Introduction to Mechanical Measurement

CHAPTER 1
Mechanical Measurement
Presentation Prepared by
Prof. Naman M. Dave
Assistant Prof. (Mechanical Dept.)
Gandhinagar Institute of Technology

Please refer this file just as
reference material. More
concentration should on class
room work and text book
methodology.

Measurement and Metrology
 Science of measurement
 Depending on field of application
 Today (in broader sense) with some
practical terms

Metrology
 Practical terms related with
measurement:
o Errors in measurements
o Methods of measurements
o Measuring Instruments
o Units of measurement and their standards
o Industrial inspection and its different
techniques
o Measuring instruments and accuracy

Needs of Measurement and
Metrology
 To ensure that the part to be measured conforms
to the established standard.
 To meet the interchangeability of manufacture.
 To provide customer satisfaction by ensuring that
no faulty product reaches the customers.
 To coordinate the functions of quality control,
production, procurement & other departments of
the organization.
 To judge the possibility of making some of the
defective parts acceptable after minor repairs.

Objectives of Measurement and
Metrology
 Although the basic objective of a measurement is to
provide the required accuracy at a minimum cost,
metrology has further objectives in a modem engineering
plant with different shapes which are:
 To minimize the cost of inspection by efficient and
effective use of available facilities,
 To minimize the cost of rejection and re-work through
application of statistical quality control techniques.
 To maintain the accuracies of measurement.
 To determine the process capabilities and ensure that
these are better than relevant component tolerances.
 To do complete evaluation of newly developed products.

Modes of Measurement
 Based upon the number of conversions, three
basic categories of measurements have been
developed.
 They are;
1. Primary measurement
2. Secondary measurement
3. Tertiary measurement

1. Primary measurement
 Direct observation and comparison
 Not involvement of any conversion
Ex. Length, Height, Depth or Width etc. measurement.

2. Secondary measurement
 >Indirect method >Involvement of one conversion
 Ex. Pressure or Temperature measurement
3. Tertiary measurement
 >Indirect method >Involvement of 2 conversion
 Ex. Measurement of rotating shaft

Methods of Measurement
Type of Method Technique to measure
With contact Instrument is placed in contact
with the object. For ex. vernier
calliper
Without contact Instrument not placed in contact
with the object. (use of sensor)
Absolute or
Fundamental
Based on the measurements of
base quantities entering into the
definition of the quantity.

Comparative Based on the comparison of the
value of a quantity to be
measured with a known value of
the same quantity.
Null measurement Here, difference between
measurand value and known
value of same quantity with
which it is compared is brought to
zero.

Substitutional Method Quantity to be measured is
replaced by a known value of the same
quantity, so selected that the effects
produced in the
indicating device by these two values
are the same (a type of direct
comparison).
Complementary Method The value of quantity to be measured is
combined with known value of the same
quantity.
Ex: Volume determination by liquid
displacement.

Transposition
Quantity to be measured
is first balanced by a
known value and then
balanced by an other
new known value.
Value of quantity measured is
first balanced by an initial known
value A of same quantity, then
measured by quantity is put in
place of this known value. Then,
it is balanced again by another
value B. If the position of
element in equilibrium.

Coincidence Measurements coincide with
certain lines and signals. Ex.
Callipers
Deflection The value of the quantity to be
measured is directly indicated by
the deflection of a pointer on a
calibrated scale

Generalized Measurement System

Elements of measuring system
1. Primary sensing element
 Quantity under measurement makes its first
contact with primary sensing element.
 Sense the condition, state or value of the process
variable by extracting a small part of energy
from the measurand,
 and then produce an output which reflects this
condition, state or value of measurand.

2. Variable conversion (transducer) element
 Convert one physical form into another form without
changing the information content of the signal.

3. Variable manipulation element
 Modifies the signal by amplification, filtration or other
means so that desired output produced according to
some mathematical rule for Ex. i/p x constant = o/p

4. Data transmission element
 Transmits the signal from one location to another
without changing its information contents.

5. Data processing element
 modifies the data before it displayed or finally
recorded
• perform mathematical operation such as addition,
subtraction, multiplication, division, etc.
• to calculate average, statistical and logarithmic values
• to convert data into desired form.
• to separate undesired signal from output signal.
• to provide correction on the output signal.

6. Data presentation elements
 Provides a record or indication of the output
(i) Transmitting information (measured quantity) to
another location or devices.
(ii) Signaling : To give a signal that the pre-defined
value has been reached.
(iii) Recording : To produce a continuous record of
measured quantity in written form.
(iv) Indicating : To indicate the specific value on
calibrated scale.

Performance characteristics
 Important to select most suitable instrument for specific
measurement.
 Static Performance characteristics:
• Desired input to the instrument not change w.r.t time.
 Dynamic Performance characteristics:

Static Performance characteristics
 Readability: This term indicates the
closeness with which the scale of
the instrument may be read.
 Susceptibility of device to have its
indications converted into
meaningful number.
 Least count: It is the smallest
difference between two indications
that can be detected on the
instrument scale.

