Introduction to Metrology
Metrology is the field of knowledge concerned with measurement and includes both theoretical and practical problems with reference to measurement, whatever their level of accuracy and in whatever fields of science and technology they occur.
It
is the science concerned with establishment, reproduction, conversion and
transfer of units of measurements and their standards.
Measurement
The
process of experimentally obtaining quantity values that can reasonably be
attributed to property of a body or a substance is called measurement. It is
the process of determining or finding the size, quantity or degree of something
by means of measuring instruments which describes the object of measurement.
Physical measurement is the act of deriving quantitative information about a
physical object by comparison with a reference.
Scope of metrology
1.
Scientific
measurement
Establishing and
maintaining standards for units of measurement such as length, mass, time and
temperature which are essential for scientific research and experimentation.
2.
Industrial
measurement
Ensuring accuracy and
precision in manufacturing process, quality control and calibration of
instruments used in production. It includes measuring dimension tolerances and
performance of component.
3.
Engineering
Various measurement
techniques are applied to design, test and validate engineering system and
components. This includes everything from structural measurement to calibration
of sensors and control systems.
4.
Legal
Metrology
Ensuring fairness and
accuracy in trade, commerce and legal applications. It involves regulation and
verification of measurement instruments used in transactions like scales.
5.
Error
Management
It involves detecting, analyzing and correcting measurement errors by assessing sources of error, quantifying uncertainty, calibrating instruments and improving measurement techniques to ensure accuracy and reliability in measurements.
Objectives of Metrology
1. To
completely evaluate the newly developed products.
2. To
determine the process capabilities and ensure that these are better than relevant
component tolerances.
3. To
determine the measuring devices capabilities and ensure that they are adequate
for their respective measurements.
4. To
minimize the cost of inspection.
5. To
reduce the costs of rejects and rework through application of statistical
quality control techniques.
6. To
standardize the measuring methods.
7. To
maintain accuracies of measurement by periodic calibration of instruments.
Functions of Legal Metrology
1.
Standards
Development
Formulating and
updating national standards for products, services and processes to ensure
quality, safety and efficiency. It involves adopting and adapting international
standards to local needs.
2.
Metrological
Services
Providing calibration
and testing services to ensure the accuracy of measurement and instruments. It
provides technical support and expertise to industries and organization on
measurement practices.
3.
Quality
Assurance
It manages the
processes that ensure products and services meet established standards. It
operates the product certification mark and quality assurance protects consumers
from unsafe products.
4.
Consumer
Protection
It safeguards customer
by ensuring that products and services meet quality standards. It seeks to
protect the public against inaccuracy in trade. It involves handling complaints
and taking actions.
5.
Public
Awareness
It educates the public
and businesses about the importance of standards and accurate measurements and
promoting the benefits of using established standards.
Dynamic Metrology
It is the technique of
measuring small variations of a continuous nature. It consider dynamic quantity
which are continuous functions of time i.e. its value at one time
instant depends on its values at previous time instant. The mathematical modeling
of dynamic measurements typically utilizes methodologies and concepts from
digital signal processing.
Examples: measurement
of high-speed electronics, medical ultrasound, mechanical quantities, etc.
Deterministic Metrology
It is a new philosophy in which part measurement is replaced by process measurement. In it, full advantage is taken of the deterministic nature of production machines. System process are monitored by temperature, pressure, flow, force, vibrations, etc. This technology is adopted in very high precision manufacturing machinery and control systems to accomplish micro technology and nanotechnology accuracies.
Examples: 3D error
compensation by CNC systems and expert systems, leading to full adaptive
control.
Necessity and Importance of Metrology
1. In
design engineering, the specifies dimensions of the designs should be checked or
measured.
2. Higher
productivity and accuracy can be achieved by proper understanding of metrology.
3. One
can improve the measuring accuracy and dimensional and geometrical accuracy of
the product.
4. Proper
gauges should be designed and used for rapid and effective inspection.
5. Automatic
control and automations are based on metrology.
Process of Measurement
Measurand: It is the physical
quantity or property like length, angle, etc to be measured.
Reference: It is the
physical quantity or property to which quantitative comparisons are made.
Comparator: It is the
means of comparing measurand with some reference.
Units of Measurement
Methods of Measurement
1.
Direct
Method
This is the simple
method of measurement, in which the value of the quantity to be measured is
obtained directly without the calculations. Examples: measurements by scale,
caliper and micrometers.
2.
Indirect
Method
In this method, the
value of the quantity to be measured is obtained by measuring other quantities,
which are functionally related to required value. Example: angle measurement by
sine bar.
