Dictionary of scientific
and technical units
A
a – Symbol for atto-, SI metrological prefix.
A – Symbol for ampere.
Ah – Symbol for ampere-hour.
ampere [from A.-M. Ampère (1775-1836),
French physicist] – The SI unit of electrical
current intensity (symbol: A), of which it is one
of the seven base units: it is the intensity of the
constant current which, when flowing in two
straight parallel conductors of infinite length
and negligible cross-section and placed 1 m
apart in vacuum, produces between these
conductors a force of 2⭈10⫺7 N per metre of
length.
ampere-hour – Practical unit of electrical
charge (symbol: Ah), used solely for electrical
accumulators: it is the quantity of electricity
which in one hour traverses a conductor
through which a current passes with a constant
intensity of 1 ampere, and is therefore equal to
3,600 coulomb.
a.n.c. – Abbreviation of at normal conditions,
with reference to the state of a gas.
ångström [from A.J. Ångström (1814-74),
Swedish physicist] – A unit of length
(symbol: Å), equal to 10⫺1 nm⫽10⫺10 m.
API [American Petroleum Institute] gravity
– The conventional international parameter to
classify the value (expressed in degrees API,
symbol: °API) of a crude oil according to its
density. It is defined by the ratio: °API
value⫽[141.5/(relative density)]⫺131.5, where
the relative density is that with respect to water,
with both substances at a temperature of 60°F
(15.5°C). As the density of the crude oil
decreases, the parameter value increases. Water
is given a parameter value of 10°. Values of
relative density from 1.076 to 0.6112
correspond to the interval 1° to 100° of the API
scale. Conventionally a crude oil is defined as
heavy for values of less than 25°API
(i.e. relative densities greater than 0.90),
and light for values of more than 40°API
(i.e. relative densities of less than 0.83).
Measurements are carried out using a
densimeter calibrated for direct readings.
atm – Symbol for atmosphere.
VOLUME V / INSTRUMENTS
atmosphere – Practical unit of pressure
(symbol: atm), equal to the pressure exerted at
sea level and 45° latitude, by a column of
mercury at a temperature of 0°C, 760 mm
high; its name derives from the fact that it
corresponds to the mean atmospheric pressure
at sea level at mean latitude; it is equal to
101,325 Pa and thus to 1.01325 bar; sometimes
it is called physical a. to distinguish it from
technical or metric a. (symbol: at), equivalent
to 1 kgp/cm2 (whereas physical atmosphere is
equal to 1.033 kgp/cm2), i.e. 98,066.5 Pa; this
unit is sometimes called absolute technical
atmosphere (symbol: ata), while effective
technical atmosphere (symbol: ate) is the unit
of measurement for pressure above or below
atmospheric pressure.
atto- [from Danish and Norwegian atten
«eighteen»] – SI metrological prefix (symbol: a)
which multiplies by 10⫺18 the value of the unit
of measurement it prefixes: e.g. 1 attometer
⫽1 am⫽1·10⫺18 m.
avoirdupois – A system used as a unit of
measurement for mass and weight, which is
neither decimal nor metric. It is the most
commonly used in the commercial and also
technical field in English-speaking countries.
The base unit is the pound.
B
b – Symbol (also bb or bbl) for barrel.
B – a) Symbol for Baumé degree; b) symbol for
bel.
bar – Unit of measurement of pressure equal to
105 Pa, accepted in the SI (symbol: bar). It is
approximately equivalent to the practical unit of
atmospheric pressure (1 bar⫽0.986 atm).
barrel – A unit of measurement of capacity used
for crude oil and its derivatives, originally in
the USA and later internationally. Symbol: bbl,
b, or also bb; thus, the daily production
capacity of a plant or the daily production of a
well or oil field is expressed in b/d or bb/d or
bbl/d (also bpd or bbpd, barrels per day). One
b. is equal to 42 US gallons, i.e. approx. 160
litres. The mass of 1 b of crude depends on
density and corresponds to approx. 310 pounds
or 0.14 t. ❏ Barrels of oil equivalent (boe): an
expression which indicates, by relating the
energy content to that of a barrel of crude, the
quantity of another fuel or the quantity of
energy produced by other means. E.g. 103 m3
of natural gas is the equivalent of approx. 6 bbl
of crude. ❏ bpcd (or bcd), barrels per calendar
day, is used to express the average daily
capacity production of a plant, and is equal to
the quantity of crude produced over the year in
barrels divided by 365. ❏ bpsd, barrels per
stream day, is used to express the average
capacity of production of a plant, and is equal to
the quantity of crude produced in a specific
period in barrels divided by the number of actual
working days. ❏ bwpd, barrels of water per day,
is used to express the daily quantity of water in
barrels used to feed heaters, plants etc. or run
into a spent oil field to recover residual oil.
barye – CGS unit of pressure (symbol: ba),
equal to 1 dyne per square centimetre and
equivalent to 0.1 Pa.
Baumé degree [from A. Baumé (1728-1804),
Fr. inventor] – Practical unit (symbol: Bé or B)
of liquid density at a temperature of 4°C,
defined with a densimeter, invented by Baumé
himself, at a constant volume and bearing a
uniformly calibrated scale. Thus, 15 Bé is the
difference between the point at which it
surfaces in water and in a 10% sodium chloride
solution. The B. scale was also used by the
petrochemical industry until 1922, which later
adopted the API scale.
bb – Symbol (also b or bbl) for barrel.
bbl – Symbol (also b or bb) for barrel.
bbpd – Abbr. of barrels per day (also bpd or
bbl/d), see barrel.
bcd – Abbr. of barrels per calendar day (also
bpcd ), see barrel.
