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Unified Code for Units of Measure

From Wikipedia, the free encyclopedia

System of codes for unambiguously representing measurement units

The Unified Code for Units of Measure (UCUM) is a system of codes for unambiguously representing measurement units. Its primary purpose is machine-to-machine communication rather than communication between humans.[1] UCUM is used by different organizations like IEEE, and standards like DICOM, LOINC, HL7, and ISO 11240:2012.[2]

The code set includes all units defined in ISO 1000, ISO 2955-1983,[3][a] ANSI X3.50-1986,[4][b] HL7 and ENV 12435, and explicitly and verifiably addresses the naming conflicts and ambiguities in those standards to resolve them. It provides for representations of units in 7 bit ASCII for machine-to-machine communication, with unambiguous mapping between case-sensitive and case-insensitive representations.

A reference open-source implementation is available as a Java applet. There is also an OSGi-based implementation at Eclipse Foundation.

Units are represented in UCUM with reference to a set of seven base units.[5] The UCUM base units are the metre for measurement of length, the second for time, the gram for mass, the coulomb for charge, the kelvin for temperature, the candela for luminous intensity, and the radian for plane angle. The UCUM base units form a set of mutually independent dimensions as required by dimensional analysis.

Some of the UCUM base units are different from the SI base units. UCUM is compatible with, but not isomorphic with, SI. There are four differences between the two sets of base units:

  1. The gram is the base unit of mass instead of the kilogram, since in UCUM base units do not have prefixes.
  2. Electric charge is the base quantity for electromagnetic phenomena instead of electric current, since the elementary charge of electrons is more fundamental physically.
  3. The mole is dimensionless in UCUM, since it can be defined in terms of the Avogadro number,
  4. The radian is a distinct base unit for plane angle, to distinguish angular velocity from rotational frequency and to distinguish the radian from the steradian for solid angles.
Metric and non-metric units[edit] UCUM Metric prefixes Prefix UCUM Symbol Factor Power yotta Y 1000000000000000000000000 1024 zetta Z 1000000000000000000000 1021 exa E 1000000000000000000 1018 peta P 1000000000000000 1015 tera T 1000000000000 1012 giga G 1000000000 109 mega M 1000000 106 kilo k 1000 103 hecto h 100 102 deca da 10 101 (none) (none) 1 100 deci d 0.1 10−1 centi c 0.01 10−2 milli m 0.001 10−3 micro u 0.000001 10−6 nano n 0.000000001 10−9 pico p 0.000000000001 10−12 femto f 0.000000000000001 10−15 atto a 0.000000000000000001 10−18 zepto z 0.000000000000000000001 10−21 yocto y 0.000000000000000000000001 10−24


Each unit represented in UCUM is identified as either "metric" or "non-metric".[5] Metric units can accept metric prefixes as in SI. Non-metric units are not permitted to be used with prefixes. All of the base units are metric.

UCUM refers to units that are defined on non-ratio scales as "special units". Common examples include the bel and degree Celsius. While these are not considered metric units by UCUM, UCUM nevertheless allows metric prefixes to be used with them where this is common practice.[5]

Binary prefixes are also supported.

UCUM Binary prefixes Prefix UCUM Symbol Factor Power tebi Ti 1099511627776 240 gibi Gi 1073741824 230 mebi Mi 1048576 220 kibi Ki 1024 210

UCUM recognizes units that are defined by a particular measurement procedure, and which cannot be related to the base units.[5] These units are identified as "arbitrary units". Arbitrary units are not commensurable with any other unit; measurements in arbitrary units cannot be compared with or converted into measurements in any other units. Many of the recognized arbitrary units are used in biochemistry and medicine.

Any metric unit in any common system of units can be expressed in terms of the UCUM base units.

