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Basic information can be found in standard ISO 31, which is under revision and will be superseded by ISO/IEC 80000: Quantities and Units.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