Bé – Symbol for Baumé degree.
becquerel [from H. Becquerel (1852-1908), Fr.
physicist] – SI unit of radioactive activity
(symbol: Bq), equal to that of a substance
which undergoes one decay per second.
bel [from A.G. Bell (1847-1922),
Scottish-American inventor] – Logarithmic
unit (symbol: B) used to express the relative
level, r, of quantities of the same type,
623
DICTIONARY OF SCIENTIFIC AND TECHNICAL UNITS
P1 and P2: r (in B) ⫽ log10(P1 /P2). The unit
most commonly used in practice is the decibel
(symbol: dB) which is equal to 1/10 of one bel.
billion – Unit of the decimal numerical system,
corresponding, according to international
convention (1951), to a million millions
[106]2⫽1012; in France and in the USA the
term billion means a thousand million, i.e. 109,
and with this meaning it is often to be found as
a metrological prefix (symbol: B).
boe – Abbr. of barrels of oil equivalent, see
barrel.
bpcd – Abbr. of barrels per calendar day (also
bcd ), see barrel.
bpd – Abbr. of barrels per day (also bbpd or
bbl/d), see barrel.
bpsd – Abbr. of barrels per stream day, see
barrel.
Bq – Symbol for becquerel.
British Thermal Unit – A unit of heat quantity
(symbol: BTU or Btu): a) international BTU
(BTUIT or also BTU) is equal to 1/180 of the
quantity of heat required to bring the
temperature of a pound of water from 32°F to
212°F (from 0 to 100°C) and is equivalent to
1.05506⭈103 J (0.252 kcal); b) BTU39, BTU59,
BTU60, have analogous definitions, and refer to
the variation of 1°F starting from the
temperature in °F indicated with the subscript.
They are equal, respectively, to 1.05967,
1.05480 and 1.05468 kJ; c) thermochemical
BTU or average BTU (BTUTC) is equal to
1.05435 kJ.
BTU (also Btu) – Symbol for British Thermal
Unit.
bwpd – Abbr. of barrels of water per day, see
barrel.
C
c – a) Symbol for centi-, metrological prefix;
b) as a superscript of an angular measurement
(e.g. a⫽35c), it indicates that this is expressed
in centesimal degrees.
C – a) Symbol for coulomb; b) symbol for
Celsius degree (°C).
cal – Symbol for calorie. In the past the symbol
Cal indicated the so-called large calorie
(Fr. grande calorie), equal to 1 kilocalorie
(symbol: kcal).
calorie – Practical unit of heat (symbol: cal): it
is the amount of heat required to raise the
temperature of 1 g of distilled water at normal
atmospheric pressure from 14.5 to 15.5°C. It is
equal to 4.1868 J.
candela – The unit of luminous intensity
(symbol: cd) in the SI (of which it is one of the
seven base units): it is the luminous intensity of
a source emitting monochromatic radiation of
5.40⭈1014 Hz frequency, in a direction for
which the radiant intensity is 1/683 W/sr.
cd – Symbol for candela.
Celsius degree [from A. Celsius (1701-44),
Swedish astronomer] – Unit of temperature
interval (symbol: °C) equal to 1 kelvin; in the
past, and still in use, the same as degree
centigrade. The Celsius temperature, t,
is defined as t⫽T⫺273.15, where T is the
thermodynamic temperature in kelvin.
cental – Mass unit of the Avoirdupois System,
equal to 100 pounds, i.e. approx. 45 kg; also
624
called centweight or more precisely new
centweight to distinguish it from two preceding
units: the long centweight, in use in the UK and
equal to 112 pounds, i.e. approx. 50 kg, and the
short centweight, in use in the USA, also equal
to 100 pounds.
centesimal degree – Unit of the amplitude of
plane angle (symbol: c), equal to 1/100 of a
right angle.
centi- [derived from Lat. centum «hundred»]
– Prefix of the Decimal Metric System accepted
in the SI (symbol: c) which divides by
a hundred the value of the unit of measurement
it prefixes: e.g. 1 centilitre⫽1 cl⫽10⫺2 litre.
centigrade degree – Unit of temperature
(symbol: °C) equal to the hundredth part of the
temperature interval between the melting point
of ice (0°C) and boiling point of pure water
(100°C) at normal atmospheric pressure.
centimetre (or centimeter) – Base unit of
length in the CGS units of measurement
system, equal to the hundredth part of a metre
(symbol: cm).
centweight – Unit of mass, synon. of cental.
CGS – Acronym, from the initials of the base
units, of the decimal system of units of
measurement based on length, mass and time,
and the relative units of measurement
– centimetre, gram and second.
chain – Unit of length, equal to 66 feet, i.e. to
approx. 20 m.
