Article 1. of Royal Decree-Law 1296/1986 of 28 June, amending Law 3/1985 of 18 March of Metrology and establishing the EEC metrological control, determines how the Legal Units of Measure basic, supplementary and derived units of the International System of Units (SI), adopted by the General Conference of Weights and Measures and in force in the European Economic Community.

Likewise, in the aforementioned article, it is available that the Government, by Royal Decree, will establish the definitions of the units, their names and symbols, as well as the rules for the formation of their multiples and submultiples, according to with the agreements of the General Conference of Weights and Measures and the rules of the European Economic Community.

In Article 2, powers are conferred on the Government to authorize the use of certain non-core and non-core units in the International System of Units (SI), which is deemed indispensable for certain measurements, with the condition that they relate directly to those of the International System.

In paragraph 5 of Article 3, the Government is also empowered to require, by means of Royal Decree, that the measuring instruments contain the indications of magnitude in a single unit of legal measure.

Consequently, the content of the Resolutions of the General Conference of Weights and Measures, the Council Directive of the European Communities 80 /181/EEC, as amended by Directive 85 /11/EEC, as well as the International Document "Legal Measures Units" of OIML (International Organization of Legal Metrology).

In its virtue, on the proposal of the Minister of Public Works and Urbanism, in agreement with the Council of State, and after deliberation of the Council of Ministers at its meeting of October 27, 1989,

DISPONGO:

Single item.

1. The Legal System of Mandatory Units of Measure in Spain is the decimal metric system of seven basic units, called the International System of Units (SI), adopted by the General Conference of Pesos and Measures and in force in the Community European Economic.

2. The basic and supplementary SI units (Chapter I), the SI units derived (Chapter II) and the rules for the formation of multiples and submultiples of such units are related and defined in the Annex to this Royal Decree.

(Chapter III).

3. The use of the units listed in Chapter IV of that Annex shall also be authorised.

TRANSIENT DISPOSITION

The instruments, apparatus, means and systems of measurement must bear their indications of magnitude in a single unit of legal measure, from 31 December 1990.

FINAL DISPOSITION

This Royal Decree shall enter into force on the day following that of its publication in the "Official Gazette of the State".

Given in Madrid to October 27, 1989.

JOHN CARLOS R.

The Minister of Public Works and Urbanism,

JAVIER LUIS SAENZ DE COSCULLUELA

ANNEX

CHAPTER FIRST

Basic and supplementary SI units

1.1 Basic SI Units.

Magnitude

Name
Length	Metro	m
Mass	Kilogram	kg
Second
Ampere
Temperature thermodynamics	Kelvin	K
of substance	Mol	mol
Candela		Cd
Cd

The definitions of the basic SI units are as follows:

1.1.1 Length unit: meter (m). The meter is the length of the journey in the vacuum by the light for a time of 1/299 792 458 of second. (17. GFCM, 1983, res. 1.)

1.1.2 Mass unit: kilogram (kg). The kilogram is the unit of mass: It is equal to the mass of the international prototype of the kilogram. (3. CGFCM, 1901, p. 70 of the minutes.)

1.1.3 Time Unit: second (s). The second is the duration of 9 192 631 770 radiation periods corresponding to the transition between the two hyperfine levels of the fundamental state of the cesium 133 atom. (13. GFCM, 1967, res. 1.)

1.1.4 Electrical current intensity unit: ampere (A). The ampere is the intensity of a constant current which, remaining in two parallel conductors, rectilinear, of infinite length, of negligible circular section and situated at a distance of 1 meter from one another, in the vacuum, would produce between These drivers a force equal to 2 x 10^-7 Newton per meter in length. (CIPM, 1946, res. 2, approved by the 9th GFCM, 1948.)

1.1.5 Thermodynamic temperature unit: kelvin (K). The kelvin, thermodynamic temperature unit, is the 1/273, 16 fraction of the thermodynamic temperature of the triple water point. (13. GFCM, 1967, res. 4.)

The 13. CGFCM, 1967, res. 3, it also decided that the unit kelvin and its symbol K, will be used to express an interval or a difference of temperature.

Observation: In addition to the thermodynamic temperature (T symbol), expressed in kelvins, the temperature Celsius (symbol t) defined by the equation is also used:

t = T-T₀

where T₀= 273.15 K by definition.

To express the temperature Celsius is used the unit "degree Celsius" which is equal to the unit "kelvin": "degree Celsius" is a special name used in this case instead of "kelvin". A temperature range or difference of Celsius can therefore be expressed in both kelvins and Celsius.

