Quantity

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Quantity

Description

A quantity is the attribute of a phenomenon, body or substance that may be distinguished qualitatively and determined quantitatively.

Reference: ISO 15189:2012 Medical laboratories — Particular requirements for quality and competence

Base quantities and count

Quantity	Symbol for quantity Q	Symbol for dimension	Name of abstract unit uQ	Symbol for unit uQ [*]
elementary entity *,$	UX	U	elementary unit	x
count *,$	NX = N·UX	X	elementary unit	x
amount of substance *,§	nX = NX·NA-1	N	mole	mol
charge *,€	Qel = zX·e·NX	I·T	coulomb	C = A·s
length	l	L	meter	m
mass	m	M	kilogram	kg
time	t	T	second	s
electric current	I	I	ampere	A
thermodynamic temperature	T	Θ	kelvin	K
luminous intensity	Iv	J	candela	cd

[*] SI units, except for the canonical 'elementary unit' [x]. The following footnotes are canonical comments, related to iconic symbols.

^* For the elementary quantities N_X, n_X, and Q_el, the entity-type X of the elementary entity U_X has to be specified in the text and indicated by a subscript: n_O2; N_ce; Q_el.

^$ Count N_X equals the number of elementary entities U_X. In the SI, the quantity 'count' is explicitly considered as an exception: "Each of the seven base quantities used in the SI is regarded as having its own dimension. .. All other quantities, with the exception of counts, are derived quantities" (Bureau International des Poids et Mesures 2019 The International System of Units (SI)). An elementary entity U_X is a material unit, it is not a count (U_X is not a number of U_X). N_X has the dimension X of a count and U_X has the dimension U of an elementary entity; both quantities have the same abstract unit, the 'elementary unit' [x].

^§ Amount n_X is an elementary quantity, converting the elementary unit [x] into the SI base unit mole [mol] using the Avogadro constant N_A.

^€ Charge is a derived SI quantity. Charge is an elementary quantity, converting the elementary unit [x] into coulombs [C] using the elementary charge e, or converting moles [mol] into coulombs [C] using the Faraday constant F. z_X is the charge number per elementary entity U_X, which is a constant for any defined elementary entity U_X. Q_el = z_X·F·n_X

Quantities and units

1.1. Quantity: "property of a phenomenon, body, or substance, where the property has a magnitude that can be expressed as a number and a reference. .. The generic concept ‘quantity’ can be divided into several levels of specific concepts". The generic concept 'length' can be divided into radius of a circle or wavelength. "A reference can be a measurement unit, a measurement procedure, a reference material, or a combination of such. .. The symbols for quantities are written in italics. .. The concept ‘quantity’ may be generically divided into, e.g. ‘physical quantity’, ‘chemical quantity’, and ‘biological quantity’, or base quantity and derived quantity."

1.2. Kind of quantity: "aspect common to mutually comparable quantities. .. The quantities diameter, circumference, and wavelength are generally considered to be quantities of the same kind, namely of the kind of quantity called length.

1.7. Quantity dimension; dimension of a quantity: "expression of the dependence of a quantity on the base quantities of a system of quantities as a product of powers of factors corresponding to the base quantities, omitting any numerical factor."

1.19. Quantity value; value of a quantity: "number and reference together expressing magnitude of a quantity"

• Reference: 1. Quantities and units. International vocabulary of metrology — Basic and general concepts and associated terms (VIM). ISO. 2008. Retrieved 2020-05-14

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