Ci – Symbol for curie.
circ mil – Symbol for circular mil.
circular mil – A unit of area (symbol: circ mil),
particularly used to express the cross-section of
wires; a unit of measurement in which the area
of a cylindrical wire with a diameter of d mil
(linear) is equal to d 2 circ mil and equivalent to
5.067⭈10⫺10 m2, i.e. to approx. 500 mm2.
cm – Symbol for centimetre.
coulomb [from C.-A. de Coulomb (1736-1806),
Fr. scientist)] – SI unit of electric charge
(symbol: C): it is the charge which flows in
1 second through a section of a conductor
traversed by a steady current with an intensity
of 1 ampere.
cps – Abbr. of cycle per second.
curie [from M. Skùodowska Curie (1867-1934)]
– A unit of radioactive activity (symbol: Ci),
equivalent to 3.7⭈1010 Bq, i.e. to 3.7⭈1010
decays per second (the activity of 1 g of 226Ra).
CV – Symbol for cavallo-vapore (Italy), cheval
vapeur (French-speaking countries) and
caballo vapor (Spanish-speaking countries),
practical unit of mechanical power, equal to 75
kilogram-metres per second, i.e. to 735.499 W.
It should not be confused with the similar
English unit HP (see horse-power), from
which it differs slightly.
cycle per second – Unit of frequency
equivalent to the hertz.
dalton [from J. Dalton (1766-1844), Engl.
scientist] – Unit of atomic mass, equal to 1/16
of the atomic mass of oxygen.
darcy [from H.-P.-G. Darcy (1803-58), Fr.
engineer] – Unit of the permeability of a
porous medium (symbol: D): it is the
permeability of a porous medium through
which a liquid of viscosity 1 centipoise (cP)
passes at a rate of 1 cm/s when subjected to a
pressure gradient of 1 atm/cm; it is equivalent,
in SI units, to 9.87⭈10⫺13 m2.
day – Unit of time (symbol: d), equal to 86,400 s.
deca- [from Gr. déka «ten»] – Prefix of the
Decimal Metric System (symbol: da) accepted
in SI, which multiplies by 10 the value of the
unit of measurement it prefixes: e.g.
1 decametre⫽1 dam⫽10 m.
deci- [derived from Lat. deci(mus) «tenth»] –
Prefix of the Decimal Metric System, accepted
in the SI (symbol: d) which divides by 10 the
value of the unit of measurement it prefixes:
e.g. 1 decimetre⫽1 dm⫽1/10 m.
Decimal Metric System – A unit of
measurement system, initially based solely on
the metre, in which the various units of an
equal size are decimal multiples and
submultiples of each other.
degree (or sexagesimal degree) – Unit of the
amplitude of plane angle (symbol: °), equal to
1/90 of a right angle. It is divided into 60⬘
(minutes or arcmins), each of which is divided
into 60⬙ (seconds). 1°⫽0.017453 rad and
1 rad⫽57° 17⬘ 44.806⬙⫽57.296°.
dram – Unit of mass (symbol: dr) of the
avoirdupois system; it is equivalent to 1/16 of
an ounce, i.e. to 1.772 g.
dwt – Symbol for pennyweight, see troy.
dyn – Symbol for dyne.
dyne – Unit of force in the CGS system
(symbol: dyn); it is equivalent to 10⫺5 newton.
D
F
d – a) Symbol for the metrological prefix deci-;
b) symbol for day, also as exponent of
a measurement of time (e.g. 4yr 2d⫽4 years
and 2 days).
D – Symbol for darcy.
Da – Symbol for dalton.
da- – Symbol for the metrological prefix deca-.
f – Symbol for femto-, SI metrological prefix.
F – a) Symbol for farad; b) symbol (°F) for the
Fahrenheit degree of temperature.
Fa – Symbol for faraday.
Fahrenheit degree [from G.D. Fahrenheit
(1686-1736), German physicist] – The unit of
the Fahrenheit thermometric scale (symbol: °F),
E
E – Symbol for exa-, SI metrological prefix.
electronvolt – Unit of energy much used in
atomic and nuclear physics (symbol: eV); it is
the energy acquired by an electron when it passes
through a potential difference of 1 volt and is
consequently equivalent to 1.606⭈10⫺19 joule.
eötvös [from R. Eötvöš (1848-1919), Hungarian
physicist] – Unit of the horizontal gradient in the
acceleration of gravity, equal to 10⫺9 s⫺2.
erg – Unit of work in the CGS system
(symbol: erg); it is equivalent to 10⫺7 joule.
eV – Symbol for electronvolt.
exa- [from Gr. héx «six»] – SI metrological
prefix (symbol: E) which multiplies by 1018 the
value of the unit of measurement it prefixes:
e.g. 1 exametre⫽1 Em⫽1018 m.
ENCYCLOPAEDIA OF HYDROCARBONS
DICTIONARY OF SCIENTIFIC AND TECHNICAL UNITS
in which the temperature at which ice melts
corresponds to 32°F and the temperature at
which water boils corresponds to 212°F. The
calculations which allow for the conversion of
a temperature value expressed in °F to that in
°C, or vice-versa, are: [°C]⫽([°F]⫺32)/1.8,
[°F]⫽1.8[°C]⫹32; to convert to the
thermodynamic temperature in kelvin:
[K]⫽([°F]/1.8)⫹255.37.
farad [from M. Faraday (1791-1867), Engl.
physicist] – SI unit of electrical capacitance
(symbol: F). It is the capacitance of a insulated
conductor (or of a capacitor) the potential
(or the potential difference) of which varies by
1 volt when the charge varies by 1 coulomb.
faraday [from M. Faraday (1791-1867), Engl.
physicist] – A unit of electric charge (symbol:
Fa), equal to the charge which, in
an electrolytic cell, releases a chemical
equivalent of matter; it is equal to 96,484.56 C
(properly C/mol).
fathom – A length unit of measurement used in
English-speaking countries, equal to
approximately 1.83 metres.
femto- [from Danish and Norwegian femten
«fifteen»] – SI metrological prefix (symbol: f ),
which multiplies by 10⫺15 the value of the unit
of measurement it prefixes: e.g.