1.1.6 Unit of quantity of substance: mol (mol). The mole is the amount of substance in a system that contains as many elementary entities as atoms are in 0.012 kilograms of carbon 12.

When the mole is used, the elementary entities, which may be atoms, molecules, ions, electrons, or other specified particles or groups of such particles (14. CGFCM, 1971, res. 3.)

Observation: In the definition of the mole it is understood that it refers to carbon atoms 12 not bound, at rest and in their fundamental state.

1.1.7 Light intensity unit: candela (cd). The candela is the luminous intensity, in a given direction, of a source that emits a monochromatic radiation of frequency 540 x 10¹² hertz and whose energy intensity in that direction is 1/683 watt by stereorradian. (16. CGFCM, 1979, res. 3.)

1.2 Supplemental S1 Units.

Magnitude

Name	Symbol	Expression in SI units basic (*)
Angle plan	Radian	rad	m-m-1 = 1
Angle	Estereorradian	sr	m2 -m-2 = 1

(*) Observation: Whereas the flat angle is generally expressed as the ratio between two lengths and the solid angle, such as the ratio between an area and the square of a length, in order to maintain internal coherence The International System, based only on seven basic units, the CIPM (1980) has specified that, in the International System, the supplementary units radiating and stereorradian are derived units without dimension. This implies that the magnitudes flat angle and solid angle are considered as non-dimensional derived magnitudes.

(11. CGFCM, 1960, res. 12.)

The definitions of the supplementary SI units are as follows:

1.2.1 Flat-angle unit: radian (rad). The radian is the flat angle between two radii of a circle which, on the circumference of that circle, intercepts an arc of length equal to that of the radius. (International Standard ISO 31-I, December 1965.)

1.2.2 Solid Angle Unit: stereorradian (sr). The stereorradian is the solid angle which, having its vertex in the center of a sphere, intercepts on the surface of said sphere an area equal to that of a square that has the radius of the sphere. (International Standard ISO 31-I, December 1965.)

CHAPTER II

SI-derived units

2.1 The derived SI units are defined in such a way that they are consistent with the basic and submplementary units, that is, they are defined by algebraic expressions in the form of power products of the basic SI units and/or supplementary with a numeric factor equal to 1. Any other more or less explicit definition may not constitute a true definition of the derived unit but rather an explanation, moreover quite useful, about the nature of a single or several magnitudes, of which it is the unit of measure.

Several of these derived SI units are simply expressed from the basic and supplementary SI units as the ones that relate to and define in 2.2.

Others have received a special name and a particular symbol such as those listed in 2.3.

These in turn can be used to express SI units derived more simply than from the basic and supplementary SI units, such as those that are related and defined in 2.4.

A single SI unit name can correspond to several different magnitudes as shown in the tables below, in which the enumeration of the above measures is not limited.

Also, a derived SI unit can be expressed differently using basic unit names and derived unit special names.

However, it should be noted that if a derived SI unit can be expressed in several equivalent forms using either basic and supplementary unit names or special names of other derived SI units, supports the preferential use of certain combinations or certain special names, in order to facilitate the distinction between magnitudes having the same dimensions.

Examples:

The hertz is used for the frequency, with preference to the second at least one power, and for the moment of force, the newton metro to the joule is preferred.

In the field of ionising radiations, for the activity, the becquerel is preferred to the second to the power minus one, and the gray or the sievert, according to the magnitude considered, to the joule per kilogram.

Hence, the groups of SI units derived below are not a classification of such units, but examples of derived SI units expressed in one way or another.

2.2 Examples of derived SI units expressed from basic and submplementary units.

Magnitude

Acceleration

kg/s

Name
	square meter	m2
Volume	metro cubic	m3
	metro per second	m/s
metro per second square	m/s2
Number of waves	meter to power minus one	m-1
Mass	kilogram per cubic meter	kg/m3
Flow	cubic meter per second	3/s
flow
speed	radian by second	rad/s
angular acceleration	radian per second square	rad/s2

Your definitions are as follows:

2.2.1 Surface unit: square meter (m²). One square meter is the area of a 1-meter-side square (1m² = 1 m-1 m).

2.2.2 Volume unit: cubic meter (m³). One cubic meter is the volume of a 1-meter-side cube (1 m³ = 1 m-1 m-1 m).

2.2.3 Speed drive: meter per second (m/s or m-s^-1). One meter per second is the speed of a body that, with uniform motion, runs, a length of 1 meter in 1 second.