Units derived from UCUM base units Name Symbol Quantity UCUM base unit
Equivalents hertz Hz frequency s−1 steradian sr[n 1] solid angle rad2 millinewton mN force, weight g⋅m⋅s−2 millipascal mPa pressure, stress g⋅m−1⋅s−2 millijoule mJ energy, work, heat g⋅m2⋅s−2 milliwatt mW power, radiant flux g⋅m2⋅s−3 ampere A[n 2] electric current C⋅s−1 millivolt mV voltage, electrical potential difference, electromotive force g⋅m2⋅s−2⋅C−1 kilofarad kF electrical capacitance g−1⋅m−2⋅s3⋅C2 milliohmelectrical resistance, impedance, reactance g⋅m2⋅s−1⋅C−2 kilosiemens kS electrical conductance g−1⋅m−2⋅s1⋅C2 milliweber mWb magnetic flux g⋅m2⋅s−1⋅C−1 millitesla mT magnetic induction, magnetic flux density g⋅s−1⋅C−1 millihenry mH electrical inductance g⋅m2⋅C−2 degree Celsius °C Celsius temperature K lumen lm luminous flux cd⋅rad2 lux lx illuminance m−2⋅cd⋅rad2 becquerel Bq radioactivity (decays per unit time) s−1 gray Gy absorbed dose (of ionizing radiation) m2⋅s−2 sievert Sv equivalent dose (of ionizing radiation) m2⋅s−2
Notes
  1. ^ In the SI, both the radian and steradian are dimensionless derived units.
  2. ^ In the SI, the coulomb is derived from the ampere. 1 C = 1 A × 1 s.
Mechanical units Name Symbol Quantity Expression in terms
of UCUM base units square metre m2 area m2 cubic metre m3 volume m3 millinewton second mN⋅s momentum, impulse m⋅g⋅s−1 millijoule second per radian mN⋅m⋅s/rad angular momentum m2⋅g⋅rad⋅s−1 millijoule per radian mN⋅m/rad = mJ/rad torque m2⋅g⋅rad⋅s−2 millinewton per second mN/s yank m⋅g⋅s−3 reciprocal metre m−1 wavenumber, optical power, curvature, spatial frequency m−1 gram per square metre g/m2 area density m−2⋅g gram per cubic metre g/m3 density, mass density m−3⋅g cubic metre per gram m3/g specific volume m3⋅g−1 millijoule second mJ⋅s action m2⋅g⋅s−1 millijoule per gram mJ/g specific energy m2⋅s−2 millijoule per cubic metre mJ/m3 energy density m−1⋅g⋅s−2 millinewton per metre mN/m = mJ/m2 surface tension, stiffness g⋅s−2 milliwatt per square metre mW/m2 heat flux density, irradiance g⋅s−3 square metre per second m2/s kinematic viscosity, thermal diffusivity, diffusion coefficient m2⋅s−1 millipascal second mPa⋅s = mN⋅s/m2 dynamic viscosity m−1⋅g⋅s−1 gram per metre g/m linear mass density m−1⋅g gram per second g/s mass flow rate g⋅s−1 milliwatt per steradian square metre mW/(sr⋅m2) radiance g⋅rad−2⋅s−3 milliwatt per steradian cubic metre mW/(sr⋅m3) radiance m−1⋅g⋅rad−2⋅s−3 milliwatt per metre mW/m spectral power m⋅g⋅s−3 gray per second Gy/s absorbed dose rate m2⋅s−3 metre per cubic metre m/m3 fuel efficiency m−2 milliwatt per cubic metre mW/m3 spectral irradiance, power density m−1⋅g⋅s−3 millijoule per square metre second mJ/(m2⋅s) energy flux density g⋅s−3 reciprocal millipascal mPa−1 compressibility m⋅g−1⋅s2 millijoule per square metre mJ/m2 radiant exposure g⋅s−2 gram square metre per steradian g⋅m2/sr moment of inertia m2⋅g⋅rad−2 millijoule second per radian per gram mN⋅m⋅s/rad/g specific angular momentum m2⋅s−1⋅rad−1 milliwatt per steradian mW/sr radiant intensity m2⋅g⋅rad−2⋅s−3 milliwatt per steradian metre mW/(sr⋅m) spectral intensity m⋅g⋅rad−2⋅s−3
  1. ^ Withdrawn without replacement.
  2. ^ Superseded by ISO 5807.

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