OF MEASUREMENT
There are two different methods for measurement
1. Direct comparison with primary or secondary
standard.
2. Indirect comparison with a standard through
calibration system.
3. Comparative method
4. Coincidence method
5. Fundamental method
6. Contact method
7. Transposition method
8. Complementary method
9. Deflection method
DIRECT METHOD
• In direct comparison the parameter to be measured is
directly compared with either a primary standard or a
secondary standard. In this method, the comparison is
done with a standard with the help of calibrated system.
• Direct method are quite common for the measurement of
physical quantities like length, mass and time.
• For eg: if we want to measure the length of the rod with
the standard length, and finding the bar which is so many
times long because that many units on the standard have
the same length as the bar. Thus, we have determined the
length by direct comparison. Generally, the direct
comparison is not always the most accurate or the best, it
is not sensitive enough also.
CLASSIFICATION OF DIRECT MEASUREMENT:
• Measurement may be classified as primary, secondary and
tertiary based on the complexity of the measurement
system.
• In primary measurement, any physical parameters are
measured by comparing directly with reference standards.
For eg:
1. matching of two lengths when determining the length
of an object with a meter rod.
2. matching of two weights when determining the mass
of grossary items.
– The primary measurement provides subjective
information only. Here, the observer indicates
only the one rod is longer than the other rod. One
object contains more or less mass than the other.
The primary measurement is under the category
of direct measurement.
SECONDARY MEASUREMENT:
• A secondary measurement involves only one translation to
be done on the quantity under the measurement. For eg: if
we want to measure the pressure of a gas in a container, it
may not be observable. Therefore, it requires,
1. an instrument to convert pressure into displacement
and
2. the change in displacement units equivalent to known
change in pressure.
Therefore, the primary signal is first transmitted to a
transducer, where it is effected to transulate into a
length change in pressure gauge. Then the secondary
signal of length change is transmitted to the observer’s
eye.
TERTIARY MEASUREMENT
• A tertiary measurement involves two translations. The
measurement of static pressure by a bourdon tube
pressure gauge is a typical example of tertiary
measurement.
• During the measurement of pressure, the free end deflects
slightly. This small deflection is made larger by using rack
and pinion arrangement for better displaying and reading.
TERITARY ARRANGEMENT
INDIRECT METHOD
• Indirect comparison makes use of some form of
transducing device, which converts the quantity
to be measured into an analogous signal. The
analogous signal is then processed by
intermediate device and displayed on the output
device as known function of the input.
• Indirect method for measurement are used in
those cases where the direst measurement is
difficult.
• In this method, an empirical relationship is
generally established between the measurement
mode and result mode that are desired.
3. COMPARATIVE METHOD
• In this method, the quantity to be measured is
compared with other known value.
• For example: comparators
4. COINCIDENCE METHOD
• The value of the quantity to be measured
is determined is coincided with certain
lines and signals.
5. FUNDAMENTAL METHOD
• Measuring a quantity is directly related
with the definition of that quantity.
6. CONTACT METHOD
• The sensor or measuring tip of the
instruments touches the area (or) diameter
(or) surface to be measured.
• For example: Vernier caliper
7. TRANSPOSITION METHOD
• In this method, the quantity to be measured
is first balanced by known value and then it
is balance by other new known value.
• For example: determination of mass by
balancing methods.
8. COMPLEMENTARY METHOD
• The value of quantity to be measured is
combination with known value of the same
quantity.
• For example: volume determination by liquid
displacement.
9. DEFLECTION METHOD
• The value to be measured is directly
indicated by a deflection of pointer.
• For example: pressure measurement.
QUERIES?
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UNITS
• To specify and perform calculations with physical
quantities, the physical quantities must be defined
both in kind and magnitude. The standard measuring
of each kind of physical quantity is the unit.
• In earlier days, the number of system of units has been
used at various times in the past days. There are
different system of units available in different
countries.
• The M.K.S and C.G.S. system of units have been used
in earlier days.
• However, for the sake of uniformity of units all over
the world, S.I. have been developed.
• Internationally system of units (S.I) is the system established in
1960 by the general conference of weight and measures
(CGPM) and abbreviated as S.I ( system international units) in
all languages.
• This system of units is based on meter (m), kilogram (kg),
seconds (s),ampere (A), degree Kelvin (K),and candela (cd).
• The S.I, like the traditional metric system, is based on decimal
arithmetic.
• For each physical quantity, units of different sizes are formed
by multiplying or dividing a single base value of powers of 10.
therefore, this system offers the great advantage because the
change (unit conversions) can be made very simple by adding
zeros or shifting decimal points.
• The seven base units are established by the general conference
of weights and measures (CGPM) are described below.
• There are two supplementary units. Other physical quantities
are derived from these basic and supplementary units.
TYPES OF S.I.UNITS:
1. Fundamental Units
2. Supplementary Units
3. Derived Units
FUNDAMENTAL UNITS:
• FUNDAMENTAL UNITS ARE
INDEPENDENTLY CHOOSEN AND NOT
DEPENDENT ON ANY OTHER UNITS.
• IT IS ALSO CALLLED AS BASE UNIT.
S.NO QUANTITY
1.
Length
2.
UNIT
UNIT SYMBOL
Meter
m
Mass
Kilogram
Kg
3.
