The SI System

Compressed Air Wiki Basic Theory

Any physical quantity is the product of a numerical value and a unit. Since 1964, the International System of Units (SI system) has gradually been adopted worldwide, with the exception of Liberia, Myanmar and the United States.

Basic information can be found in standard ISO 31, which is under revision and will be superseded by ISO/IEC 80000: Quantities and Units. Units are divided into four different classes :

Base units
Supplementary units
Derived units
Additional units

Base units, supplementary units and derived units are called SI units. The additional units are not SI units, although they are accepted for use with SI units. Base units are any of the established, independent units in which all other units can be expressed.

Length	meter	m
Mass kilogram	kilogram	kg
Time	second	s
Electrical current	ampere	A
Temperature	kelvin	K
Luminous intensity	candela	cd
Amount of substance	mole	mol

Derived units are formed as a power or product of powers of one or more base units and/or supplementary units according to the physical laws for the relationship between these different units. Additional units: A limited number of units outside the SI system cannot be eliminated for different reasons, and continue to be used along with the SI as additional units.

Quantity	Unit	Symbol	Expressed in other SI units
frequency	hertz	Hz	s^-1
force	newton	N	kg x m x s^-2
pressure / mechanical stress	pascal	Pa	N / m²
energy / work	joule	J	N x m
power	watt	W	J / s
electric quantity / charge	coulomb	C	A x s
electric voltage	volt	V	W / A
capacitance	farad	F	C / V
resistance	ohm	Ω	V / A
conductivity	siemens	S	A / V
magnetic flux	weber	Wb	V x s
magnetic flux density	tesla	T	Wb / m²
inductance	henry	H	Wb / A
luminous flux	lumen	lm	Cd x sr
light	lux	lx	Lm / m²
angle	radian	rad	m / m
solid angle	steradian	sr	m² / m²

Quantity	Unit	Symbol	Remark
volume	liter	l	1 l = 1 dm³
time	minute	min	1 min = 60 s
time	hour	h	1 h = 60 min
mass	metric ton	t	1 t = 1.000 kg
pressure	bar	bar	1 bar = 105 Pa
plane angle	degree	.°	1° = π/180 rad
plane angle	minute	.'	1' = 1°/60
plane angle	second	."	1" = 1'/60

Prefixes may be added to a unit to produce a multiple of the original unit. All of these multiples are integer powers of ten, for example: kilo-denotes a multiple of thousand (10³) milli-denotes a multiple of one thousandth (10-3)

Power	Prefix Designation	Prefix Symbol	Example	Symbol
1012	tera	T	1 terajoule	1 TJ
109	giga	G	1 gigahertz	1 GHz
106	mega	M	1 megawatt	1 MW
103	kilo	k	1 kilometer	1 km
102	hecto	h	1 hectoliter	1 hl
101	deca	da	1 decalumen	1 dalm
10-1	deci	d	1 decibel	1 dB
10-2	centi	c	1 centimeter	1 cm
10-3	milli	m	1 milligram	1 mg
10-6	micro	µ	1 micrometer	1 µm
10-9	nano	N	1 nanohenry	1 nH
10-12	pico	p	1 picofarad	1 pF
10-15	femto	f	1 femtometer	1 fm
10-18	atto	a	1 attosecond	1 as

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The SI System

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