1 femtometre⫽1 fm⫽10⫺15 m.
foot (pl. feet) – Unit of length (symbol: ft),
equal to 1/3 of a yard and therefore to 0.3048 m.
It is divided into 12 inches.
frigorie – Unit of refrigeration, sometimes used
to denote the calorie when it expresses a
quantity of heat extracted from a given system.
ft – Symbol for foot.
H – Symbol for henry.
ha – Symbol for hectare.
hectare – Unit of area (symbol: ha) equal to
10,000 m2.
hecto- [from Fr. hecto, derived from Gr. ekatón
«hundred»] – Prefix of the Decimal Metric
System (symbol: h), accepted in the SI, which
multiplies by 100 the value of the unit of
measurement it prefixes: e.g. 1 hectometre
⫽1 hm⫽100 metres.
henry [from J. Henry (1797-1878), American
mathematician and physicist] – SI unit of
(self- and mutual) inductance (symbol: H); a
circuit has the inductance of 1 H if, by varying
the intensity of its current by 1 ampere per
second, an electromotive force equal to 1 volt
is thereby induced.
hertz [from H.R. Hertz (1857-94), German
physicist] – SI unit of frequency (symbol: Hz),
equal to 1 s⫺1. It is the frequency of a periodic
phenomenon with period equal to one second.
horse-power – Practical unit of mechanical
power used in English-speaking countries
(symbol: HP); it is the power exerted by a 550
pound-force which moves its point of application
by 1 foot per second in the same direction as the
force; 1 HP⫽745.7 W⫽1.014 CV.
hour – Unit of time (symbol: h; as a
superscript in numerical measures, e.g. 17h),
equal to 60 minutes each with 60 seconds
(in all 3,600 seconds).
HP – Symbol for horse-power.
Hz – Symbol for hertz.
G
in – Symbol for inch; inH2O and inHg are
respectively the symbols for inch of water and
of inch of mercury.
inch – Unit of length used in English-speaking
countries (symbol: in), equal to 1/12 of one
foot and equivalent to 2.54 cm. I. of mercury
and i. of water: practical units of pressure
(symbol: inHg and inH2O), respectively
equal (at 0°C) to 3,386 and 249 Pa.
g – Symbol for gram.
G – Symbol for giga-, SI metrological prefix.
Gal [from G. Galilei (1564-1642)]
– Denomination of the CGS unit of acceleration,
equal to 1 cm/s2 and therefore to 10⫺2 m/s2.
gallon – Unit of capacity (symbol: gal).
a) Imperial gallon, used for liquids in the
United Kingdom, in countries of the British
Commonwealth and in most of the former
colonies of the British empire, equal to
4.546 dm3. b) United States gallon, used for
liquids in the USA, equal to 3.790 dm3.
giga- [from Gr. gígas «giant»] – SI metrological
prefix (symbol: G) which multiplies by 109 the
value of the unit of measurement it prefixes:
e.g. 1 gigahertz⫽1 GHz⫽109 hertz.
gon – Another denomination and symbol for the
centesimal degree.
grain – Unit of mass of the Avoirdupois System
(symbol: gr), so-called as it corresponds to
the average mass of a grain of wheat; it is equal
to 1/7,000 of 1 pound and to 64.8 mg.
gram – Unit of mass in the CGS system
(symbol: g), equal to 1/1,000 of kilogram.
H
h – a) Symbol for hecto-, metrological prefix;
b) symbol for hour, sometimes as a superscript
for units of time: e.g. 16.5 h⫽16.5h⫽16.5 hours.
VOLUME V / INSTRUMENTS
I
J
J – Symbol for joule.
joule [from J.P. Joule (1818-89), British
physicist] – The SI unit of work and of energy
(symbol: J), equal to the work carried out by
the force of 1 newton which moves the point of
application by 1 m in its own direction.
K
k – Symbol for kilo-, SI metrological prefix.
K – Symbol for kelvin.
kelvin [from W. Thomson, Lord Kelvin (18241907), Brit. physicist] – The unit of thermodynamic temperature (symbol: K) in the SI
(of which it is one of the seven base units): it is
equal to the fraction 1/273.16 of the thermodynamic temperature of the triple point of water.
kg – Symbol for kilogram; kgf (or kgf) is the
symbol of the kilogram-force.
kgm – Symbol for kilogram[force]-metre.
kilo- [derived from Fr. kilo, adaptation from Gr.
kìlioi «thousand»] – SI metrological prefix
(symbol: k) which multiplies by 103 the value
of the unit of measurement it prefixes: e.g.
1 kilowatt⫽1 kW⫽103 W.
kilogram – The unit of mass (symbol: kg) in
the SI (of which it is one of the seven base
units), equal to the mass of the International
Prototype held at the Bureau International
de Poids et Mesures (BIPM) in Sèvres (Paris).
