introduction to Mechanical Measurement and Metrology.

WHAT IS MEASUREMENT?
WHAT IS NEED OF MEASUREMENT?
MEASUREMENT IS THE
ASSIGNMENT OF A NUMBER TO
A CHARACTERISTIC OF AN
OBJECT OR EVENT…

BASIC DEFINITIONS RELATED TO
MEASUREMENT:

ACCURACY:
• Accuracy is defined as the closeness of the
measured value with true value.
OR
• Accuracy is defined as the degree to which
the measured value agrees with the true
value.
• Practically it is very difficult to measure
the true value and therefore a set of
observations is made whose mean value is
taken as the true value of the quantity
measured.

PRECISION:
• A measure of how close repeated trials are
to each other.
OR
• The closeness of repeated measurements.
• Precision is the repeatability of the
measuring process. It refers to the group
of measurements for the same
characteristics taken under identical
conditions.
• If the instrument is not precise it will give
different results for the same dimension
when measured again and again.

DISTINCTION BETWEEN PRECISION
AND ACCURACY:

• The amount of error change throughout
an instrument's measurement range.
OR
• Linearity is also the amount of deviation
from an instrument's ideal straight-line
performance.
LINEARITY:

SENSITIVITY:
• Sensitivity may be defined as the rate of
displacement of the indicating device of an
instrument, with respect to the measured
quantity.
• Sensitivity of thermometer means that it is the
length of increase of the liquid per degree rise
in temperature. More sensitive means more
noticeable expansion.

READABILITY:
• Readability refers to the ease with which the
readings of a measuring instrument can be
read.
• Fine and widely spaced graduation lines
improve the readability.
• To make the micrometers more readable they
are provided with Venier scale or magnifying
devices.

MAGNIFICATION:
• Magnification is the process of enlarging
something only in appearance, not in
physical size so that it is more readable.
(The stamp appears larger with the use of a magnifying glass.)

CALIBRATION:
• Calibration is process of comparison of
measurement values delivered by a device
under test with those of a standard of known
accuracy.
• It is carried out by making adjustments such
that the device produces zero output for zero
input.

CONTINUE…
• The process whereby the magnitude of the
output of a measuring instrument is
related to the magnitude of the input force
driving the instrument (i.e. Adjusting a
weight scale to zero when there is nothing
on it).
• The accuracy of the instrument depends
on the calibration.
• If the output of the measuring instrument
is linear and repeatable, it can be easily
calibrated.

REPEATABILITY:
• It is the ability of the measuring instrument to
repeat the same results for the measurements
for the same quantity, when the
measurements are carried out…
 By the same observer,
 With the same instrument,
 Under the same conditions,
 Without any change in location,
 Without change in the method of
measurement,
 The measurements are carried out in
short intervals of time.

REPRODUCIBILITY:
• Reproducibility is the closeness of the
agreement between the results of
measurements of the same quantity, when
individual measurements are carried out…
 By different observer,
 With different instrument,
 Under different conditions,
 With change in location,
 With change in the method of
measurement,
 The measurements are carried out in
longer intervals of time.

BACKLASH:
• In Mechanical Engineering, backlash, is
clearance between mating components,
sometimes described as the amount of lost
motion due to clearance or slackness when
movement is reversed and contact is re-
established.

HYSTERESIS:
• It is the difference between the indications
of a measuring instrument when the same
value of measured quantity is reached by
increasing or decreasing that quantity.
• It is caused by friction, slack motion in the
bearings and gears, elastic deformation,
magnetic and thermal effects.

introduction to Mechanical Measurement and Metrology.

DRIFT:
• It is an undesirable gradual deviation of
the instrument output over a period of
time that is unrelated to changes in input
operating conditions or load.
• An instrument is said to have no drift if is
reproduces the same readings at different
times for same variation in measured
quantity.
• It is caused by wear and tear, high stress
developed at some parts etc.

THRESHOLD:
• The min. value below which no output
change can be detected when the input of
an instrument is increased gradually from
zero is called the threshold of the
instrument.
• Threshold may be caused by backlash.

RESOLUTION:
• When the input is slowly increased from
some non-zero value, it is observed that
the output does not change at all until a
certain increment is exceeded; this
increment is called resolution.
• It is the min. change in measured variable
which produces an effective response of
the instrument.
• It may be expressed in units of measured
variable

DEAD ZONE AND DEAD TIME:
Dead Zone:
• The largest change of input quantity for
which there is no change of output of the
instrument is termed as dead zone.
• It may occur due to friction in the
instrument which does not allow the
pointer to move till sufficient driving force
is developed to overcome the friction loss.
• Dead zone caused by backlash and
hysteresis in the instrument.

CONTINUE…
Dead Time:
• The time required by a measurement
system to begin to respond to a change in
the value to be measured is termed as
dead time.
• It represents the time before the
instrument begins to respond after the
measured quantity has been changed.

PROCESS OF MEASUREMENT:
• The sequence of operations necessary for
the execution of measurement is called
process of measurement.
• There are main three important elements
of measurement,
(1) Measurand: Is the physical quantity or
property like length, angle, diameter,
thickness etc. to be measured.
(2) Reference: It is the physical quantity or
property to which quantitative comparisons
are made.
(3)Comparator: It is the means of
comparing measured value with some
reference.

CONTINUE…
• Suppose a fitter has to measure the
length of M.S. plate- he first lays his rule
along the flat. He then carefully aligns
the zero end of his rule with one end of
M.S. flat and finally compares the length
of M.S. flat with the graduations on his
rule by his eyes. In this example, the
length of M.S. flat is a measurandmeasurand, steel
rule is the referencereference and eye can be
considered as a comparatorcomparator.

