Arbitrary unit
Encyclopedia
In science
and technology
, an arbitrary unit (abbreviated arb. unit, see below)
or procedure defined unit (p.d.u.)
is a relative unit of measurement to show the ratio of amount of substance
, intensity
, or other quantities, to a predetermined reference
measurement. The reference measurement is typically defined by the local laboratories or dependent on individual measurement apparatus. It is therefore impossible to compare "1 arb. unit" by one measurer and "1000 arb. unit" by another measurer without detailed prior knowledge on how the respective "arbitrary units" were defined; thus, the unit is sometimes called an unknown unit. The unit only serves as a placeholder unit to compare multiple measurements performed in similar environment.
Since the unit expresses the ratio between the measurement and the reference, it can be considered a unit for dimensionless quantities.
The unit is commonly used in fields such as physiology to indicate substance concentration, and spectroscopy
to express spectral intensity.
When the reference measurement is precisely defined and internationally agreed upon, arbitrary units can also be a unit capable of public comparison. One example of a publicly defined arbitrary unit is the WHO International Unit
.
Among these, "AU" and "a.u." are common abbreviations for astronomical unit
s and atomic units
. For this reason, at least one academic journal
explicitly recommends against using "a.u." and recommends "arb. unit" instead.
While arbitrary unit is not a formally recognized unit, IUPAC and IFCC recognize the need to deal with "unknown units", and decided in 2008 to use the term "procedure defined unit" in their database. The decision forbids using factors or denominators in conjunction with p.d.u. (such as "p.d.u./L").
Science
Science is a systematic enterprise that builds and organizes knowledge in the form of testable explanations and predictions about the universe...
and technology
Technology
Technology is the making, usage, and knowledge of tools, machines, techniques, crafts, systems or methods of organization in order to solve a problem or perform a specific function. It can also refer to the collection of such tools, machinery, and procedures. The word technology comes ;...
, an arbitrary unit (abbreviated arb. unit, see below)
or procedure defined unit (p.d.u.)
is a relative unit of measurement to show the ratio of amount of substance
Amount of substance
Amount of substance is a standards-defined quantity that measures the size of an ensemble of elementary entities, such as atoms, molecules, electrons, and other particles. It is sometimes referred to as chemical amount. The International System of Units defines the amount of substance to be...
, intensity
Intensity (physics)
In physics, intensity is a measure of the energy flux, averaged over the period of the wave. The word "intensity" here is not synonymous with "strength", "amplitude", or "level", as it sometimes is in colloquial speech...
, or other quantities, to a predetermined reference
Reference
Reference is derived from Middle English referren, from Middle French rèférer, from Latin referre, "to carry back", formed from the prefix re- and ferre, "to bear"...
measurement. The reference measurement is typically defined by the local laboratories or dependent on individual measurement apparatus. It is therefore impossible to compare "1 arb. unit" by one measurer and "1000 arb. unit" by another measurer without detailed prior knowledge on how the respective "arbitrary units" were defined; thus, the unit is sometimes called an unknown unit. The unit only serves as a placeholder unit to compare multiple measurements performed in similar environment.
Since the unit expresses the ratio between the measurement and the reference, it can be considered a unit for dimensionless quantities.
The unit is commonly used in fields such as physiology to indicate substance concentration, and spectroscopy
Spectroscopy
Spectroscopy is the study of the interaction between matter and radiated energy. Historically, spectroscopy originated through the study of visible light dispersed according to its wavelength, e.g., by a prism. Later the concept was expanded greatly to comprise any interaction with radiative...
to express spectral intensity.
When the reference measurement is precisely defined and internationally agreed upon, arbitrary units can also be a unit capable of public comparison. One example of a publicly defined arbitrary unit is the WHO International Unit
International unit
In pharmacology, the International Unit is a unit of measurement for the amount of a substance, based on biological activity or effect. It is abbreviated as IU, as UI , or as IE...
.
Abbreviations and nomenclature
Abbreviations for "arbitrary unit" include: arb. unit, arb. u., AU, and a.u.Among these, "AU" and "a.u." are common abbreviations for astronomical unit
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....
s and atomic units
Atomic units
Atomic units form a system of natural units which is especially convenient for atomic physics calculations. There are two different kinds of atomic units, which one might name Hartree atomic units and Rydberg atomic units, which differ in the choice of the unit of mass and charge. This article...
. For this reason, at least one academic journal
Academic journal
An academic journal is a peer-reviewed periodical in which scholarship relating to a particular academic discipline is published. Academic journals serve as forums for the introduction and presentation for scrutiny of new research, and the critique of existing research...
explicitly recommends against using "a.u." and recommends "arb. unit" instead.
While arbitrary unit is not a formally recognized unit, IUPAC and IFCC recognize the need to deal with "unknown units", and decided in 2008 to use the term "procedure defined unit" in their database. The decision forbids using factors or denominators in conjunction with p.d.u. (such as "p.d.u./L").
External links
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