3.
Absolute
or Fundamental Method
Measuring a quantity
directly in accordance with the definition of that quantity. Example: force and
area are measured for pressure measurement.
4.
Comparative
Method
In this method, the
value of quantity to be measured is compared with the known value of the same
quantity or other quantity practically related to it. So in it, only deviation
from the master gauge is determined. Example: calibration of the vernier caliper
using standard slip gauge.
5.
Coincidence
Method
It is a differential
method of measurement in which a very small difference between value of the
quantity to be measured and the reference is determined by the observation of
the coincidence of certain lines/signals. Example: measurement using vernier
caliper, micrometer.
6.
Complementary
Method
In this method, value
of quantity to be measured is combined with a known value of the same quantity.
The combination is so adjusted that these two values is equal to predetermined
comparison value. Example: volume of solid by liquid displacement.
7.
Deflection
Method
In this method, value of the quantity to be measured is directly indicated by the deflection of a pointer on the calibrated scale. Example: dial indicator
8. Transportation Method
It is a method of measurement by direct comparison in which the value of the quantity measured is first balanced by an initial known value A of the same quantity, and then the value of the quantity measured is put in place of this known value and is balanced again by another known value B. If the position of the element indicating equilibrium is the same in both cases, the value of the quantity to be measured is √AB . For example, determination of a mass by means of a balance and known weights, using the Gauss double weighing method.
9. Contact Method
In this method the sensor or measuring tip of the instrument actually touches the surface to be measured. e.g., measurements by micrometer, vernier caliper, dial indicators etc. In such cases arrangement for constant contact pressure should be provided to prevent errors due to excessive contact pressure.
10. Contactless Method
In contactless method of measurement, there is no direct contact with the surface to be measured. e.g., measurement by optical instruments, such as tool makers microscope, projection comparator etc.
11. Null Method
In this method the quantity to be measured is compared with a known source and the differences between these two is made zero. E.g., measurement by potentiometer.
Accuracy
Accuracy is defined as
the closeness of the measured value with true or accepted value. It can be
defined as the degree to which the measures value agrees with the true value. It
is the closeness of agreement between a test result and the accepted reference
value.
Precision
It is the measure of
how close repeated trials are to each other. It is 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.
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/operator,
·
with
the same instrument,
·
under
the same conditions,
·
without
any change in location,
·
without
change in the method of measurement,
·
and
the measurements are carried out in short intervals of time.
Repeatability
conditions are where independent test results are obtained with the same method
on identical test items in the same laboratory by the same operator using the
same equipment within short intervals of time.
Reproducibility
It is the consistency
of pattern of variability in measurement i.e., closeness of the agreement
between the results of measurements of the same quantity, when individual
measurements are carried out:
·
by
different observer/operator,
·
By
different method,
·
Using
different instruments,
·
Under
different conditions, locations, times etc.
It may also be
expressed quantitatively in terms of dispersion of the results. Reproducibility
is a precision under reproducibility conditions.
Sensitivity
It refers to the ability of measuring device
to detect small differences in a quantity being measured. In other words,
sensitivity denotes the maximum change in an input signal that will not
initiate a response on the output. It may be defined as the rate of
displacement of the indicating device of an instrument, with respect to the
measured quantity. In other words, sensitivity of an instrument is the ratio of
the scale spacing to the scale division value.
·
the
smallest change in the signal that can be detected.
Resolution
It is the smallest
change of the measured quantity which changes the indication of a measuring
instrument.
·
the
smallest portion of the signal that can be observed
Stability
It
refers to the ability of a measuring instrument to constantly maintain its
metrological characteristics with time.
Readability
It refers to the ease with which the readings
of the measuring instrument can be read It is the susceptibility of the
measuring device to have its indicators converted into meaningful numbers. If
the graduation lines are very finely spaced, the scale will be more readable by
using a microscope, but the readability will be poor with the naked eye.
Magnification
It
is the process of enlarging something only in appearance, not in physical size
so that it is readable.
Drift
It is an undesirable deviation of the
instrument output over a period of time that is unrelated to changes in input
operating conditions of load.
Threshold
The
minimum 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.
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 bearing and gears,
elastic deformation, magnetic and thermal effects.
Calibration
It
is a set of operations that establish, under specified conditions, the
relationship between values of quantities indicated by a measuring instrument
or values represented by a material measure and the corresponding values
realized by standards. The result of a calibration may be recorded in a
document, e.g., a calibration certificate. The result can be expressed as
corrections with respect to the indications of the instrument.
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