Introduction to Metrology
Metrology:
Metrology has come from Greek word “metron” (measure) and
–logy, meaning science of measurement. Engineering metrology is the measurement
of dimensions: length, thickness, diameter, taper angle, flatness, profiles,
etc.
Objectives of metrology:
The basic objective of metrology is to provide the required
accuracy at minimum cost while taking measurements. Further objectives-
1) To maintain accuracy and precision at the time of
inspection.
2) It can be used for selection of proper measuring
instruments in industries.
3) It is used for minimizing cost of inspection.
4) To find/decide tolerances.
5) To achieve standardization.
6) To achieve
interchangeability and selective assembly.
Terms used in measurement:
1.
Accuracy
2.
Precision
3.
Calibration
4.
Sensitivity
5.
Readability.
6.
Magnification
1. Accuracy:-
“Accuracy is the degree to which measured value agrees
with the true value of quantity of interest.” Accuracy denotes the closeness of
measured value with the true value. For example if an instrument like micrometer
measures dimension of part as 25mm and if stated accuracy is +0.01mm then the
true dimension could lie between 24.99mm and 25.01mm.
2. Precision .
“Precision means the degree of repeatability of measuring
process.” Precision has no meaning for only one measurement but it exists when
set of measurements is carried out. Suppose 10 measurements are performed.
Then, precision means how well these 10 measured values agree with each other
i.e. how nearer the readings of these 10 measurements are and how less is the
variation between each value.
Distinction between Precision and Accuracy:
Accuracy
is very often confused with precision, but both are different.
In figure (a), since the gunshots have hit the target region
and all the gunshots are nearer to each other, they are precise and accurate. In
figure (b), gunshots are far away from the target region, so the gunshots are inaccurate
but they are nearer to each other, so they are precise.
· represents the
location of average position of shots. 
In fig (c), the gunshots are much apart from each other
and nor is their average position within the target region, so the gunshots are
neither accurate nor precise. In figure (d), suppose the gunshots are divided
in a sample. Each sample consists of five gunshots. Then the gunshots are
nearer to each other in a sample, hence gunshots are precise within a sample.
But the samples are far away from each other, hence the gunshots on the whole
are not precise nor are they accurate
3. Calibration .
Calibration is the procedure in which the measuring instrument
is checked against a known standard.” For e.g. Calibrating a Thermometer involves
checking its reading in ice (of pure water). Instruments must be calibrated often
since they lose accuracy on frequent uses.
4. Sensitivity
Sensitivity is the ability of measuring device to detect
small differences in quantity being measured. The indication of this
characteristic is the smallest variation of the quantity that can be detected
by the instrument. Sensitivity is also called Resolution. The greater the
resolution of device, the smaller will be the things it can resolve and greater
will be magnification required to expand these measurements upto the point where
they can be observed by naked eye. Sensitivity is also related to measuring instrument/device.
5. Readability .
Readability is the susceptibility of a measuring device
to have its indications converted to a meaningful number.” It refers to ease
with which the readings of a measuring instrument can be read.
Characteristics/Requirements of precision measuring
instrument:
1) High degree of
sensitivity.
2) High degree of
accuracy.
3) There should be no
errors, in transferring accurate measurements.
4) It must give same
results, any number of times, in hands of different operators ( i.e. good
precision)
5) Materials from which
it is made must be seasoned and stabilized, to reduce minimum error in accuracy
due to changes that take place normally in metals.
6) Abbe’s principle of
alignment should be followed in the design.
“It states that the axis or line of measurement should
coincide with the axis of measuring instrument or line of measuring scale.”
