Chapter 1.pptx.pdf

1. Mechanical
Measurement
1
By: Mudit M. Saxena, Dept. of Mech. Engg.

Measurement and Metrology
• Science of measurement
• Depending on field of application
• Today (in broader sense) with some practical terms
2

Metrology
Practical terms related with measurement:
• Errors in measurements
• Methods of measurements
• Measuring Instruments
• Units of measurement and their standards
• Industrial inspection and its different techniques
• Measuring instruments and accuracy
3

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.
4

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.
5

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
6

1. Primary measurement
• Direct observation and comparison
• Not involvement of any conversion
Ex. Length, Height, Depth or Width etc. measurement.
7

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
8

Methods of Measurement
9

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.
10

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.
11

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.
12

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.
13

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
14

Generalized Measurement System
15

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.
16

17

2. Variable conversion (transducer) element
Convert one physical form into another form without changing
the information content of the signal.
18

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
19

4. Data transmission element
Transmits the signal from one location to
another without changing its information contents.
20

5. Data processing element
the databefore it displayed or finally
modifies
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.
21

22

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 givea 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.
23

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:
24

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.
25

• 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.
26

• 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.
27

• 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.
28

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
for maximum
decrcased
value.
✔ Maximum
diff. between
increasing input value and the
decreasing input value at the same
output. 29

• 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.
30

Zonal Drift: Combination of both
Zero Drift:
calibration
whole
gradually
shifts due to slippage
• Drift: gradual variation or undesired change in o/p during constant i/p.
• Span: proportional change in the indication or change along the upward scale
31

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.
32

• 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.
33

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
34

Measurement Errors
• Types (Classification of Errors)
35

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
36

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.
37

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.
38
By: Mudit M. Saxena, Dept. of
Mech. Engg.

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.
39

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.
40

Measurement Errors
Environmental 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
41

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.
42

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.
43

Measurement Errors
Observation errors :
• Due to poor capabilities and carelessness of operators.
2. Personal bias: Observer tendency to read high or low, anticipate a signal and
read too
3. Wrong reading, wrong calculations, wrong recording data, etc.
44

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.
45

ASSIGNMENT 1
Q 1. What is Metrology ? Explain its need and objectives.
Q 2. Explain the modes of measurement.
Q 3. Explain the methods of measurement.
Q 4. Depict by a line diagram generalized measurement
system.
Q 5. Explain the elements of measurement system in detail.
Q 6. Explain performance charactristics.
Q 7. Explain measurement errors.
46

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