❏ Kilogram-force: a technical unit of force
(symbol: kgf or kgf), equal to the force which
imparts to a 1 kilogram mass an acceleration
equal to the value of normal acceleration due to
gravity, i.e. a force equal to the weight of 1 kg
at sea level and 45° latitude; it is equivalent to
9.807 newton.
kilogram-metre – A practical unit of work
(symbol: kgm), equal to the work carried out
by a 1 kilogram-force which moves its point of
application by 1 m in its own direction; it is
equivalent to 9.807 J.
kilowatt-hour – Technical unit of electrical
energy (symbol: kWh); it is the energy
developed in one hour by an electric current
with the constant power of 1 kilowatt; it is
equivalent to 3.6⭈106 J.
kWh – Symbol for kilowatt-hour.
L
l, L – Symbol for litre.
lb – Symbol for pound.
line – A unit of length equal to: a) 1/12 of one
inch (i.e. 2.117 mm) in the United Kingdom
and associated countries; b) 1/40 of one inch
(i.e. 0.635 mm) in the USA.
litre (var. liter) – Unit of capacity in the Decimal
Metric System (symbol: l or also L), equal to
the volume of a 1 kg mass of distilled water at
a temperature of 4°C, and thus equivalent to
1.000028 dm3. In the SI, litre is the permitted
term to indicate dm3.
lm – Symbol for lumen.
long ton – Unit of mass, equal to 2,240 lb and
consequently to 1,016 kg, see ton.
lumen – The SI unit of luminous flux
(symbol: lm), equal to the luminous flux
emitted by a point source through a unit solid
angle which in all the directions included in
that angle has an intensity of 1 candela.
❏ Lumen per watt: unit (symbol: lm/W) of
luminous efficacy of electric lamps, as a ratio
of the luminous flux emitted, in lumen, and the
electrical power absorbed, in watts.
lumen-second – The SI unit of quantity of light
(symbol: lm s), equal to the quantity of light
transmitted in 1 second by a beam of light with
a flux of 1 lumen.
lux – The SI unit of illumination (symbol: lx),
equal to 1 lumen per square metre.
lx – Symbol for lux.
M
m – a) Symbol for metre; b) symbol for the SI
metrological prefix milli-; c) as a superscript
for a unit of time, symbol for minute: e.g.
625
DICTIONARY OF SCIENTIFIC AND TECHNICAL UNITS
45m⫽45 minutes; d) mH2O is a symbol of metre
of water, see metre.
M – Symbol for metrological prefix mega-.
mc – Symbol incorrectly used for cubic metre,
instead of the correct symbol m3.
mega- [from Gr. mégas «big»] – SI metrological
prefix (symbol: M) which multiplies by 106 the
value of the unit of measurement it prefixes:
e.g. 1 megaohm⫽1 MW⫽106 W.
metre (var. meter) – The SI unit of length
(symbol: m), of which is one of the seven base
units: it is defined as the distance travelled by
light in a vacuum during a time interval of
1/299,792,458 seconds. ❏ Cubic metre: SI unit
of volume (symbol: m3). ❏ Metre of water:
practical unit of pressure, equal to the pressure
exerted by a column of water 1 m high and thus
equal to g⭈103 Pa, i.e. approx. 9,860 Pa.
❏ Square metre: SI unit of area (symbol: m2).
mho – Another denomination in Englishspeaking countries (formed by inverting the
letters of ohm) of the siemens, SI unit of
electric conductance.
micro- [from Gr. mikrós «little»] – SI
metrological prefix (symbol: m) which
multiplies by 10⫺6 the value of the unit of
measurement it prefixes, thus reducing it to a
millionth: e.g. 1 micrometre⫽1 mm⫽10⫺6 m.
micrometre – Unit of length (symbol: mm)
equal to 10⫺6 metre; the improper use of the
symbol m and the name micron still persists.
micron – An old denomination (symbol: m) of
the micrometre (symbol: mm).
mil – A unit of length, equal to 1/1,000 of 1 inch
and consequently to 25.4 mm. Two units
of area derive from this unit: a) circular mil,
an area of a circle of 1 mil diametre and thus
equal to 507 mm2; b) square mil, an area of
a square whose sides are 1 mil and thus equal
to 645 mm2.
mile – a) International nautical mile, unit of
length defined as the average length of a
meridian arc of one sexagesimal minute and
consequently equal to 1,852 m.
b) International (land) mile or statute mile,
unit of length equal to 5,280 feet and
consequently to 1,609 m.
milli- [from Fr. milli-, from Lat. mille «thousand»]
– SI metrological prefix (symbol: m), which
multiplies by 10⫺3 the value of the unit of
measurement it prefixes, reducing its value to
1 thousandth: e.g. 1 millivolt⫽1 mV⫽10⫺3 V.
millibar – Sub-multiple of the bar (symbol:
mbar), extensively used in meteorology; it is
equal to 1/1,000 of a bar and consequently to
102 pascal.
millimetre – Sub-multiple of the metre, equal to
1/1,000 of a metre (symbol: mm). ❏ Millimetre
of mercury: practical unit of pressure
(symbol: mmHg) also called torr, used in
vacuum techniques, equal to the hydrostatic
pressure exerted by a column of mercury 1 mm
high; it is equivalent to 133.322 pascal.
min – Symbol for minute.
minute – Unit of time (symbol: min; sometimes
as a superscript of the measurement: e.g. 12min
or, more often, 12m), equal to 1/60 of an hour
and to 60 seconds.
mol – Symbol for mole.
mole – Unit of quantity of matter (symbol: mol)
of the SI of which it is one of the base units: it
is the amount of substance (in a wide sense)
which contains as many elementary entities
626
(molecules, atoms, ions, electrons, photons,
etc.) as there are atoms in 0.012 kg of 12C, i.e.
equal to Avogadro’s number (6.023⭈1023).
mq – Symbol incorrectly used for square metre,
instead of the correct symbol m2.
