Reefkeepers need to keep track of
many measurements in their aquaria. The most important of
these include temperature, salinity (or specific gravity),
calcium and alkalinity. Just as important as the actual values,
however, are the units of measurement involved. Is an alkalinity
of five a "good" value for a reef aquarium? The answer, obviously,
depends on the units of measure. An alkalinity of 5 meq/L
is on the high side, while an alkalinity of 5 dKH is on the
low side. Even more confusing to aquarists are test kits that
report results in unusual units. The Hach kit that reveals
the magnesium concentration in units of ppm calcium carbonate
equivalents probably takes the prize in the "dubious units"
contest.
In order to assist aquarists in understanding their reef
aquaria, this article provids an encyclopedic listing of the
most common units of measure used in reef keeping. In most
cases, the entries include definitions and conversions to
other units that might be preferable to use. Many online conversion
calculators can be employed when converting different units,
and many are linked in the listings. In general, this link and this link
have very good conversion calculators.
Happy Reefing!
Note: For quick navigation throughout this
page, use the slideout menu on the left of the screen.
Units of Measure:
A
See amp.
Å
See Ångstrom.
absorbance (AU)
Absorbance is a logarithmic unit used to measure light penetration
through a medium, such as aquarium water. If T is the percentage
of light transmitted, then the absorbance is defined to be
log^{10} T absorbance units.
An increase in absorbance of 1.0 AU corresponds to a reduction
in transmittance by a factor of 10.The table below shows the
relationship between AU and light transmission:
Absorbance (AU)

Transmission (T)

Light Transmission
(%)

0.000043

0.9999

99.99

0.00043

0.999

99.9

0.0044

0.99

99

0.046

0.9

90

0.30

0.5

50

0.70

0.2

20

1

0.1

10

2

0.01

1

3

0.001

0.1

4

0.0001

0.01

alkalinity
Alkalinity
is a measure of the amount of acid necessary to reduce water's
pH to the point where all carbonate and bicarbonate have been
converted into carbonic acid (about pH 4.2 in seawater).
In seawater, bicarbonate and carbonate provide nearly all
of the alkalinity. Consequently, reef aquarists can use it
as a surrogate measure for bicarbonate and carbonate which
are taken up by calcifying organisms (Figure 1). The units
of alkalinity can be meq/L (milliequivalents per liter), dKH
(degrees of carbonate hardness), or ppm (meaning ppm of calcium
carbonate equivalents). One meq/L = 2.8 dKH = 50 ppm CaCO_{3}
equivalents. Seawater has an alkalinity of about 2.5 meq/L
= 7 dKH = 125 ppm CaCO_{3}
equivalents.
Figure 1. This green mushroom is not likely influenced
strongly by whether the alkalinity is 4 dKH or 4 meq/L,
but the coralline algae behind it likely will be.

Alkalinity is not a measure of pH, although some people talk
of it that way, nor is it a very useful indicator of a solution's
buffering capacity, although it is related strongly to buffering
capacity. An article explaining in detail what alkalinity
is and what its units mean is online here.
There is a calculator for converting between different alkalinity
units online here. Here
is an online calculator for determining how much of different
supplements to add to boost alkalinity, as well as calcium
and magnesium.
amp (A)
The ampere (amp or A) is a unit of electrical current flow.
It is named after the French physicist AndréMarie Ampère
(17751836). The definition of the ampere is complicated,
but it can be related simply to other electrical units. One
ampere represents a current flow of one coulomb of charge
per second. One ampere of current also results from a charge
of one volt across one ohm of resistance, or from a power
production rate of one watt per volt. Electrical circuit breakers
and fuses allow a certain amount of current to pass up to
a design limit (often 15 or 20 amps for normal household circuits
in the United States), and they then terminate the current
flow by burning out (a fuse) or tripping a breaker (circuit
breaker).
Ångstrom (Å)
The Ångstrom (or Angstrom; named after the Swedish physicist
Anders Jon Ångström (18141874)) is a metric unit of length,
equal to 0.1 nanometer or 10^{10}
meter. Angstroms are used most often to measure the wave length
of light, or sometimes in measuring the distance between atoms
in a molecule. The distance between adjacent atoms in a typical
molecule is 13 Ångstroms. The table below shows a typical
wavelength of different colors of light. A calculator for
various lengthrelated units can be found here.
Wavelength
(Ångstroms)