METHODS OF MEASUREMENT:
 The methods of measurement can be
classified as:
(1) Direct Method:
• This is a simple method of measurement, in
which the value of the quantity to be measured
is obtained directly without the calculations.
• For example, measurements by scales, Vernier
Calipers, micrometers, bevel protector etc.
• This method is most widely used in
production. This method is not very accurate
because it depends on human judgment.

(2) Indirect Method:
•In indirect method the value of quantity to be
measured is obtained by measuring other
quantities which are functionally related to
required value.
•For example, angle measurement by sine bar,
measurement of shaft power by dynamometer
etc.
CONTINUE…

Measuring
System
Standard
Instrument Workpiece
Person
Environment
Most basic elements
(Without this Measurement is not possible)
With which
Measurement
will be done
To carry out
Measurement
Predefined conditions
for Measurement
On which
Measurement
is carried out

CONTINUE…
 Types of errors:
During measurement several types of
error may arise, these are:
1. Static errors which includes:
(a) Reading errors
(b) Characteristic errors
(c) Environmental errors
2. Instrumental loading errors
3. Dynamic errors

CONTINUE…
1. Static errors:
- These errors result from the physical nature of
various components of measuring system.
There are three basic sources of static errors:
(a) Reading errors:
These errors occur due to
carelessness of operators. These do not have
any direct relationship with other types of
errors within the measuring system.

CONTINUE…
Reading errors include:
 Parallax error:
Parallax errors arise on account of
pointer/observer and scale not being in same
plane, we can eliminate this error by having
the pointer and scale in same plane.
 Wrong scale reading and wrong recording of
data.
 Inaccurate estimates of average reading.
 Incorrect conversion of units in calculations.

CONTINUE…
(b) Characteristics error:
 It is defined as the deviation of the output
of the measuring system from the
theoretical predicated performance or
from nominal performance specifications.
 Linearity errors, repeatability, hysteresis
are the characteristics errors if theoretical
output is straight line. Calibration error is
also included in characteristics error.

CONTINUE…
(c) Environmental errors:
 These error result from the effect of
surrounding such as temperature,
pressure, humidity etc. on measuring
system.
 It can be reduced by controlling the
atmosphere according to the specific
requirement.

CONTINUE…
2. Instrument loading error:
 Loading errors results from the change in
measurand itself when being measured.
 Instrument loading error is the difference
between the value of measurand before
and after the measurement.
 For example a soft or ductile component
is subjected to deformation during
measurement due to the contact pressure
of the instrument and cause a loading
error. The effect of this error is
unavoidable.

CONTINUE…
3. Dynamic errors:
 Dynamic error, also called measurement
error, is the difference between the true
value of measuring quantity and value
indicated by measurement system if no
static error is assumed.
 These errors can be broadly classified as:
(a) Systematic or controllable errors:
1. Calibration errors
2. Atmospheric errors
3. Stylus pressure errors
4. Avoidable errors
(b) Random errors

CONTINUE…
(a) Systematic or controllable errors:
These errors are controllable in
both their magnitude and stress. These can
also be determined and reduced. These are
due to:
(1) Calibrations errors:
The actual length of
standards such as scales will vary from
nominal value by small amount. This will
cause an error in measurement of constant
magnitude.

CONTINUE…
(2) Atmospheric error:
Variation in atmospheric
condition (i.e. temperature, pressure and
moisture content) at the place of
measurement from that of internationally
agreed standard values (20° temp. and
760 mm of Hg pressure) can give rise to
error in measurand size of the component.

CONTINUE…
(3) Stylus pressure:
Another common source of
error is the pressure with which the work
piece is pressed while measuring. Though
the pressure involved is generally small
but this is sufficient enough to cause
appreciable deformation of both the stylus
and the work piece.
Variations in force applied by the
anvils of micrometer on the work to be
measured results in the difference in its
readings. In this case error is caused by
the distortion of both micrometer frame
and work piece.

CONTINUE…
(4) Avoidable errors:
These errors may occur due
to parallax, non alignment of work piece
centers, improper location of measuring
instruments such as a thermometer in
sunlight while measuring temperature.

CONTINUE…
(b) Random errors:
The random errors occur randomly
and the specific causes of such errors cannot be
determined. The likely sources of this type of
error are:
• Small variations in the position of setting
standard and work piece.
• Slight displacement of lever joints in the
measuring instrument.
• Friction in measuring system.
• Operator errors in reading scale.

DIFFERENCE BETWEEN
SYSTEMATIC AND RANDOM
ERRORS:
Systematic error
- These errors are
repetitive in nature and
are of constant & similar
form.
- These errors result from
improper conditions.
Random error
- These are non
consistent. The sources
giving rise to such errors
are random.
- Such errors are inherent
in the measuring system.

CONTINUE…
- Expect personal errors
all other systematic
errors can be
controlled in
magnitude and sense.
- If properly analyzed
these can be
determined and
reduced or eliminated.
- Specific causes,
magnitudes and sense
of these errors cannot
be determined from
the knowledge of
measuring system.
- These errors cannot
be eliminated, but the
results obtained can
be corrected.

CONTINUE…
- These errors includes
calibration errors,
variation in
atmosphere, pressure,
misalignment error
etc.
- These errors includes
Small variations in the
position of setting
standard and work
piece, Slight
displacement of lever
joints in the measuring
instrument, Friction in
measuring system,
Operator errors in
reading scale.

DIFFERENCE BETWEEN RANGE
AND SPAN:
 It is region between
the limits within an
instrument is
designed to operate
for measuring,
indicating or
recording a
measurand.
 For pressure guage:
1bar To 50 bar
 It is a algebraic
difference between the
largest and smallest
reading of instrument
 For pressure guage:
50-1 = 49 bar

introduction to Mechanical Measurement and Metrology.

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