 Range: It represents the highest possible value that can be
measured by an instrument or limits within which
instrument is designed to operate.
 Linearity: A measuring system is said to be linear if the
output is linearly proportional to the input.

 Repeatability: It is defined as the ability of a measuring
system to repeat output readings when the same input is
applied to it consecutively, under the same conditions, and
in the same direction.
 Reproducibility: It is defined as the degree of closeness
with which the same value of a variable may be measured
at different times.
 System response: Response of a system may be defined as
the ability of the system to transmit & present all the
relevant information contained in the input signal.

 Threshold: Min. value of i/p required to cause a
detectable change from ‘0(zero)’ o/p.
 If i/p increased gradually from ‘0(zero)’, there will
be some min. value below which no o/p change
can be detected.

 Hysteresis:
 Hysteresis is the maximum
differences in two output
(indicated values) at same
input (measurand) value
within the specified range
when input is continuously
increased from zero and
when input is continuously
decrcased for maximum
value.
 Maximum diff. between
increasing input value and
the decreasing input value at
the same output.

 Calibration: Procedure of
making, adjusting or checking
a scale so that readings of an
instrument conforms to an
accepted standard.
 Sensitivity: Ratio of o/p
response to a specific range in
i/p.
 Dead zone: Largest change of
i/p quantity for which
instrument does not indicate
output.

 Drift: gradual variation or undesired change in o/p
during constant i/p.
Zonal Drift: Combination
of both
Zero Drift: whole
calibration gradually
shifts due to slippage
• Span: proportional change in the indication or change
along the upward scale

 Loading effect:
Any instrument, invariably extracts energy from
system, the original signal should remain
undistorted. This is incapability of system to
faithfully measure signal in undistorted form.

 Accuracy: It is degree to which the measured
value agrees with true value. Max. amount by
which result differs from the true value.
 Precision: It is repeatability or reproducibility of
the measurement. If instrument is not precise,
great difference in dimension measured again
and again.

Measurement Errors
 What is Error ?
 It is difference between indicated or
measured value and true value.
 It is impossible to made measurement
with perfect accuracy

Measurement Errors
 Types (Classification of Errors)

Measurement Errors
 Gross errors
• Human mistakes
• Careless readings, mistake in recordings,
• improper application of instrument
• Can not treated mathematically
• Can be avoided only by taking care in reading
and recording

Measurement Errors
 Systematic error
• Have definite magnitude and direction.
• Can be repeated consistently with repetition of
experiments.
• To locate these errors: repeated measurements
under different conditions or with different
equipment or possible by an entirely different
methods.

Measurement Errors
 Instrumental error
 Due to design or construction /assembly of
instruments
 Limiting accuracy
 Improper selection of instrument
 Poor maintenance
 For Ex. Errors due to friction, wear, slips,
vibration
 Errors due to incorrect fitting of scale at zero,
non-uniform division of scale, bent pointer.

Measurement Errors
 Operational error
 Misuse of instrument
 Poor operational techniques
 For Ex. Errors in flow measurement if flow-
meter is placed immediately after a valve or
a bend.

Measurement Errors
 Environmental errors
 due to conditions external to the measuring
instrument, including conditions in the area
surrounding the instrument,
 such as effects of change in temperature,
humidity, barometric pressure, or magnetic
or electrostatic fields.
 For ex. Buoyant effect of the wind causes
errors on precise measurement of weights by
pan balance.

Measurement Errors
 These errors may be avoided by
 (i) Use instrument under conditions for
which it was design and calibrated. This
atmospheric condition can be maintain by air
conditioning.
 (ii) Provide sealing certain components in
the instrument.
 (iiii) Make calibration of instrument under
the local atmospheric conditions
 Environmental errors

Measurement Errors
 System interaction errors
 Interaction between system (to be measured)
and instrument body. So it change the
condition of the system.
 For Ex. A ruler pressed against a body
(system) resulting the deformation of the
body.

Measurement Errors
 Observation errors :
 Due to poor capabilities and carelessness of
operators.
i. Parallax : These errors may arise when the
pointer and scale not in same plane or line of
vision of observer is not normal to the scale.

Measurement Errors
 Observation errors :
 Due to poor capabilities and carelessness of
operators.
ii. Personal bias: Observer tendency to read high
or low, anticipate a signal and read too
iii. Wrong reading,
wrong
calculations,
wrong recording
data, etc.

Measurement Errors
 Random Error
 Accidental in their incidence
 Variable in magnitude and usually follow a
certain statistical (probability) law.
 Friction and stickiness in instrument
 Vibration in instrument frame or supports
 Elastic deformation
 Large dimensional tolerances between the
mating parts.
 Supply power fluctuations
 Backlash in the movement.

CHAPTER 1
Mechanical Measurement

Introduction to Mechanical Measurement

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