1.1 Meaning of Metrology
Metrology is the branch of science, which deals with measurement depending upon the field of application. It can be classified as
- Industrial Metrology
- Technical Metrology
- Astronomical Metrology
- Medical Metrology etc.
1.2 Units and Measurements
Unit : Measurement involves comparison of a quantity to be measured with a reference standard of the same kind. The chosen reference standard is called unit.
International system of units: It is the system established in 1960 by the general conference of weights and measures and abbreviated as SI. SI means “System International” d’ units which has been adopted as standard in most of the countries in the world.
1.2.1 Basic units: SI system is built on arbitrarily selected units called basic units.
The basic SI units are given below
Unit Unit symbol Physical quantity
meter m length
kilogram kg mass
second s time
ampere A electric current
kelvin K temperature
candela cd luminous intensity
mole mol amount of substance
1.2.2 Supplementary units:
‘Radian’ for plain angle
‘Staradian’ for solid angle
The radian is the plane angle between two radii of a circle that cut off on the circumference an arc equal in length to the radius.
The steradian is defined as the solid angle that, having its vertex in the center of a sphere, cuts off an area of the surface of the sphere equal to that of a square with sides of length equal to the radius of the sphere.
1.2.3 Derived units:
These are units of combination of two or more quantities, which usually require compound word to name them.
1.2.4 Definitions:
Meter: By international agreement, the standard meter had been defined as the distance between two fine lines on a bar of platinum-iridium alloy. The 1960 conference redefined the meter as 1,650,763.73 wavelengths of the reddish-orange light emitted by the isotope krypton-86. The meter was again redefined in 1983 as the length of the path traveled by light in vacuum during a time interval of 1/299,792,458 of a second.
Kilogram: The kilogram in SI is defined as a quantity of mass of the international prototype of platinum – iridium cylinder.
Second: The second, the basic unit of time, was defined as 1/86,400 of a mean solar day or one complete rotation of the earth on its axis. Scientists discovered, however, that the rotation of the earth was not constant enough to serve as the basis of the time standard. As a result, the second was redefined in 1967 in terms of the resonant frequency of the cesium atom—that is, the frequency at which this atom absorbs energy, or 9,192,631,770 hertz (cycles per second).
Ampere: It is that constant current, if maintained in two straight parallel conductors of infinite length of negligible circular cross section and placed one meter apart in vacuum would produce between these conductors a force equal to 2 x 10 –7N/m
Kelvin: It is the fraction 1/273.16 of thermodynamic temperature of the triple point of water. (The point at which the three phases of water (vapor, liquid, and ice) are in equilibrium)
Candela: It is defined as the intensity of a light source, in a given direction, with a frequency of 540 x 1012 hertz and a radiant intensity of 1/683 watt per steradian in that direction.
Mole: It is the amount of a substance that contains as many elementary particles (atoms, molecules, ions, electrons, or other particles) as the number of carbon atoms in 0.012 kg (12 g) of carbon-12.
1.2.5 SI Units – The Do’s & Don’ts
In connection with SI units, some rules of style, abbreviations, writing and drafting practices are applied, some of which are given below.
1) Symbols specified for various units are mandatory for adoption and no modification is allowed. For example,
Qty Correct Incorrect
second s sec
meter m mtr
2) The use of small and capital letters for various symbols should be correctly given otherwise they will create confusion. Eg:
Qty Correct Incorrect
Mega M m
meter m M
3) The proper space (single space) is to be left between numerical quantity and symbol or
between two symbols. Eg:
Qty Correct Incorrect
5 newton meter 5 N m 5Nm.
10 meter Kelvin 10 m K 10mK
4) The No dots, commas etc. are used after SI symbols except at the end of sentences. Eg:
Qty Correct Incorrect
32 meter 32 m 32m.