2.2.4 Acceleration unit: meter per second square (m/s² or m s^-2). One meter per second square is the acceleration of a body, animated of uniformly varied motion, whose speed varies every second, 1 m/s.

2.2.5 Wave Number Unit: Metro to power minus one (m^-1). One meter to the power minus one is the number of waves of a monochromatic radiation whose wavelength is equal to 1 meter.

2.2.6 Volume mass unit: kilogram per cubic meter (kg/m³ or kg-m^-3). One kilogram per cubic meter is the mass in volume of a homogeneous body whose mass is 1 kilogram and the volume of 1 m³.

2.2.7 Volume flow unit: cubic meter per second (m³/s or m³.s^-1). One cubic meter per second is the volume flow of a uniform current such that, a substance of 1 cubic meter of volume crosses a given section in 1 second.

2.2.8 Mass flow unit: kilogram per second (kg/s or kg-s^-1). One kilogram per second is the mass flow of a uniform current such that, a substance of 1 kilogram of mass crosses a given section in 1 second.

2.2.9 Angular speed unit: radius per second (rad/s or rad-s^-1). A radian per second is the angular velocity of a body which, with a uniform rotation around a fixed axis, rotates in 1 second, 1 radian.

2.2.10 Angular acceleration unit: radius per second square (rad/s² or rad-s^-2). A radian per second square is the angular acceleration of a body, animated from a uniforme-mind rotation varied around a fixed axis, whose angular velocity varies 1 radian per second, in 1 second.

2.3 SI units derived with special names and symbols.

Magnitude

lux

Activity (of a radionuclide)

Name	Symbol	Expression on other SI units	Expression in basic SI
Frequency	hertz		Hz	-	-1
	Force	newton	N	-	m.kg.s-2
Pressure, Tension	pascal	Pa	N. m-2	m-1.kg. s-2
	Energy, job, amount of heat	Joule	J	N. m-2	2.kg. s-2
(*), radiant flow	watt	W	J. s-1	2.kg. s
Amount of electricity, electric charge	couloomb	C	-	s.A
power, electrical potential, electromotive force	volt	V	W. A-1	m2.kg. s-3. A-1
Resistance	ohm		Table_table_izq"> V. A-1	m2.kg. s-3. A-2
conductance	siemens	S	A. V-1	m-2.kg-1.s3. A2
Capacity	farad	F	C. V-1	m-2.kg-1.s4. 2
flow, magnetic induction flow	weber	Wb	V. s	m2.kg. s-2. A-1
Induction, Magnetic Flow Density	tesla	T	Wb.m2	kg. s-2. A-1
Inductance	henry	H	-1	m2.kg. s-2. A-2
Flow	lumen	lm	cd.sr
	lux	lx	lm.m-2	-2.cd.sr
becquerel	Bq	-	-1
absorbed dose, Power communicated mass, kerma, absorbed dose index	gray	Gy	J. kg-1	2.s-2
	Dose equivalent, equivalent dose index	sievert	Sv	J. kg-1	2.s

(*) Note: In electrotechnics the drive is called:

In the case of active power: Watt (W).

In the case of the apparent power: Voltampere (VA).

In the case of reactive power: Var (var).

Your definitions are as follows:

2.3.1 Frequency Unit: Hertz (Hz). A hertz is the frequency of a periodic phenomenon whose period is 1 second.

2.3.2 Force Unit: newton (N). A newton is the force that, applied to a body that has a mass of 1 kilogram, communicates to it an acceleration of 1 meter per second square.

(1 N = 1 kg-1 m/s²)

2.3.3 Pressure unit, voltage: pascal (Pa). A pascal is the uniform pressure which, acting on a flat surface of 1 square meter, exerts a total force of 1 newton perpendicular to this surface.

It is also the uniform tension that, acting on a surface of 1 square meter, exerts on this surface a total force of 1 newton.

2.3.4 Energy unit, work, amount of heat: Joule (J). A joule is the work produced by a force of 1 newton, whose point of application shifts 1 meter in the direction of force.

(1 J = 1 N-1 m)

2.3.5 Power unit, radiant flow: watt (W). A watt is the power that results in an energy production equal to 1 Joule per second.

2.3.6 Electricity quantity unit, electric charge: couloomb (C). A coulomb is the amount of electricity carried in 1 second by an intensity current 1 ampere.