Mtoe – Symbol for the multiple of the toe
(tons of oil equivalent) equal to 106 toes.
N
n – Symbol for nano-, SI metrological prefix.
N – Symbol for newton.
nano- [from It. nano «dwarf»] – SI metrological
prefix (symbol: n), which multiplies by 10⫺9
the value of the unit of measurement it prefixes
(i.e. it reduces the unit to a billionth): e.g.
1 nanometre⫽1 nm⫽10⫺9 m.
nautical mile – Unit of length (see mile) equal
to 1,852 m.
newton [from I. Newton (1642-1727)] – SI unit
of force (symbol: N): it is the force which
imparts an acceleration of 1 metre per second
squared to a mass of 1 kilogram.
nit – SI unit of luminance (symbol: nt); it is
equal to 1 candela per square metre.
Nm3 – Symbol for normal cubic metre.
normal cubic metre – Unit of volume
(symbol: Nm3), used to express in m3 the
volume of a gas measured under the normal or
standard conditions of pressure (1 atm) and of
temperature (0°C).
nt – Symbol for the unit of luminance nit.
O
peta- [from prefix peta- in some Gr. verbs
indicating an increase in size] – SI metrological
prefix (symbol: P) which multiplies by 1015 the
value of the unit of measurement it prefixes:
e.g. 1 petajoule⫽1 PJ⫽1015 joule.
pico- [from Spanish pico «little quantity»] – SI
metrological prefix (symbol: p) which
multiplies by 10⫺12 the value of the unit of
measurement it prefixes: e.g. 1 picometre
⫽1 pm⫽10⫺12 m.
Pm – Symbol for perm.
poise [from J.-L.-M. Poiseuille (1799-1869), Fr.
scientist)] – CGS unit of dynamic viscosity; it
is equivalent to 1/10 Pa·s.
pound – Unit of mass in English-speaking
countries (symbol: lb); in 1963 it was defined
in the UK as being exactly equal to
0.453592370 kg (divisible by 7, since
1 lb⫽7,000 grains). It has a slightly different
value in the USA, i.e. 0.4535924277 kg.
pound-force per square inch – Practical unit
of pressure (symbol: psi), equal to 6,894.757 Pa
⫽0.068 atm.
ppm – Abbr. of parts per million.
practical – Applied to units of measurement of
a specific discipline, especially technical, and
not organized in a system, as for example is the
case of units containing the calorie in
thermology. ❏ P. system (also, technical system):
any mechanical unit of measurement system
(p. unit) in which together with length and
time, the base units are force rather than mass,
as is the case with absolute systems.
prefix – Metrological prefix: term (and relative
symbol) which, placed before the name
(symbol) of a unit of measurement, changes
its value (in the Decimal Metric System and
in the SI, by positive or negative powers of
10). E.g. the metrological prefix milli(symbol: m) divides by 1,000 the value of the
unit it prefixes: 1 millimetre⫽1 mm
⫽10⫺3 metres.
psi – Symbol for pound-force per square inch.
ohm [from G.S. Ohm (1787-1854), German
physicist] – The SI unit (symbol: W) of
electrical resistance: it is the resistance of a
conductor through which a constant current of
1 ampere flows when a potential of 1 volt is
applied between its extremities.
ounce – Unit of mass (symbol: oz) in Englishspeaking countries, differentiated as follows:
ounce avoirdupois equal to 1/12 of pound
(avdp) and consequently to 0.02835 kg, the
most common; ounce troy equal to 0.03110 kg.
❏ Ounce-force: unit of force (symbol: ozf),
equal to the weight of 1 ounce (avdp) at sea
level at 45° latitude, equivalent to approx.
0.278 newton.
oz – Symbol for ounce.
ozf – Symbol for ounce-force, see ounce.
q – Symbol for quintal.
qt – Symbol for quart.
quart – Unit of capacity (symbol: qt), equal to a
quarter of a gallon and therefore to 1.137 l in
the United Kingdom and 0.946 l in the USA.
quintal (or metric centner) – Unit of mass of
the Decimal Metric System (symbol: q), equal
to 100 kg.
P
R
p – Symbol for pico-, SI metrological prefix.
P – Symbol for peta-, SI metrological prefix.
Pa – Symbol for pascal.
pascal [from B. Pascal (1623-62), Fr. physicist
and philosopher] – The SI unit of pressure
(symbol: Pa), equal to the pressure of 1 newton
per square metre (1 Pa⫽1 N/m2).
perch – Unit of length, equal to 16.5 feet and
consequently to 5.029 m.
perm – CGS unit (symbol: Pm) of specific
permeability, equal to 1 cm2 and consequently
to 10⫺4 m2.
R – Symbol for the Réaumur thermometric
degree (°R).
