To date, the International System of Units, the SI, is made up of seven base units (between brackets the sole/single symbol representing it) :
- The metre (m)
- The kilogram (kg)
- The second (s)
- The ampere (A)
- The kelvin (K)
- The candela (cd)
- The mole (mol)
metre (m) The metre is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second. kilogram (kg) The kilogram is the mass of the platinum-iridium prototype which was approved by the Conférence Générale des Poids et Mesures, held in Paris in 1889, and kept by the Bureau International des Poids et Mesures. second (s) The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom. ampere (A) The ampere is the intensity of a constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 metre apart in vacuum, would produce between these conductors a force equal to 2 x 10-7 newton per metre of length. kelvin (K) The kelvin is the fraction 1/273,16 of the thermodynamic temperature of the triple point of water. candela (cd) The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 x 1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian mole (mol) The mole is the amount of substance of a system which contains as many elementary entities as there are atoms in 0,012 kilogram of carbon 12.
The derived units (including the dimensionless units)
There are numerous derived units that are complementary to the base units. They may have special names (e.g. hertz, pascal, becquerel, etc.) but can always be expressed in terms of the base units. There are also dimensionless derived units.
It should be noted that these units are linked together to form a consistent system.
These units are linked together to form a coherent system.
Lastly, each quantity may need to cover a vast range of values. To avoid the need for multiplying factors or values with a large number of zeros, prefixes are used. The prefixes cover a range extending from 1024 to 10-24 times the units.
- ELECTRICITY AND MAGNETISM
Current intensity : The ampere (A)
| potential difference, U : | volt (V = W/A) |
| electrical capacitance, C : | farad (F = C/V) |
| electrical resistance, R : | ohm (Ω = V/A) |
| inductance, L : | henri (H = Wb/A) |
| quantity of electricity, Q : | coulomb (C = A.s) |
| power, P : | watt (W = J/s) |
| energy, W : | joule (J = N.m) |
| magnetic induction, B : | tesla (T = Wb/m2) |
| electric field, E : | volt per metre (V/m) |
| magnetic field strength, H : | ampère per metre (A/m) |
| electric conductance, G : | siemens (S = A/V) |
| attenuation, η : | decibel (dB) |
- MASS AND RELATED QUANTITIES
The mass : the kilogram (kg)
| density : ρ | kg.m-3 |
| volume : V | m-3 |
| force : F | newton (N) |
| torque : M | N.m |
| pressure : p | pascal (Pa) |
| dynamic viscosity : η | Pa.s |
| kinematic viscosity : υ | m2.s-1 |
| acoustic pressure : p | pascal (Pa) |
| dynamic volume : v | m3 |
| mass flow-rate : qm | kg.s-1 |
| volume flow-rate : qv | m3.s-1 |
| air flow-rate : V | m.s-1 |
- LENGTH AND DIMENSIONAL QUANTITIES
Length : the metre (m)
| wavelength : λ | metre (m) |
| length of material standards : L | metre (m) |
| lplane angle : α | radian (rad) |
| form measurement : | metre (m) |
- RADIOMETRY – PHOTOMETRY
Photometry
Luminous intensity : the candela (cd) (m)
| luminous flux : Φ | lumen (lm) |
| illuminance : E | lux (lx) |
| luminance : L | cd.m-2 |
Radiometry of detectors
| spectral responsivity : S(λ) | A.W-1 |
Radiometry of sources
| energy flow : Φe | watt (W) |
| radiance : Le | W.m-2.sr-1 |
| irradiance : Ee | W.m-2 |
| laser source power : P | watt (W) |
| laser source energy : Q | joule (J) |
Radiometry of materials
| regular spectral transmittance : t(Φ) | flux ratio |
| diffuse spectral reflectance : R(λ) | flux ratio |
Fibre optics
| energy flow : P | watt (W) |
| wavelength : λ | metre (m) |
| propagation time : t | second (s) |
| fibre length | metre (m) |
| linear attenuation factor : | dB.m-1 |
| reflectance | dB |
| detector (or fibre) passband | hertz (Hz) (ou Hz.m-1) |
- TEMPERATURE AND THERMAL QUANTITIES
Thermometry and radiation thermometry
| temperature in the ITS-90 or in the PLTS-2000 : T | kelvin (K) |
| or t | degree Celsius (°C) |
Metrology of thermal quantities
| thermal conductivity : λ = α.ρ.Cp | (ρ = density) | W.m-1.K-1 |
| thermal diffusivity : α =λ/ρ.Cp | (ρ = density) | m2.s-1 |
| specific heat capacity : Cp = (∂H/∂T)p | (H = enthalpy) | J.kg-1.K-1 |
| spectral directional emissivity : ελ | dimensionless ratio | |
| normal spectral emissivity : ελ | dimensionless ratio | |
| total hemispherical emissivity : ελ | dimensionless ratio |
Hygrometry
| temperature | dew point : Td frost point : Tf | degree Celsius (°C) degree Celsius (°C) |
| l’humidité relative | with respect to water : Uw with respect to ice : Ui | percentage (%) percentage (%) |
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