(1 C = 1 A. 1 s = 1 A. s)

2.3.7 Electrical voltage unit, electric potential, electromotive force: volt (V). -A volt is the difference of electrical potential that exists between two points of a wire conductor that carries a current of intensity constant of 1 ampere when the power dissipated between these points is equal to 1 watt.

2.3.8 Electrical resistance unit: ohm (Endurance). An ohm is the electrical resistance that exists between two points of a conductor when a constant potential difference of 1 volt applied between these two points produces, in said conductor, a current of intensity 1 ampere, when there is no force Electromotive in the driver.

2.3.9 Electrical conductance unit: siemens (S). A siemens is the conductance of a conductor having an electrical resistance of 1 ohm.

2.3.10 Electrical capacity unit: farad (F). A farad is the capacity of an electric capacitor that among its armor appears a difference of electric potential of 1 volt, when charged with an amount of electricity equal to 1 couloomb.

2.3.11 Magnetic flux unit, magnetic induction flow: weber (Wb). A weber is the magnetic flux which, when traversing a single-coil circuit, produces in the same a 1 volt electromotive force if said flow is voided in 1 second by uniform decrease.

(1 Wb = 1 V-1 s)

2.3.12 Magnetic induction unit, magnetic flux density: tesla (T). A tesla is the split uniform magnetic induction which, normally on a surface of 1 square metre, produces through this surface a total magnetic flux of 1 weber.

2.3.13 Inductance Unit: henry (H). A henry is the electrical inductance of a closed circuit in which an electromotive force of 1 volt occurs when the electric current running through the circuit varies evenly at the rate of one ampere per second.

2.3.14 Light Flow Unit: lumen (lm). A lumen is the luminous flux emitted at a solid angle of a stereo by a uniform spot source which, at the apex of the solid angle, has a luminous intensity of 1 candela.

(1 lm = 1 cd-1 sr)

2.3.15 Lighting Unit: lux (lx). A lux is the illuminance of a surface that receives a luminous flux of 1 lumen, evenly spread over 1 square meter of the surface.

2.3.16 Unit of activity (of a radionuclide): becquerel (Bq). A becquerel is the activity of a radioactive source in which 1 transformation or 1 nuclear transition occurs per second.

2.3.17 Unit of absorbed dose, energy communicated mass, kerma, absorbed dose index: gray (Gy). A gray is the absorbed dose in a 1 kilogram mass material element to which the ionizing radiations uniformly communicate a 1-Joule energy.

2.3.18 Unit of equivalent dose, equivalent dose rate: sievert (Sv). Special name of the Joule per kilogram.

Note: The equivalent magnitude of H dose is the product of the absorbed dose D of ionising radiation and two factors without dimension Q (quality factor) and N (product of several factors) prescribed by the Commission. International Radiation Protection:

H = Q-N-D

Thus, for given radiation, the numeric value of H in joules per kilogram may be different from the numeric value of D in joules per kilogram, since it is a function of the value of Q and N.

In order to avoid the risks to which human beings subjected to under-estimated radiation could be exposed, risks that could result from confusion between absorbed doses and equivalent doses, although the proliferation of Special names represent a danger to the International System of Units and should be avoided as far as possible, however, in order to safeguard human health, the 16 th General Conference of Weights and Measures (1979) adopted the name special "sievert", symbol Sv, for the unit SI of equivalent dose in the field of radiation protection. Subsequently, and not considering this enough, the International Committee on Weights and Measures (ICRP) in 1984 recommends [Recommendation I (CI 1984)] the name "gray" instead of the joule per kilogram, for the unit of absorbed dose D and the sievert place of the joule per kilogram, for the equivalent dose unit H.

2.4 Examples of derived SI units expressed from those with special names.

Magnitude

W/sr

Name	Symbol	Expression in SI units basic
Viscosity dynamic	second step	Pa-s	m-1-kg-s-1
Entropy, thermal capacity	J/K		2-kg. s-2-K-1
Mass thermal capacity, mass entropy	Joule per kilogram kelvin.	J/ (kg. K)	m2-s -2-K-1
conductivity.	watt per meter kelvin	W/ (m-K)	m-kg-s-3-K-1
electric field strength	volt metro	V/m	m-kg-s-3-A-1
radiant intensity	W/sr

2.4.1 Dynamic Viscosity Unit: Pascal Second (Pa-s). A second step is the dynamic viscosity of a homogeneous fluid in which the rectilinear and uniform movement of a flat surface of 1 square meter gives rise to a delay force of 1 newton, when there is a difference of speed of 1 meter per second between two parallel planes separated by 1 meter apart.