5) No space is to be left between prefix and symbol of the units. Eg:
Qty Correct Incorrect
5 millimeter 5 mm 5 m m
10 kilogram 10 kg 10 k g
6) Plurals are never used in connection with SI unit symbols. Eg:
Qty Correct Incorrect
32 meter 32 m 32ms
8 kilograms 8 kg 8kgs
7) While expressing the temperature in Kelvin, the word degree or symbol [ 0 ] is not used. Eg:
Correct Incorrect
5 kelvin 5 degree kelvin
5K 50K
8) When the quantities are less than unit, the decimal point must be preceded by zero.
Correct Incorrect
0.75 .75
0.385 .385
9) The time may be indicated in hours with zero at midnight.
Correct Incorrect
04 00h 4 AM
15 00h 3 PM
10) If a set of dimension is in the same unit, the unit symbol is to be given in the last dimension only.
Correct Incorrect
25X10X5mm 25mmX10mmX5mm
11) Units with the name of scientist should not be capitalized when written in full.
Correct ncorrect
N(newton) n(Newton)
W(watt) w(Watt)
12) The number/quantity are not to be written in fraction, but always to be written in decimals
Correct Incorrect
0.75m 3/4m
1.25kg 1 1/4kg
13) Use of multiples and sub – multiples. Eg;
Correct Incorrect
Nano meter (nm) milli micro meter
giga meter(Gm) kilo mega meter
14) In case of mass unit i.e. Kilogram (kg) the prefix should be attached to root name gram (g) and not to base unit kilogram.
Correct Incorrect
Mega gram (Mg) kilo kilogram
gram (g) milli kilo gram
15) When the quantities are less than unit, the decimal point should be preceded by zero. Eg.
Correct Incorrect
0.75 .75
0.385 .385
16) The terms lakhs, crores and arabs are not to be used in technical reports. The term million can be used. Eg.
Correct Incorrect
05 million 5 lakhs
40 million crores
1.1.6 Prefixes used for multiples and sub-multiples
Multiple or unit-part Prefix SI symbol Multipl.factor
1 000 000 000 000 000 000 exa E 1018
1 000 000 000 000 000 peta P 1015
1 000 000 000 000 tera T 1012
1 000 000 000 giga G 109
1 000 000 mega M 106
1 000 kilo k 103
1 00 hekto h 102
1 0 deca da 10
0.1 deci d 10-1
0.01 centi c 10-2
0.001 milli m 10-3
0.000 001 micro μ 10-6
0.000 000 001 nano n 10-9
0.000 000 000 001 pico p 10-12
0.000 000 000 000 001 femto f 10-15
0.000 000 000 000 000 001 atto a 10-18
1.2 Importance of inspection in industry
In order to determine the fitness of any thing made, man has always used inspection. But industrial inspection is of recent origin and has scientific approach behind it. It came into being because of mass production which involved interchangeability of parts. The modern production techniques require that production of complete article be broken up into various component parts so that the production of each component part becomes an independent process .The various parts to be assembled together in assembly shop comes from various shops. So it is very essential that the dimensions of component part must be confined with in the prescribed limits. Thus industrial inspection assumed its importance due to necessity of suitable mating of various components manufactured separately.
Inspection led to development of precision inspection instruments which caused the transition from crude machines to better designed machines. Inspection has also introduced a spirit of competition and led to production of quality products.
1.3 Precision vs. Accuracy
Precision is the repeatability of a measuring process or simply, how well identically performed measurements agree with each other. This concept applies to a process or set of measurements, not a single measurement.
Accuracy is the agreement of the result of a measurement with the true value of the measured quality. It would be clear from the example given below,
Eg. If a carpenter had to cut a board to fit the shelf into two projections in the wall, it does not matter whether his scale is accurate or not, provided he uses the same scale for the measurement of board and the distance between projections in the wall. Here the precision with which he measures two is of importance. Now supposing he had to order for the board from market then it is necessary that the scale used by him and the one in market are in agreement with each other. One way to achieve this is that both use the accurate scales in accordance with the standard scales. In this case accuracy of the scale is important and it should be manufactured such that it’s units are in accordance with the standard units set.
COMMENTS