Ra – Symbol for the Rankine thermometric
degree (°Ra).
rad – Symbol for radian.
radian – SI unit (symbol: rad) of plane angles,
equal to the angle subtended at the centre
of a circle by an arc the length of which
is equal to the radius; it is 1 rad⫽57° 17⬘ 44.81⬙
⫽57.296°.
Rankine degree [from W.J.M. Rankine
(1820-72), Scottish engineer] – The unit
Q
ENCYCLOPAEDIA OF HYDROCARBONS
DICTIONARY OF SCIENTIFIC AND TECHNICAL UNITS
(symbol: °Ra) of the Rankine thermometric
scale (absolute temperature scale, derived from
the Fahrenheit scale, equivalent to that scale
but starting from thermodynamic zero); the
ratio is 1.8°Ra⫽1 K.
Réaumur degree [from R.-A. Ferchault
de Réaumur (1683-1757), French scientist]
– The unit (symbol: °R) of the Réaumur
thermometric scale in which zero is taken to be
the temperature at which ice melts and the
value of 80°R is the temperature at which water
boils, at a pressure of 1 atm.
rhe – CGS unit of fluidity (the reciprocal of
dynamic viscosity); it is equal to the reciprocal
of the poise.
rpm – Abbr. of revolution per minute, practical
unit of rotational frequency.
S
s – Symbol for second.
S – Symbol for siemens.
second – The unit of time (symbol: s) of the SI,
of which it is one of the seven base units: it is
defined as the interval of time containing
9,192,631,770 periods of the electromagnetic
radiation emitted in a clearly-defined transition
between two energy levels of the cesium 133
atom.
short ton – Unit of mass, equal to
2,000 pounds and consequently to 907.18 kg,
see ton.
SI – Abbr. in all languages of the Système
International [d’Unités] (International System
[of Units]). The SI base units are:
metre [length], kilogram [mass], second [time],
ampere [intensity of electric current], kelvin
[thermodynamic temperature], mole [amount
of substance], candela [luminous intensity].
siemens [from E.W. von Siemens (1816-92),
German engineer] – The SI unit of electrical
conductance (symbol: S): it is the conductance
of a conductor with electrical resistance equal
to 1 ohm.
sievert [from R.M. Sievert (1896-1966),
Swedish radiologist] – The SI unit
(symbol: Sv) of radioactive dose equivalent,
equal to 1 joule/kg.
SMD – Abbr. of Système Metrique Décimal
[d’Unités] (Decimal Metric System [of Units]).
sr – Symbol for steradian.
St – Symbol for stokes.
steradian – The SI unit of solid angle (symbol:
sr): it is equal to the solid angle subtended at
the centre of a sphere by an area of surface
equal to the square of the radius.
stokes [from G.G. Stokes (1819-1903), Engl.
mathematician] – The CGS unit of kinematic
viscosity (symbol: St); it is equivalent to
10⫺4 m2/s.
Sv – Symbol for sievert.
system of units – A coordinated set of units
of measurement, based on the definition
of base quantities and relative base units of
measurement, and the consequent definition
of the quantities and units of measurement
derived from those bases. Numerous
systems of units are still applied in various
sectors and different parts of the world;
for example in English-speaking countries
the following are still used: Apothecaries’
VOLUME V / INSTRUMENTS
System, Avoirdupois System, Imperial
System (in the UK), Troy System.
❏ International System of Units (Système
International d’Unités), designated SI
(see SI) in all languages: it is the system
of units generally used in the scientific field.
to 1/12 of the mass of an atom of the carbon
isotope 12C.
unit – When referring to the taking of
measurements, it is the value of a physical
quantity to which the magnitudes of other
quantities of the same kind are related.
T
V
t – Symbol for tonne.
T – a) Symbol for tesla; b) symbol for tera-, SI
metrological prefix.
tce [abbr. of (metric) tons of coal equivalent]
– Unit of energy used for large-scale energy
predictions or balances, equal to the energy
obtained from the combustion of 1 t of carbon;
conventionally it is assumed that on average
1 t of carbon produces 7⭈106 kcal, it follows
that 1 tce⫽0.7 toe and 1 toe⫽1.43 tce,
see toe.
technical system of units – A system of
mechanical units in which the base sizes are
length, force and time.
tera- [from Gr. tèras «monster»] – SI
metrological prefix (symbol: T) which
multiplies by 1012 the value of the unit of
measurement it prefixes, e.g. 1 terametre
⫽1 Tm⫽1012 m.
tesla [from N. Tesla (1856-1943), Croatian
physicist] – The SI unit of magnetic
induction (symbol: T), equal to 1 weber/m2
(1 T⫽1 Wb/m2).
toe [abbr. of tons of oil equivalent] – Unit of
measurement of energy used in technology and
economics for large-scale energy predictions or
balances, equal to the average thermic energy
obtainable from the combustion of 1 tonne
(metric ton) of oil, conventionally assumed as
being equal to 9.8⭈106 kcal⫽4.19⭈1010 J
⫽11,400 kWh.
ton – Unit of mass, equal: in the UK to 2,240
pounds and consequently to 1,016 kg
(long ton); in the USA to 2,000 pounds and
consequently to 907.18 kg (short ton). For
metric ton, see tonne. ❏ Ton-force: unit of
force, equal to the weight of 1 ton at sea level
and 45° latitude.
tonne (or metric ton) – A multiple of the
kilogram in the metric systems of units, equal
to 1,000 kg (symbol: t). For the long ton and
the short ton, see ton.
torr [from E. Torricelli (1608-47), Italian
scientist] – Denomination and symbol of the
millimetre of mercury, unit of pressure
equivalent to 133.322 pascal.
troy – System of units of measurement of
mass, used in English-speaking countries for
precious metals and stones; its base is the
troy grain equal to approx. 64.8 mg and the
main units are: the pennyweight equal
to 24 grains, the troy ounce equal to 480
grains, troy pound equal to 5,760 grains
(0.373 kg).