2.4.2 Entropy drive: Joule by kelvin (J/K or J-K^-1). A joule by kelvin is the entropy increase of a system that receives a heat amount of 1 Joule, at the constant thermodynamic temperature of 1 kelvin, provided that no irreversible transformation takes place in the system.

2.4.3 Mass thermal capacity unit, mass entropy: Joule per kilogram kelvin [J/ (kg-K) or J-kg^-1-K^-1]. A joule per kilogram kelvin is the mass thermal capacity of a homogeneous body of a mass of 1 kg, in which the input of a heat quantity of 1 joule produces a thermodynamic temperature elevation of 1 kelvin.

2.4.4 Thermal conductivity unit: watt per metre kelvin [W/ (m-K) or W-m^-1 -K^-1]. A watt per metre kelvin is the thermal conductivity of a homogeneous isotropic body, in which a temperature difference of 1 kelvin between two parallel planes, of area of 1 meter square and distant 1 meter, produces between these planes a thermal flow of 1 watt.

2.4.5 Electrical field strength unit: volt per meter (V/m). One volt per meter is the intensity of an electric field that exerts a force of 1 newton on a charged body with an amount of electricity of 1 couloomb.

2.4.6 Radiant intensity unit: watt by stereorradian (W/sr or W. sr^-1). A watt by stereorradian is the radiant intensity of a point source that uniformly sends a 1 watt energy flow at a solid angle of 1 stereorradian.

CHAPTER III

Rules for the formation of multiples and submultiples of SI units

3.1 Writing the symbols, names, and numbers.

3.1.1 The symbols of the SI units, with rare exceptions such as the ohm case, are expressed in general Roman characters; however, if these symbols correspond to units derived from names own, its initial letter is uppercase.

The symbols are not followed by a point, nor do they take the s for the plural.

When the symbol of a multiple or a submultiple of a unit carries an exponent, it affects not only the part of the symbol that designates the unit, but the symbol set. For example, km² means (km)², a square area that has a km of side, that is, 10⁶ square meters and never k (m²), which would correspond to 1,000 square meters.

The symbol of the drive follows the prefix symbol, without space.

The product of the symbols of two or more units is preferably indicated by a point, as a multiplication symbol. This point can be deleted if confusion with another symbol of unity is not possible. For example, newton-meter can be written N-m N-m or Nm, never mN, meaning milinewton.

When a derivative unit is the quotient of two other units, the obliquely bar (/), horizontal bar, or negative powers can be used to avoid the denominator.

You should never enter on the same line more than one oblique bar, unless parentheses are added, in order to avoid any ambiguity. In complex cases, parentheses or negative powers can be used. This will be written:

m/s² or m-s^-2 but never m/s/s

(Pa-s)/(kg/m³) or Pa-m³-kg^-1-s but never Pa-s/kg/m³

3.1.2 The names of the units due to the names of eminent scientists must be written with the same spelling as the name of the units, but with an initial lower case.

Notwithstanding the above, their usual castellanized denominations will also be acceptable, provided that they are recognized by the Royal Spanish Academy (examples: ampere, culombio, faradio, hercio, July, ohmio, voltio, watatio, weberio).

The names of the drives take a s in the plural (example: 10 newtons), except that they end in s, x, or z.

3.1.3 In numbers, the comma is used only to separate the entire portion of the decimal part. To make it easier to read, the numbers can be divided into three-digit groups (from the comma, if any); these groups are never separated by points or commas. Group separation is not used for four-digit numbers that designate a year.

3.2 Multiples and decimal submultiples.

3.2.1 The decimal multiples and submultiples of the SI units are formed by prefixes, which designate the decimal numeric factors by which the unit is multiplied, and which are shown to the left of the table.

The 11 th GFCM (1960, Resolution 12) adopted a first set of prefixes and multiples symbols and decimal submultiples of SI units. The 12 th GFCM (1964, Resolution 8) added the prefixes for 10^-5 and 10^-18 and the 15 th GFCM (1975, Resolution 10) added those corresponding to factors 10¹⁵ and 10¹⁸.