Color

<3800

Ultraviolet

38004350

Violet

43505000

Blue

50005200

Cyan

52005650

Green

5320

Green laser pointer

56505900

Yellow

59006250

Orange

6328

Red He/Ne laser (laser
pointer)

62507400

Red

>7400

infrared

atm (atmosphere)
The atmosphere is a unit of pressure designed to match the
average pressure of the Earth's atmosphere at sea level.
One atmosphere equals 1013 millibars (mb), 101.3 kilopascals
(kPa), 14.7 pounds per square inch (lb/in^{2}).
29.92 inches of mercury (in Hg), and 760.0 millimeters of
mercury (mm Hg), and 14.7 pounds of force per square inch
(lbf/in^{2}). The standard
atmosphere equals 1.033 technical atmosphere. A calculator
for various pressurerelated units can be found here.
AU
See absorbance.
bar (b)
The bar is a metric unit of atmospheric pressure. It equals
one million dynes per square centimeter, 100 kilopascals,
750 torr, 1.02 kilograms per square centimeter (kg/cm^{2}),
29.53 inches of mercury (in Hg) and 14.5 pounds per square
inch (lb/in^{2}). One bar
is slightly less than the average pressure of the Earth's
atmosphere, which is 1.01 bar. Meteorologists often report
atmospheric pressure in millibars. A calculator for various
pressurerelated units can be found here.
Baumé (°B or °Bé)
Baumé or degree Baumé is a unit of relative density, typically
implying a concentration of a material dissolved in water.
It can be read on a special type of hydrometer invented by
the French chemist Antoine Baumé (17281804). Two scales are
used, depending on whether the liquid is lighter than water
or heavier than water. For liquids lighter than water, the
relative density d in degrees Baumé is related to specific
gravity S by the formula d = (144.3 / S)  144.3. For liquids
heavier than water the formula is d = 144.3  (144.3 / S).
Reef aquarists will most likely encounter this unit as a unit
of concentration of certain chemicals, such as hydrochloric
acid.
British thermal unit
See BTU.
BTUh
See British thermal unit per hour
buffering capacity
See meq/L/pH unit.
BTU (British thermal unit or Btu)
The BTU is a unit of heat energy defined as the amount of
heat required to raise the temperature of one pound of water
by one degree Fahrenheit. It is most often used in reef keeping
as a way to gauge the power of chillers (see BTUh below).
One Btu is equal to about 251.996 calories, or 0.251996 kilocalories
(the “calories” counted by dieters). One BTU equals approximately
778.169 foot pounds, 1.055 056 kilojoules or 0.293 071 watt
hour. A calculator for various energyrelated units can be
found here.
BTUh or BTU/h (British thermal unit per Hour)
BTUh is the symbol used in the U.S. heating and air conditioning
industry for British thermal units per hour, a unit of power.
The correct symbol is Btu/h. One BTUh represents the capacity
to generate (or remove) one BTU each hour. Reef aquarists
encounter it most in the power rating of chillers. The higher
the rating, the more cooling capacity. One Btuh equals 0.293071
watt.
Brix (degree Brix, °Bx)
Brix is a unit of proportion sometimes encountered by reef
keepers using hydrometers intended for other industries.
Brix is equal to percent, used in measuring the concentration
of sugar in fruit juices, maple syrup, and similar solutions.
One brix equals 1 weight percent sugar in the solution. The
unit is named for the Austrian scientist Adolf Brix (d. 1870),
who invented a hydrometer that reads directly the percentage
of sugar in the juice. Brix is most often measured with
a hydrometer. A Brix hydrometer cannot be directly used to
measure aquarium salinity, however, without knowing the relationship
between Brix and seawater salinity.
buffering capacity (meq/L/pH unit)
See meq/L/pH unit.
calorie
The calorie (also called the small calorie or gram calorie)
is the CGS unit of heat energy. This calorie is the amount
of heat required to raise the temperature of one gram of water
by one degree Celsius. One calorie equals 4.1868 joules or
0.00396832 British thermal units (Btu). There are other definitions
that vary slightly from this one, with very slightly differing
numbers of joules per calorie. A calculator for various energyrelated
units can be found here.
Calorie (the large or dietary calorie)
See kcal.
candle
See candela.
candela (cd)
The candela is the SI for measuring the intensity of light.
Candela is the Latin word for "candle." Originally,
one candela represented the intensity of an actual candle.
The definition has evolved over the years to make it moiré
reproducible, but is still about the same intensity. A calculator
for various lightrelated units can be found here. A
calculator for various lightrelated units can be found here.
An article with detailed information on such lighting units
and their application to reef aquarists can be found here.
cc
See cm^{3}.
CCT (correlated color temperature)
See color temperature.
Celsius
See degree Celsius. A calculator
for various temperaturerelated units can be found here.
centigrade
See degree centigrade. A calculator
for various temperaturerelated units can be found here.
centimeter
See cm.
centipoise (cP, cPs, or cPo)
Centipoise is a common metric unit of dynamic viscosity equal
to 0.01 poise,
which is also 1 millipascal second (mPa·s). The viscosity
of water at 20°C (68°F) is approximately 1 cP. A calculator
for various viscosityrelated units can be found here.
cft
See ft^{3}.
A calculator for various volumerelated units can be found
here.
CGS Units (MKS units)
The CGS systems of units is based on the centimeter, the
gram, and the second, is called the CGS system. The
similar MKS system of units is based on the meter, kilogram,
and second. These basic units are then used to derive other
units used in those systems. Since the basic units of these
the systems differ only by factors of 10, the same is true
for the derived units.
In 1954, the Tenth General Conference on Weights and Measures
adopted the meter, kilogram, second, ampere, degree Kelvin,
and candela as the basic units for all weights and measures,
and in 1960 the Eleventh General Conference adopted the name
International System of Units (SI) for this collection of
units. In effect, these decisions gave the basic units of
the MKS system preference over the CGS system. Although some
of the CGS units remain in use for a variety of purposes,
they are being replaced gradually by the SI units selected
from the MKS system.
The following is a table of some common CGS units with their
SI equivalents.
CGS
unit