Time
Second
S
4.
Temperature
Kelvin
K
5.
Electric current
Ampere
A
6.
Luminous Intensity
Candela
cd
METER (m)
• It is the unit of length.
• It is the length equal to 1,650,763,73
wavelength of light emitted in vacuum by the
atom Kryton-86 on its transition between the
levels 2P10 and 5d5.
Kilogram (Kg)
• It is the fundamental unit of mass.
• It is equal to the mass of the international
prototype preserved at the IBWNS
(international bureau of weights and measures
in serves).
• The prototype is made of cylinder of 90%
platinum and 10% iridium alloy.
Second (S)
• It is the unit of time.
• A second can be defined as 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 cesium133 atom.
Kelvin (K)
• It is the fundamental unit of temperature.
Kelvin is defined as the fraction 1/273.16 of the
thermodynamics temperature of the triple
point of water.
AMPERE (A)
• It is the unit of electric current.
• The ampere is the constant current which is
produced between the two straight parallel
conductors of finite length and negligible cross
section placed in vacuum would produce
between these conductors a force equal to 2*107 N/m.
Candela (Cd )
• It is the fundamental unit of luminous
intensity.
• Candela is defined as the luminous intensity of
a surface of (1/6,00,000 ) m2Of a black body at
the temperature of freezing platinum under an
atmospheric pressure (1.01325 Mpa).
SUPPLEMENTARY UNITS:
• There are two supplementary units added to the
S.I.Unit system in addition to fundamental units.
These two shown in table.
S.NO
1.
QUANTITY
Plane Angel
UNIT
Radian
UNIT
SYMBOL
rad
2.
Solid Angle
Steradian
sr
Radian (rad):
• One radian is defined as the plane angel subtended
as the center of an arc of unit length at unit radius.
Steradian (sr):
• one Steradian is defined as the solid angle subtended
at the centre by unit area of spherical surface at unit
radices.
DERIVED UNITS:
• The derived unit are expressed in terms of
fundamental and supplementary units by
defining equations. These derived units can be
categorized as follows:
• a) mechanical units: units for force, pressure,
stress, weight, torque, acceleration, velocity,
density, etc.
• b) electric and magnetic units: units for power,
energy, electric resistance, electric field strength,
magnetic flux density.
• c) thermal units: units for specific heat capacity,
latent heat, sensible heat, etc.
S.NO
DERIVED UNITS
QUANTITY
UNIT
UNIT SYMBOL
1.
AREA
m2
A
2.
Volume
m3
V
3.
Density
Kg/m3
Ƿ
4.
Velocity
m/s
v
5.
Angular velocity
rad/s
Ѡ
6.
Acceleration
m/s2
a
7.
Angular Acceleration
rad/s2
α
8.
Force
N
F
9.
Pressure
N/m2
P
10.
Work, energy
Joule
W
11.
Power
Watt
p
12.
Frequency
Hertz
f
13.
Latent heat
KJ/Kg
hfg
14.
Sensible heat
KJ/Kg
Hf
15.
Specific heat capacity
KJ/Kg-K
C
STANDARDS
• Standard is a physical representation of a unit of
measurement. A known accurate measure of physical
quantity is termed as standard.
• These standards are used to determine the values of
physical quantities by comparison method.
• Different standard have been developed for various
units including fundamental as well as derived units.
• All these standards are preserved at the
international bureau of weight and measures at
sever, Paris.
• Depending on the function and applications,
different types of standards of measurement are
classified as follows.
TYPES OF STANDARDS:
1.
2.
3.
4.
INTERNATIONAL STANDARDS
PRIMARY STANDARDS
SECONDARY STANDARDS
WORKING STANDARDS.
INTERNATIONAL STANDARDS
• International standards are defined by
agreement. they are periodically evaluated and
checked by absolute measurement in terms of
fundamental units of physics.
• They represent certain units of measurement
to the closest possible accuracy attainable by
the science and technology of measurement.
• These international standards are not available
to ordinary uses like measurement and
calibrations.
PRIMARY STANDARDS:
• The main function of primary standard is the
calibration and verification of secondary
standard. Primary standards are maintained at
the national standard laboratories in different
countries.
• The primary standards are not available for
the use outside the national laboratory. These
primary standards are absolute standard of
high accuracy that can be used as ultimate
reference standards to check, calibrate and
certify the secondary standards.
SECONDARY STANDARD:
• Secondary standards are basis reference standards
used by the measurement and calibration
laboratories in industries. These secondary
standards are maintained by the particular industry
to which they belong.
• Each industry has its own secondary standard each
laboratory periodically sends its secondary standard
to the national standard laboratory for calibration
and comparison against the primary standard.
• After comparison and calibration, the national
standard laboratory returns the secondary standard
to the particular industrial laboratory with a
certification of measuring accuracy in terms of
primary standards.
WORKING STANDARDS:
• Working standard are the main tools of a
measuring laboratory. These standards are used to
check and calibrate laboratory instrument for
accuracy and performance.
• For eg: manufacturing of mechanical components
such as shafts, bearings, gears, etc, use a standard
called working standard for checking the
components dimensions.
• For example: plug gauge is used for checking the
bore diameter of bearings.
QUERIES?
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