Standards method of Symbol for Units of Measurement
Conventions
1. The symbol derived by the name of the scientist is in upper case letters e.g.
Voltage – V, Ampere –A, Power – W, Newton – N, Energy-J ( Joule), Charge – Q (Coulomb), Capacitance – F (Farad), Inductance – H (Henry), Conductance – S(Siemens), Magnetic Flux Density – T (Tesla), Thermodynamic Temperature – K (Kelvin), Noise – dB (deci-Graham Alexender Bel), Curie –C, Roentgen-R
2. When the unit is of two letters derived from the name of the scientists, then first letter is capital and the second is small
Pressure – Pa (Pascal), Magnetic Flux- Wb (Weber), Frequency – Hz (Hertz), Gauss – Ga
3. There were debate for the symbol for resistance as the first letter is O which may confuse with Zero/Nil, therefore, slashed from the bottom and written as Ω
4. All other symbols are written in small with single and double alphabet as follows
Distance – meter (m, cm, km)
Mass – gram (g, kg, t-1000 g)
Time – second (s, min-60 s, h-60 min, d-24 h
Volume – liter (UK) and liter (USA) and US, Canada and Australia preferred letter L to avoid confusion with number 1 where as other countries follows l)
5. Multiple and sub-multiple are denoted by small letter affix except Mega and above e.g.
µA (0.0000001 A), mA (0.0001 A) and kA (1000 A)
MVA (1000000 VA), GHz (1000000000 Hz)
6. A space is always left between numeral value and unit e.g.
230 V and not 230V
7. pH is power of hydrogen
8. For instantaneous values, lower case alphabet is used such as i and v for current and voltage .
9. Litre, tonne and electron-volt are not SI but used very commonly due to acceptance and widely followed.
SI system
SI is also called System International, a system of unit adopted since 1960 and generally called as the metric system as well. The system provided a basis for worldwide standardisation of the measurement units in all fields of science and technology. The system was adopted by India in 1961. The SI system had the following advantages
1. To adopt only one system of units for use in all spheres of education, trade and industry.
2. Advantage of Coherence means that the derived units may be expressed as a combinations of based units without the need for the connecting numerical factors. The division of distance in meter and time in s gives velocity as m/s without requiring any conversion factor.
3. The base units are length – meter (m), mass – kilogram (kg), time-second (s), electric current – ampere (A), thermodynamic temperature – Kelvin (K), luminous intensity – candela (cd ) and amount of substance – mole (mol)
4. The derived unit are are formed by combination of base units according to the algebraic relations linking the corresponding quantities. Pressure, Energy, Power, Capacitance, Electric Resistance, magnetic flux, Inductance, luminous flux, temperature in Celsius are some of the derived units.