measuring

SI
unit equivalent

barye
(ba)

pressure

0.1 pascal
(Pa)

biot
(Bi)

electric current

10 amperes
(A)

calorie
(cal)

heat energy

4.19 joule
(J)

dyne
(dyn)

force

10^{5} newton
(N)

erg

work, energy

10^{7} joule
(J)

franklin
(Fr)

electric charge

3.3 x 10^{10} coulomb
(C)

galileo
(Gal)

acceleration

0.01 meter per second squared (m·s^{2})

lambert
(Lb)

luminance

3183 candelas
per square meter (cd·m^{2})

phot
(ph)

illumination

10^{4} lux
(lx)

poise
(P)

dynamic viscosity

0.1 pascal
second (Pa·s)

stilb
(sb)

luminance

10^{4} candelas
per square meter (cd·m^{2})

stokes
(St)

kinematic viscosity

10^{4}
square meters per second (m^{2}·s^{1})

CRI (color rendering index)
Color rendering index a scale used to measure the ability
of an artificial lighting system to show the "true"
colors of objects (the colors those objects display in natural
daylight). The scale ranges from 0 to 100, with higher numbers
representing a higher fidelity of color reproduction. Reef
aquarists encounter it with respect to aquarium lighting systems,
although few reef specialty bulbs provide such information
(or at least accurately). A calculator for various lightrelated
units can be found here. An
article with detailed information on such lighting units and
their application to reef aquarists can be found here.
cm (centimeter)
A centimeter is a unit of length, equal to 0.01 meter, or
about 0.39 inches. A calculator for various lengthrelated
units can be found here.
cm^{2} (square
centimeter)
A square centimeter is the CGS unit of area equal in size
to a square 1 cm x 1 cm, although it can be any shape of equal
area. 1 cm^{2} = 0.16 in^{2}=
100 mm^{2}= 10^{4}
m^{2}. A calculator for various
arearelated units can be found here.
cm^{3} (cc; cubic
centimeter)
A cubic centimeter is a unit of volume equal in size to a
cube 1 cm x 1 cm x 1 cm, although it can be any shape of equal
volume. 1 cm^{3} = 0.061
in^{3}. A calculator for various
volumerelated units can be found here.
color rendering index
See CRI.
color temperature (CCT)
Color temperature is a measure of the overall "color"
of a light source, such as a lamp used over an aquarium.
The measurement is obtained by comparing the spectrum that
it emits (i.e., the wavelengths of length) to the spectrum
of a perfect emitter at a specific temperature. CCT is expressed
in kelvin (K). Lower temperatures indicate more red and yellow
light, higher temperatures more blue. "CCT" stands
for "correlated color temperature." A calculator
for various lightrelated units can be found here.
An article with detailed information on such lighting units
and their application to reef aquarists can be found here.
cP, cPs, or cPo
See centipoise.
cps
See cycles per second.
cubic foot
See ft^{3}.
cfm (cubic foot per minute)
CFM is a unit of flow rate equal to one cubic foot of volume
each minute. It is often used in the rating of cooling fans
used in aquarium canopies (often in the 10100 CFM range),
and sometimes also for water pumps. A calculator for various
flow raterelated units can be found here.
cubic inch
See in^{3}.
cubic foot per minute
See cfm.
cubic meter (m^{3})
See m^{3}.
cup
The measuring cup a traditional unit of volume used in the
United States. One cup equals 1/2 (liquid) pint, 8 fluid ounces,
14.4 cubic inches, or 236.6 milliliters. In the United States,
the same size cup is used for measuring both liquid and dry
substances. In Canada, a cup is equal to 8 Imperial fluid
ounces (13.9 cubic inches; 227.3 milliliters). In Britain,
a similar but larger unit called the breakfast cup is sometimes
used, equal to 10 Imperial fluid ounces.
cycles per second (cps)
Cycles per second is a traditional unit of frequency equal
to one per second, or one hertz. A calculator for various
frequencyrelated units can be found here.
Dalton (Da; D)
A Dalton is an alternate name for the atomic mass unit (u
or amu). The dalton is often used in microbiology and biochemistry
for the masses of large organic molecules such as proteins.
