MEASUREMENTS AND INSTRUMENTATION

UNITS AND STANDARDS OF MEASUREMENTS

BY:-

ANJAN P.SELVAM

R.V.GANAPATHY

SYSTEMS OF MEASURMENT

A system of measurement is a set of units which can be used to specify anything which can be measured and were historically important, regulated and defined because of trade and internal commerce. Scientifically, when later analyzed, some quantities are designated as fundamental units meaning all other needed units can be derived from them, whereas in the early and most historic eras, the units were given by the ruling entities and were not necessarily well inter-related or self-consistent.

HISTORY OF SCIENTIFIC SYSTEM

The French revolution – scientific system – steady significant pressure – convert to a scientific basis – so called customary units of measure. Most systems– length (distance)– weight – time – fundamental quantities . Some systems – changed to recognize – improved relationship – notably – 1824 legal changes – imperial system.

Later science developments – added – electric current – complete – minimum set of fundamental quantities – by which all other metrological units may be defined. Other quantities – power – speed, etc. – derived from – fundamental set – for example, speed is distance divided by time. Historically – wide range of units – same quantity; for example, in several cultural settings, length was measured in inches, feet, yards, fathoms, rods, stadia, leagues, with conversion factors which are not simple powers of ten or even always simple fractions within a given customary system.

DIFFERENT SYSTEMS OF MEASUREMENTS

METRIC SYSTEM Metric systems of units – evolved since –

adoption – first well-defined system in France 1791. During this – use of these systems – spread throughout – first to the non-English-speaking countries – more recently to the English speaking countries.

Multiples and submultiples of metric units are related by powers of ten; the names for these are formed with prefixes. This relationship is compatible with the decimal system of numbers and it contributes greatly to the convenience of metric units.

In the early metric system there were two fundamental or base units, the metre and the gram, for length and mass. The other units of length and mass, and all units of area, volume, and compound units such as density were derived from these two fundamental units.

IMPERIAL AND U.S.CUSTOMARY UNITS

Both the Imperial units and U.S. customary units derive from earlier English units. Imperial units were mostly used in the British Commonwealth and the former British Empire. They are still used in common household applications to some extent and so are also sometimes called common units, but have now been mostly replaced by the metric system in commercial, scientific, and industrial applications.

Contrarily, however, U.S. customary units are still the main system of measurement in the United States. The customary units have a strong hold due to the vast industrial infrastructure and commercial development. The metric system is preferred in certain fields such as science, medicine and technology.

These two systems are closely related, however they differ between them. Units of length and area are identical except for surveying purposes. The Avoirdupois units of mass and weight differ for units larger than a pound (lb.).

NATURAL UNITS Natural units are physical units of measurement

defined in terms of universal physical constants in such a manner that some chosen physical constants take on the numerical value of one when expressed in terms of a particular set of natural units. Natural units are natural because the origin of their definition comes only from properties of nature and not from any human construct. Various systems of natural units are possible.

Some examples: Geometric unit systems Planck units Stoney units Schrödinger units Atomic units(au) Electronic units Quantum electrodynamical units

NON – STANDARD UNITS Non-standard measurement units

found in books, newspapers etc., include:AreaEnergyMassVertical distanceVolume

ELECTROSTATIC UNITS The electrostatic system of units is a system of

units used to measure electrical quantities of electric charge, current, and voltage, within the centimeter gram second (or "CGS") metric system of units. In electrostatic units, electrical charge is defined via the force it exerts on other charges. Although CGS units have mostly been supplanted by the MKS or "International System of Units" (SI) units, electrostatic units are still in use in some applications, most notably physics.