V – Symbol for volt.
VA – Symbol for volt-ampere.
volt [from A. Volta (1745-1827), Italian
physicist] – The SI unit of electrical potential
difference or of electromotive force
(symbol: V). It is equal to the difference in
electrical potential between two sections
of a conductor carrying a constant current of
1 ampere when the power dissipated between
these sections is 1 watt; or also to the
electromotive force between two points for
which 1 coulomb of electric charge will absorb
1 joule of work passing from one point to the
other.
volt-ampere – SI unit of the amount
of apparent power in an alternating electrical
current (symbol: VA), equal to
the apparent power of an alternating current
of 1 ampere (root-mean-square value)
which flows in a circuit under the effect
of an electromotive force of 1 volt
(root-mean-square value).
volt-ampere-hour – Unit of the apparent
energy of an alternating electric current
(symbol: VAh), equal to the energy carried in
1 hour by an alternating current with apparent
power of 1 volt-ampere.
U
u – Symbol for unified atomic mass unit.
unified atomic mass unit – Unit of atomic
mass (symbol: u, often omitted) equal
W
W – Symbol for watt, the SI unit of power;
where it is necessary to differentiate,
We is the symbol for electrical watt, Wt for
thermal watt.
watt [from J. Watt (1736-1819), British
inventor] – The SI unit of power (symbol: W),
equal to 1 joule per second (1 W⫽1 J/s). In
electric power station technology the
expressions thermal watt (symbol: Wt) and
electrical watt (symbol: We ) are used to
indicate, respectively, the thermal power
produced by the fuel and the electrical power
supplied by the electric generator.
watt-hour – Practical unit of energy (symbol:
Wh), equal to the energy provided in 1 hour
by a power of 1 watt; 1 Wh⫽3.6⭈103 J.
A multiple frequently used is the kilowatt-hour
(kWh), equal to 103 Wh.
Wb – Symbol for weber.
weber [from W.E. Weber (1804-91), German
physicist] – The SI unit of magnetic flux
(symbol: Wb), equal to 1 tesla · m2
(1 Wb⫽1 T·m2).
Y
y – Symbol for yocto-, SI metrological prefix.
Y – Symbol for yotta-, SI metrological prefix.
627
DICTIONARY OF SCIENTIFIC AND TECHNICAL UNITS
yard – Unit of length (symbol: yd), equal
to 3 feet and consequently to 36 inches and to
0.9144 m.
yd – Symbol for yard.
year – A unit of time (symbol: yr, or also a
from Lat. annus, often as a superscript),
which is the same as the interval
of time between two consecutive passages
of the Sun at the spring equinox
(solar or tropical year) and equals
365d 5h 48m 46.98s (3.156⭈107 s).
yocto- (or yotto-) [derived from the Lat. octo
«eight», with y placed before it as a
distinguishing feature] – SI metrological prefix
(symbol: y), which multiplies by 10⫺24
(the eighth power of a thousandth) the value
of the unit of measurement it prefixes:
e.g. 1 yoctometre⫽1 ym⫽10⫺24 m.
yotta- (or yocta-) [the same derivation as yocto-]
– SI metrological prefix (symbol: Y),
which multiplies by 1024 (the eighth power
of a thousand) the value of the unit
628
of measurement it prefixes:
e.g. 1 yottametre⫽1 Ym⫽1024 m.
yr – Symbol for year.
Z
z – Symbol for zepto-, SI metrological prefix.
Z – Symbol for zetta-, SI metrological prefix.
zepto- (or zetto-) [derived from the Gr. heptá
«seven», with z placed before it as a
distinguishing feature] – SI metrological prefix
(symbol: z), which multiplies by 10⫺21
(the seventh power of a thousandth) the value
of the unit of measurement it prefixes: e.g.
1 zeptometre⫽1 zm⫽10⫺21 m.
zetta- (or zepta-) [the same derivation as zepto-]
– SI metrological prefix (symbol: Z), which
multiplies by 1021 (the seventh power of a
thousand) the value of the unit of measurement
it prefixes: e.g. 1 zettametre⫽1 Zm⫽1021 m.
Bibliography
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Dizionario, in: Enciclopedia delle scienze
fisiche, Roma, Istituto della Enciclopedia
Italiana, 1992-1996, 7v.
Fazio M. (1995) SI, MSKA, CGS & Co.
Dizionario e manuale delle unità di misura,
Bologna, Zanichelli.
Jerrard H.G., McNeill D.B. (1992)
A dictionary of scientifics units including
dimensionless numbers and scales,
London-New York, Chapman & Hall.
Il McGraw-Hill Zanichelli. Dizionario
enciclopedico scientifico e tecnico
inglese-italiano e italiano-inglese (2004),
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Melaragno M. (1991) Quantification in science.
The VNR dictionary of engineering units and
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ENCYCLOPAEDIA OF HYDROCARBONS