Standards and Units
- 1. Standards & Units M. Afnan Habibi
- 2. Lesson Summary • Introduction Concept Measurement • Fundamental and Derived Units • Standards Classification
- 3. Introduction • Measurement is the result of a quantitative comparison between a given quantity and a quantity of the same kind chosen as a unit. • The result of the measurement is expressed by a number representing the ratio between the unknown quantity and the standard. • A standard is defined as the physical personification of the unit of measurement. • The device or instrument used for comparing the unknown quantity with the unit of measurement or a standard quantity is called a measuring instrument
- 4. Introduction • In direct measurement methods, the unknown quantity is measured directly instead of comparing it with a standard. Examples: current by ammeter, voltage by voltmeter, resistance by ohmmeter, power by wattmeter, etc. • In indirect measurement methods, the value of the unknown quantity is determined by measuring the functionally related quantity and calculating the desired quantity rather than measuring it directly. Example: R=V/I
- 5. Fundamental and Derived Units • The fundamental units in mechanics are measures of length, mass and time. Because they are fundamental to most other physical quantities. • Measures of certain physical quantities in the thermal, electrical and illumination disciplines are also represented by auxiliary fundamental units. • All other units which can be expressed in terms of the fundamental units are called derived units. • Every derived unit originates from some physical law defining that unit.
- 6. Dimensions • A derived unit is recognized by its dimensions, which can be defined as the complete algebraic formula for the derived unit. • The dimensional symbols for the fundamental units of length, mass and time are L, M and T respectively. • The dimensional formulas of the derived units are particularly useful for converting units from one system to another. Example: Newton
- 7. Standards • A standard of measurement is a physical representation of a unit of measurement. • A unit is realized by reference to an arbitrary material standard or to natural phenomena including physical and atomic constants. • The term ‘standard’ is applied to a piece of equipment having a known measure of physical quantity.
- 8. The Classifications of Standards 1) International standards 2) Primary standards 3) Secondary standards 4) Working standards 5) Current standards 6) Voltage standards 7) Resistance standards 8) Capacitance standards 9) Time and frequency standards
- 9. International Standards • The international standards represent certain units of measurement to the closest possible accuracy that production and measurement technology allow. • These standards are maintained at the International Bureau of Weights and Measures and are not available to the ordinary user of measuring instruments for purposes of comparison or calibration Symbol Name Quantity A ampere electric current K kelvin temperature s second time m metre length kg kilogram mass cd candela luminous intensity mol mole amount of substance
- 10. Primary Standards • The primary standards are maintained by national standards laboratories in different places of the world. • The National Bureau of Standards (NBS) in Washington is responsible for maintenance of the primary standards in North America. • Other national laboratories include the National Physical Laboratory (NPL) in Great Britain and the oldest in the world, the Physikalisch Technische Reichsanstalt in Germany. • Primary standards are not available for use outside the national laboratories.
- 11. Secondary Standards • Secondary standards are the basic reference standards used in the industrial measurement laboratories. • These standards are maintained by the particular involved industry and are checked locally against other reference standards in the area.
- 12. Working Standards • Working standards are the principle tools of a measurement laboratory. • They are used to check and calibrate general laboratory instruments for accuracy and performance or to perform comparison measurements in industrial applications.
- 13. Current Standard • The fundamental unit of electric current (Ampere) is defined by the International System of Units (SI) as the constant current • Which maintained in two straight parallel conductors of infinite length and negligible circular cross section placed 1 meter apart in vacuum, will produce between these conductors a force equal to 2 × 10-7 newton per meter length
- 14. Voltage Standard • In early times, the standard volt was based on an electrochemical cell called the saturated standard cell or simply standard cell. Standard cell of emf of 1.0183 volt at 20°C (Courtesy, physics.kenyon.edu)
- 15. Voltage Standard • In 1962, based on the work of Brian Josephson, a new standard for the volt was introduced. A thin-film junction is cooled to nearly absolute zero and irradiated with microwave energy. • A voltage is developed across the junction, which is related to the irradiating frequency by the following relationship: V=hf/2e • h= 6.63 x 10-34 Js • e= 1.602 x 10-19 C • f= frequency of microwave irradiation.
- 16. Resistance Standard • The standard resistor is a coil of wire of some alloy like manganin which has a high electrical resistivity and a low temperature coefficient of resistance. • The resistance coil is mounted in a double walled sealed container to prevent changes in resistance due to moisture conditions in the atmosphere. Resistance standard
- 17. Capacitance Standard • The unit of capacitance (the farad) can be measured with a 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. Commutated dc method for measuring capacitance
- 18. Time Standard and Frequency Standard • A mean solar second is then equal to 1/86400 of the mean solar day. • In the year 1956, the ephemeris second has been defined by the International Bureau of Weights and Measures as the fraction 1/31556925.99747 of the tropical year for 1900 January 01 at 12 h ET (Ephemeris Time), and adopted as the fundamental invariable unit of time. • The International Committee of Weights and Measures has now defined the second in terms of frequency of the cesium transition, assigning a value of 9192631770 Hz to the hyperfine transition of the cesium atom unperturbed by external fields.