Often the values are reported in kilodaltons (kDa). The Dalton
is named after the English chemist John Dalton (17661844),
who first developed the atomic theory of matter in 1803.
dB (decibel)
The decibel is a logarithmic unit most commonly used for
measuring sound. The unit is logarithmic because experiment
has shown that when humans perceive one sound to be twice
as loud as another, the louder sound is about ten times as
intense as the fainter one. Consequently, sound is measured
on a logarithmic scale. If one sound is 10 decibels "louder"
than another, this means the louder sound appears to be 10
times louder than the fainter one. A difference of 20 decibels
corresponds to an increase of 10 x 10 or 100 times in intensity.
A quiet room has a normal sound intensity of around 40 decibels,
ten thousand times louder than the faintest perceptible sound,
and thunder may have an intensity of 120 decibels, a trillion
times louder than the faintest sound. An online article with
a detailed definition and explanation of the decibel is available
here.
dB A
Decibel A is a unit of sound intensity, like the decibel,
except that sounds of high and low frequencies that are heard
poorly by the human ear have been filtered out prior to measurement.
An online article with a detailed definition and explanation
of the decibel A is available here.
degree Baumé (°B or °Bé)
See Baumé.
degree Brix
see Brix.
degree Celsius (°C)
Degrees Celsius is a metric unit of temperature. The Celsius
temperature scale is named for the Swedish astronomer and
physicist Anders Celsius (17011744). The freezing point of
water was originally defined to be 0°C, while the boiling
point was 100°C. Thus the Celsius degree is 1/100 of the difference
between these two temperatures. The scale now has a more precise
and reproducible definition relating to the physical properties
of water, but the size of the units is the same. A calculator
for various temperaturerelated units can be found here.
degree centigrade (°C)
Degrees centigrade an old name for the degree Celsius. The
Celsius scale is sometimes called "centigrade" because
it has 100 degrees between the freezing point and boiling
point of water. In 1850, the name was changed to "degree
Celsius" in honor of its inventor. A calculator for
various temperaturerelated units can be found here.
degree Kelvin (K)
Kelvin is the SI unit of temperature, previously called the
degree Kelvin (°K), but that term is no longer used. See
Kelvin. A calculator for various temperaturerelated units
can be found here.
degree Fahrenheit(°F)
Degrees Fahrenheit is a unit of temperature used primarily
in the United States. The unit was named after by the German
physicist Daniel G. Fahrenheit (16861736), who invented the
system as well as the mercury thermometer. He definedt 0°
to be the coldest temperature he could conveniently achieve
using an ice and salt mixture, and he intended to set 96°
as the temperature of the human body. On this scale, the freezing
point of water turned out to be about 32°F and the boiling
point 212°F. Later, the scale was more precisely defined by
these two temperatures. To convert a temperature in °F to
the Celsius scale, first subtract 32° and then multiply by
5/9. To convert a temperature in °C to the Fahrenheit scale,
we must first multiply by 9/5 and then add 32° (or alternately
to go both ways, F =(1.8)C + 32). A calculator for various
temperaturerelated units can be found here.
dKH
dKH stands for the German term “degrees of carbonate hardness.”
It is a unit of alkalinity, and is equivalent to 0.36 meq/L
or 17.8 ppm calcium carbonate equivalents. There are other
related units that have similar names, such as Clark degrees,
but they are rarely used by reef aquarists. Hardness is most
often used to refer to calcium and magnesium in solution,
but “carbonate hardness” evolved from the assumption that
in fresh water, much of the carbonate comes from weathering
of calcium and magnesium carbonates. So some units of measure
(like dKH and ppm calcium carbonate equivalents) refer to