10¹⁵

Factor	Prefix	Symbol	Factor	Prefix
18	exa	E	-1	deci	d
	15	peta	P	-2	centi	c
12	T	10	10-3	10-3	10	m
9	giga	giga	G	10-6	micro	u
6	mega	M	10	nano	n
	kilo	k	10-12	peak	p
102	hecto	h	10-15	f
	f
101	deca	da	-18	atto

The symbol of a prefix is considered to be combined with the symbol of the unit to which it is directly bound, without intermediate space, thus forming the symbol of a new unit, which may be affected by a positive exponent or negative, and can be combined with other drive symbols to form composite unit symbols.

Examples:

1 cm³ = (10^-2 m)³ = 10^-6 m³

1 µ s^-1 = (10^-6 s)^-1 = 10⁶ s^-1

1 mm²/s = (10^-3 m)²/s = 10^-6 m²/s

1 V/cm = (1V)/(10^-2 m) = 10² V/m

3.2.2 Compound prefixes, formed by the juxtaposition of multiple SI prefixes, are not supported; for example, nm (nanometer) and no m µ m should be written.

Among the basic units of the International System, the mass unit is the only one whose name, for historical reasons, contains a prefix. The names of the multiples and decimal submultiples of the mass unit are formed by prefixing the prefix to the word "gram" and its symbols to the symbol "g".

For example:

10^-6 kg = 1 milligram (1 mg)

but not 1 microkilogram (1 µ kg)

3.2.3 To designate decimal multiples and submultiples of a derived unit, the expression of which is present as a fraction, it is indifferent to attach a prefix to the units listed in the numerator, in the denominator or in the both.

CHAPTER IV

Other units

4.1 Special multiples and submultiples decimal names and submultiples of SI units authorized.

Magnitude

Name	Symbol	Relationship
Volume	liter	l or L (1).	1 l = 1 dm3 = 10-3 m3
Masa	tonne	t.......... .	1 t = 1 Mg = 103
and tension	bar	bar (2)	1 bar = 105

(1) The two symbols "l" and "L" are usable for the unit "litre" (16. CGFCM, 1979, Resolution 5).

(2) Unit temporarily admitted by the International Pesses and Measures Committee (1978).

These units and symbols can be applied to the prefixes and symbols set out in point 3.2.1 of the previous chapter.

4.2 Units defined from SI units, but are not multiples or decimal submultiples of those units.

Magnitude

Unit
Name	Symbol
Angle .....	return *		1 return = 2πrad
	grade (centesimal or gon *)	gon		rad
200
	grade		1	rad
rad
180
	angle minute	'	1 ' =	π	rad
				10,800
	angle second	"	1" =	π	rad
				648,000
Time ............	minute	min	1 min = 60 s
	time	h	1 h = 3,600 s
	day	d	1 d = 86,400 s

The * sign after a name or a unit symbol means that they are not set by the General Conference of Pesses and Measures. This warning is also applicable to the table in point 4.4.

Note: The prefixes and their symbols set in point 3.2.1 apply only to the name "degree (centesimal)" or "gon", and the symbols will only be applied to the "gon" symbol.

4.3. Units in use with the International System whose value in SI units has been obtained experimentally.

Magnitude

Name	Symbol	Value in SI
	Atomic Mass Unit	u	1 or 1.660 540 2-10-27
Energy	electronic volt	eV	1 eV 1.602 177 33-10-19

Your definitions are as follows:

4.3.1 The atomic mass unit (unified) is equal to 1/12 of the mass of a nucleid atom ¹²C.

1 or 1.660 540 2-10^-27 kg (approximately)

4.3.2 Electronvolt is the kinetic energy acquired by an electron when traversing a potential difference of 1 volt in the vacuum.

1 eV 1.602 177 33-10^-19 J (approximately)

4.4 Units supported only in specialized application sectors.

Magnitude

Name	Symbol	Value
of Optical Systems	dioptria *		1 dioptria = 1 m-1
Mase of the gemstones	metric carat		1 carat metric = 2-10-4
Area of farm and farm surfaces	area	a	1 a = 102m2
longitudinal mass of textile fibers and threads	tex *	tex *	1 tex = 10-6 kg-m-1
pressure and pressure from other body fluids	millimeter mercury	mm Hg *	1 mm Hg = 133,322 Pa
Section	barn	b	1 b = 10-28 m2

The prefixes and their symbols set out in point 3.2.1 shall apply to these units and to their symbols, with the exception of the mercury millimeter and its symbol. However, the multiple: 10² a, will be named "hectare".