The main electrostatic units are: Statcoulomb or "esu" for charge Statvolt for voltage Gauss for magnetic induction

Dimension Unit Definition SI

charge

electrostatic unit of charge,

Franklin, statcoulomb

1 esu = 1 statC = 1 Fr = √(g·cm³/s²)

= 3.33564 × 10−10 C

electric current Biot 1 esu/s = 3.33564 × 10−10 C/s

electric potential Statvolt 1 statV = 1 erg/esu = 299.792458 V

electric field 1 statV/cm = 1 dyn/esu = 2.99792458 × 104 V/m

magnetic field strength (H) Oersted 1 Oe= 1000/(4π) A/m = 79.577

A/m

magnetic flux Maxwell 1 Mw = 1 G·cm² = 10−8 Wb

magnetic induction (B) Gauss 1 G = 1 Mw/cm² = 10−4 T

resistance 1 s/cm = 8.988 × 1011 Ω

resistivity 1 s = 8.988 × 109 Ω·m

capacitance 1 cm = 1.113 × 10−12 F

InductaNce

statH = 8.988 × 1011 H

wave number kayser 1 /cm = 100 /m

CGS AND MKS UNITS Scientists have adopted the metric system to simplify their

calculations and promote communication across national boundaries.

The result was two clusterings of metric units in science and engineering. One cluster, based on the centimeter, the gram, and the second, is called the CGS system. The other, based on the meter, kilogram, and second, is called the MKS system.

The CGS system was introduced formally by the British Association for the Advancement of Science in 1874. It found almost immediate favor with working scientists, and it was the system most commonly used in scientific work for many years.

During the 20th century, metric units based on the meter and kilogram--the MKS units--were used more and more in commercial transactions, engineering, and other practical areas. By 1950 there was some discomfort among users of metric units, because the need to translate between CGS and MKS units went against the metric ideal of a universal measuring system.In 1960 the Eleventh General Conference adopted the name International System of Units (SI) for this collection of units.

CGS unit   measuring SI equivalent 

barye (ba) pressure 0.1 pascal (Pa)

biot (Bi) electric current 10 amperes (A)

calorie (cal) heat energy 4.1868 joule (J)

darcy permeability 0.98692 x 10-12 square meter (m2)

debye (D) electric dipole moment 3.33564 x 10-30 coulomb meter (C·m)

dyne (dyn) force 10-5 newton (N)

emu magnetic dipole moment 0.001 ampere square meter (A·m2)

erg work, energy 10-7 joule (J)

franklin (Fr) electric charge 3.3356 x 10-10 coulomb (C)

galileo (Gal) acceleration 0.01 meter per second squared (m·s-2)

gauss (G) magnetic flux density 10-4 tesla (T)

gilbert (Gi) magnetomotive force 0.795 775 ampere-turns (A)

kayser (K) wave number 100 per meter (m-1)

lambert (Lb) luminance 3183.099 candelas per square meter (cd·m-2)

langley heat transmission 41.84 kilojoules per square meter (kJ·m-2)

line (li) magnetic flux 10-8 weber (Wb)

maxwell (Mx) magnetic flux 10-8 weber (Wb)

oersted (Oe) magnetic field strength 79.577 472 ampere-turns per meter (A·m-1)

phot (ph) illumination 104 lux (lx)

poise (P) dynamic viscosity 0.1 pascal second (Pa·s)

stilb (sb) luminance 104 candelas per square meter (cd·m-2)

stokes (St) kinematic viscosity 10-4 square meters per second (m2·s-1)

unit pole magnetic flux 1.256 637 x 10-7 weber (Wb)

DIFFERENT STANDARDS OF MEASUREMENTS

THE INTERNATIONAL SYSTEM OF UNITS (SI)

All systems of weights and measures, metric and non-metric, are linked through a network of international agreements supporting the International System of Units. The International System is called the SI, using the first two initials of its French name Système International d'Unités. The key agreement is the Treaty of the Meter (Convention du Mètre), signed in Paris on May 20, 1875. 48 nations have now signed this treaty, including all the major industrialized countries. The United States is a charter member of this metric club, having signed the original document back in 1875.

The SI is maintained by a small agency in Paris, the International Bureau of Weights and Measures (BIPM, for Bureau International des Poids et Mesures), and it is updated every few years by an international conference, the General Conference on Weights and Measures (CGPM, for Conférence Générale des Poids et Mesures), attended by representatives of all the industrial countries and international scientific and engineering organizations. The 22nd CGPM met in October 2003; the next meeting will be in 2007. As BIPM states on its web site, "The SI is not static but evolves to match the world's increasingly demanding requirements for measurement."

