2. Methods of Measurement
• Direct method
• In this method, the quantity to be measured is directly
compared with the primary or secondary standard.
• This method is widely employed in production field.
• In this method, a very slight difference exists between the
actual and the measured values because of the limitation of
the human being performing the measurement.
• Indirect method
• In this method, the value of quantity is obtained by
measuring other quantities that are functionally related to
the required value.
• Measurement of the quantity is carried out directly and then
the value is determined by using a mathematical relationship.
• Eg: angle measurement using sine bar
3. • Fundamental or absolute method
• In this method, the measurement is based on the
measurements of base quantities used to define the
quantity.
• The quantity under consideration is directly measured and is
then linked with the definition of that quantity.
• Comparative method
• The quantity to be measured is compared with the known
value of the same quantity or any other quantity practically
related to it.
• The quantity is compared with the master gauge and only
the deviations from the master gauge are recorded after
comparison.
• Eg. Dial indicators
4. • Transposition method
• This method involves making the measurement by direct
comparison wherein the quantity to be measured(V) is
initially balanced by a known value (X) of the same quantity.
Then, ‘X’ is replaced by the quantity to be measured and
balanced again by another known value (Y). If the quantity to
be measured is equal to both X and Y, then
• V=
• Eg. Determination of mass by balancing methods
• Coincidence methods
• In this method, a very minute difference between the
quantity to be measured and the reference is determined by
careful observation of certain lines and signals
• Eg: Vernier caliper
5. • Deflection method
• This method involves the indication of the value of the
quantity to be measured by deflection of a pointer on a
calibrated scale.
• Eg. Pressure measurement
• Null measurement method
• In this method, the difference between the value of the
quantity to be measured and the known value of the same
quantity with which comparison is to be made is brought to
be zero.
• Substitution method
• This method involves the replacement of the value of the
quantity to be measured with a known value of the same
quantity, so selected that the effects produced in the
indicating device by these two values are the same.
6. • Contact method
• In this method, the surface to be measured is touched by the
sensor or measuring tip of the instrument.
• Eg. Micrometer, Vernier calliper and dial indicator
• Contactless method
• As the name indicates, there is no direct contact with the
surface to be measured
• Eg. Tool makers microscope, profile projector
• Composite method
• The actual contour of a component to be checked is
compared with its maximum and minimum tolerance limits.
• Cumulative errors of the interconnected elements of the
component which are controlled through a combined
tolerance can be checked by this method.
• This method is very reliable to ensure interchangeability and
is effected through the use of composite GO gauges.
7. General characteristics in metrology
Sensitivity: It is the ratio of the magnitude of output signal to the
magnitude of input signal. It denotes the smallest change in the
measured variable to which the instrument responds.
Sensitivity=(Infinitesimal change of output signal)/(Infinitesimal
change of input signal)
If the input-output relation is linear, the sensitivity will be constant
for all values of input.
If the instrument is having non-linear static characteristics, the
sensitivity of the instrument depends on the value of the input
quantity.
8. Hysteresis: All the energy put into the stressed component
when loaded is not recovered upon unloading. Hence, the output
of a measurement system will partly depend on its previous input
signals and this is called as hysteresis.
Range: It is the minimum and maximum values of a
quantity for which an instrument is designed to measure/ The
region between which the instrument is to operate is called
range.
Range = Lower Calibration Value – Higher Calibration
Value = Lc to Hc
Span: It is the algebraic difference between higher
calibration value and lower calibration value.
Span = Hc - Lc
Ex: If the range of an instrument is 10 C to 15 C, its
0̊ 0̊
span is 15 C – 10 C = 50
0̊ 0̊ C
̊
9. Response Time: It is the time which elapses after a sudden change in
the measured quantity, until the instrument gives an indication
differing from the true value by an amount less than a given
permissible error.
Speed of response of a measuring instrument is defined as the
quickness with which an instrument responds to a change in input
signal.
Repeatability: It is the ability of the measuring instrument to give the
same value every time the measurement of a given quantity is
repeated.
It is the closeness between successive measurements of the
same quantity with the same instrument by the same operator over a
short span of time, with same value of input under same operating
conditions.
10. Stability: The ability of a measuring instrument to retain its
calibration over a long period of time is called stability. It determines
an instruments consistency over time.
Backlash: Maximum distance through which one part of an
instrument may be moved without disturbing the other part.
Accuracy: The degree of closeness of a measurement compared to
the expected value is known as accuracy.
Precision: A measure of consistency or repeatability of
measurement. i.e. successive reading does not differ. The ability of
an instrument to reproduce its readings again and again in the same
manner for a constant input signal.
Magnification: Human limitations or incapability to read
instruments places limit on sensitiveness of instruments.
Magnification of the signal from measuring instrument can make it
better readable.
11. Resolution: Minimum value of input signal required to cause an
appreciable change or an increment in the output is called
resolution/ Minimum value that can be measured when the
instrument is gradually increased from non-zero value.
Error: The deviation of the true value from the desired value is
called error.
Drift: The variation of change in output for a given input over a
period of time is known as drift.
Threshold: Minimum value of input below which no output can be
appeared is known as threshold.
Reliability: Reliability may be explicitly defined as the probability
that a system will perform satisfactory for at least a given period of
time when used under stated conditions. The reliability function is
thus same probability expressed as a function of the time period.
12. Standards
Types of standards
• Line standard
• Standard yard
• Standard metre
• End standard
• End bar
• Slip gauges
• Wavelength standard
13. Line standard
The measurement of distance may be made between two
parallel lines or two surfaces.
When a length is measured between as the distance between
centres of two engraved lines, it is called line standard
Standard yard
14. • Yard is made of a one inch square cross section
bronze bar and is 38 inches long
• The bar has a round recess (gold plug) of 0.5 inches
diameter and 0.5 inches deep. The gold plug is 1 inch
away from both the ends
• The highly polished top surfaces of these plugs
contain three transversely and two longitudinally
engraved lines lying on the neutral axis
• The yard is the distance between two central
transverse lines on the plugs when the temperature
of the bar is constant at 62O
F
• To protect the gold plug from accidental damage, it
is kept at the neutral axis as the neutral axis remains
unaffected even if the bar bends
15. Standard metre
• The metre is the distance between the centre portions
of two lines engraved on the polished surface of bar
made up of platinum (90%) and iridium (10%) having a
unique cross section
• The web section gives maximum rigidity and economy
in the use of costly material
• The upper surface of the
web is inoxidizable and
needs a good finish for
quality measurement
• the bar is kept at 0O
C and
under normal
atmospheric pressure.
16. End standard
The need of an end standard arises as the use of line standards and their
copies was difficult at various places in workshops
These are usually in the form of end bars and slip gauges
• End bar
• End bars made of steel having cylindrical section of 22.2 mm diameter
with the faces lapped and hardened at the ends are available in various
lengths.
• Flat and parallel faced end bars are firmly used as the most practical end
standard used for measurement.
• These are used for measurement of larger sizes
• Slip gauges
• Slip gauges are rectangular blocks of hardened and stabilized high grade
cast steel
• The length of a slip gauge is strictly the dimension which it measures
• The blocks after being manufactured are hardened to resist wear and are
allowed to stabilize to release internal stresses
• A combination of slip gauges enables the measurements to be made in
the range of 0.0025mm to 100mm but in combinations with end bars,
the measurement range upto 1200mm is possible.
17. Wavelength standards
• Line and end standards are physical standards and
are made up of materials that can change their size
with temperature and other environmental
conditions
• In search for such suitable unit of length, wave length
source is established
• Laser is used as primary level wavelength standard
• According to this standard, a metre is defined as
equal to 1650763.73 wavelength of the red orange
radiation of krypton isotope gas
1 metre =1650763.73 wavelengths
1 yard=0.9144m
=0.9144x1650763.73
=1509458.3 wavelength.
18. Measurement uncertainty – Types, Estimation,
Problems on Estimation of Uncertainty,
Measurement uncertainty is a parameter used in data
processing for the description of both the dispersion of
the result and its estimated difference from the
accurate value. Frequently, this is simplified to only
dispersion, and measurement uncertainty is obtained
using statistical data variance.
20. Measurement Systems Analysis (MSA)
Accuracy is the difference between the true average and the observed average. If the
average value differs from the true average, then the system is inaccurate. This is an
indication of an inaccurate system.