At the heart of the SI is a short list of base units defined in an absolute way without referring to any other units. The base units are consistent with the part of the metric system called the MKS system.

In all there are seven SI base units: the meter for distance, the kilogram for mass, the second for time, the ampere for electric current, the kelvin for temperature, the mole for amount of substance, and the candela for intensity of light.

SI derived units, are defined algebraically in terms of these fundamental units.Currently there are 22 SI derived units. They include: the radian and steradian for plane and solid angles,

respectively; the newton for force and the pascal for pressure; the joule for energy and the watt for power; the degree Celsius for everyday measurement of

temperature; units for measurement of electricity: the coulomb

(charge), volt(potential), farad(capacitance), ohm (resistance), and siemens (conductance);

units for measurement of magnetism: the weber (flux), tesla (flux density), and henry (inductance);

the lumen for flux of light and the lux for illuminance; the hertz for frequency of regular events and the

becquerel for rates of radioactivity and other random events;

the gray and sievert for radiation dose; and the katal, a unit of catalytic activity used in

biochemistry.

In addition to the 29 base and derived units, the SI permits the use of certain additional units, including: the traditional mathematical units for measuring angles (degree,

arcminute, and arcsecond); the traditional units of civil time (minute, hour, day, and year); two metric units commonly used in ordinary life: the liter for

volume and the tonne (metric ton) for large masses; the logarithmic units bel and neper (and their multiples, such as

the decibel); and three non-metric scientific units whose values represent

important physical constants: the astronomical unit, the atomic mass unit or dalton, and the electronvolt.

The SI currently accepts the use of certain other metric and non-metric units traditional in various fields. These units are supposed to be "defined in relation to the SI in every document in which they are used," and "their use is not encouraged." These barely-tolerated units might well be prohibited by future meetings of the CGPM. They include: the nautical mile and knot, units traditionally used at sea and in

meteorology; the are and hectare, common metric units of area; the bar, a pressure unit, and its commonly-used multiples such

as the millibar in meteorology and the kilobar in engineering; the angstrom and the barn, units used in physics and astronomy.

ELECTRICAL STANDARDS ABSOLUTE AMPERE:

The international system of units defines the ampere as constant current which, if maintained in two straight parallel conductors of infinite length and negligible circular cross section placed 1m apart in a vacuum, will produce between these conductors a force equal to 2x10^-7 Newton per meter length.

Early – absolute value measured – current balance – force between two parallel conductors.

In 1948 – absolute ampere – measured using – current balance – weighs the force exerted between two current carrying conductors.

Resistance Standards: The absolute value of ohm in the SI system is

defined in terms of the fundamental units of length , mass, and time.

The standard resistor is a coil of wire some alloy like manganin which has a high electrical resistivity and a low temperature coefficient of resistance.

Voltage Standards: For years the standard volt was based on an

electrochemical cell called saturated cell (or) standard cell.

The standard cell has a temperature dependence and the output voltage changes about -40µV/ºC from the nominal of 1.01858 V.

New standard came in 1962 by the work of Brian Josephson. A voltage is developed across the junction, which is related to the irradiating frequency by:v=(hf/2e)

The accuracy including all of the system inaccuracies is one part in 10^8.

Capacitance Standards: Farad(unit of capacitance) can be measured with

Maxwell dc commutated bridge , where the capacitance is computed from the resistive bridge arms and the frequency of the dc commutation.

Standard capacitors are usually constructed from interleaved metal plates with air as the dielectric material.

Inductance Standards: Derived from the ohm and farad , rather than from

the large geometrically constructed inductors used to determine absolute value of ohm.

A typical set of fixed inductance standards includes values from approximately 100µH to 10H.

IEEE STANDARDS INSTITUTE OF ELECTRICAL AND

ELECTRONICS ENGINEERS (IEEE) an engineering society headquartered in New York City.

Found in 1963 by the merger of the Institute of Radio Engineers (ire)and American Institute of Electrical Engineers(AIEE).

These standards are not physical items available for comparison and checking of secondary standards but are standard procedures, nomenclature, definitions, etc.

NOT OVER YET………