Bias: Bias is the difference between the observed average measurement to the true or
reference value. To measure the Bias, first, you need to measure the same part several times
and then calculate the measurement average. Simply Bias = Average of measurement value –
Reference or true value.
Stability: Stability refers to the capacity of the measurement system to produce the same
values over time when measuring the same sample.
Precision
The precision of the measurement system is the degree to which repeated measurements
show the same result when under unchanged conditions. In other words, precision refers to
the closeness of two or more measurements to each other.
Repeatability: Repeatability is the variation between successive measurements of the same
part or characteristic by the same person using the same gauge.
Reproducibility: Reproducibility is the difference in the average of the measurements by
different people using the same instrument when measuring the identical characteristics on
the same part.
21. Calibration of measuring instruments
Calibration is the process of checking and adjusting a measuring
instrument to ensure that it produces accurate results. It's an important
part of any measurement system and is typically done by comparing the
instrument's output to a standard device or process.
Here are some reasons why calibration is important:
Accuracy: Calibration ensures that the instrument's measurements are
accurate and reflect reality.
Quality management: Calibration helps ensure the quality of products and
equipment.
Reduced risk of failure: Calibration can help reduce the risk of failure in
production.
Cost savings: Calibration can help save costs.
Calibration is usually performed by comparing the instrument to a
standard device that has been calibrated against international standards.
The calibration interval should be determined in advance based on the
manufacturer's recommendations and how often the instrument is used.
23. Air Gauging
Air gauging is a non-contact comparative measuring method. It has been used in the
industry for more than 80 years. The concept of air gauging is based on the law of
physics that states flow and pressure are directly proportionate to clearance and react
inversely to each other. Clearance in this case refers to the distance between the nozzle
of the air gauge probe and the workpiece. As clearance increases, air flow also
increases, and air pressure decreases proportionately. As clearance decreases, air flow
also decreases, and air pressure increases.
This is made possible by having a regulated air flow through the nozzle in the air jets of
the air gauges. The nozzle acts as a restrictor. As the measured product is brought
closer to the nozzle, air flow is reduced, and the back pressure is increased. When the
nozzle is completely obstructed, the flow is zero, and the back pressure is equal to the
regulated air. Conversely, when the nozzle is open to the atmosphere, air flow is at a
maximum, and the back pressure is at a minimum. The pressure differences are then
converted electronically to get an accurate dimensional value.
Air gauging is highly suited for the measurement of soft, highly polished, thin wallet,
delicate components that require high accuracy. The smaller the range, the better
repeatability, up to a few nanometers. A multitude of features can be measured by air:
inside and outside diameters, but also many geometrical features such as taper,
flatness, roundness, run-out, squareness, straightness, etc.
25. ISO standards
The International Organization for Standardization (ISO) has a
number of standards related to measurement, including:
ISO 10012:2003: This standard provides requirements for
measurement processes and equipment, and is intended to help
organizations manage measurement processes and ensure
compliance with metrological requirements. It was last reviewed
and confirmed in 2022.
ISO 17025: This standard defines requirements for the quality
management system of laboratories that can be accredited.
26. CGS, FPS, MKS & SI SYSTEM
CGS System
CGS stands for Centimeter-Gram-Second. It is a system of measurement in which the fundamental
units are the centimetre for length, the gram for mass, and the second for time. This system is
commonly used in the fields of physics, chemistry, and engineering.
FPS System
FPS stands for Foot-Pound-Second. It is a system of measurement in which the fundamental units are
the foot for length, the pound for force or weight, and the second for time. This system is commonly
used in the United States for engineering and physics applications, but it is less common
internationally.
MKS System
MKS stands for Meter-Kilogram-Second. It is a system of measurement in which the fundamental
units are the meter for length, the kilogram for mass, and the second for time. This system is
commonly used in scientific and engineering applications, and it is the primary system of
measurement used in most countries outside of the United States.
SI System
SI stands for International System of Units. It is the modern form of the metric system and is the most
widely used system of measurement in the world. The SI system is based on the MKS system, but it
has expanded to include additional units and prefixes for very large and very small quantities. The SI
system is used in science, industry, and commerce, and it provides a standard for measurement that
is consistent and internationally recognized.