the concentration with respect to the amount of calcium carbonate
that would need to dissolve into the water to produce that
alkalinity. An article explaining in detail what alkalinity
is and what the units mean is online here.
There is a calculator for converting between different alkalinity
units online here. There
is a calculator for determining how much of different supplements
to add to boost alkalinity, as well as calcium and magnesium,
online here.
drop (gtt)
The drop is an old unit of volume used in pharmaceutical
science. The drop was also known as the minim, a unit of volume
equal to 1/60 fluid dram or 1/480 fluid ounce (0.061 milliliter
in the U.S.; 0.059 milliliter in Britain). Today, prescriptions
are written in metric units, and the drop is defined as being
equal to exactly 0.05 milliliter (20 drops/ml). The actual
size of drops of water depend on how they are formed, including
the size of the opening that they come out of, the nature
of the tubing material, and what is in the water. In hospitals,
intravenous tubing is used to deliver medication in drops
of various sizes ranging from 10 drops/ml to 60 drops/ml.
Consequently, kits using drops in titrations may be less accurate
than measured volume measurements (using syringes or volumetric
pipettes, for example). If using a drop titration kit, it
is especially important to create the drops using the same
device included with the kit, as a different device may create
different size drops and hence give incorrect answers. A calculator
for various volumerelated units can be found here.
einstein
The einstein is a unit of light energy. One einstein is the
energy per mole (6.02 x 10^{23})
of photons carried by light. Light has frequency v
(in hertz) and wavelength l (in meters). One einstein
is then equal to (4.0 x 10^{10})v
or (0.12)/l joules per mole (J/mol). The unit is named
for the physicist Albert Einstein (18791955). A calculator
for various energyrelated units can be found here.
An article with detailed information on such lighting units
and their application to reef aquarists can be found here.
equivalent (equivalent weight; eq; Eq)
An equivalent is a unit of relative amount of material used
in chemistry. It is a complicated unit that can actually vary
with the intended use. One equivalent of an element, compound,
or ion is the amount (in moles) of that substance which would
react in the intended reaction multiplied by the number of
times that it can react. Most often, reef aquarists will
encounter this unit in alkalinity. For
monovalent ions that contribute one unit of alkalinity
per mole (e.g., bicarbonate, HCO_{3}^{}
and borate, B(OH)_{3}O^{}),
the number of equivalents is the same as the number of moles.
For a divalent ion that can contribute two units of alkalinity
per mole (e.g., carbonate, CO_{3}^{}),
the number of equivalents is twice the number of moles. For
ions that provide no alkalinity
(e.g., sodium, Na^{+}
or chloride, Cl^{}), the number
of equivalents is zero regardless of the number of moles.
Reef aquarists may also encounter equivalents and equivalent
concentrations in measurements of acids, where hydrochloric
acid (HCl) contributes one equivalent per mole, sulfuric acid
(H_{2}SO_{4}) contributes two equivalents
per mole, and phosphoric acid (H_{3}PO_{4})
can contribute 3 equivalents per mole. Aquarists may also
encounter equivalents in electrochemical reactions, where
the number of equivalents per mole depends on how many electrons
the particular ion can release or take up.
Equivalent weight is the number of equivalents multiplied
by the molecular weight of the ion or molecule. In practice,
one equivalent is a large unit and measurements such as alkalinity
are more likely to be in milliequivalents (mEq or meq).
erg
The erg is the unit of work or energy in the CGS system.
It is equal to the work done by a force of one dyne acting
through a distance of one centimeter. One erg is the kinetic
energy of a mass of 2 grams moving at a velocity of 1 cm/sec.
This is equal to 0.1 microjoule, or 7.4 x 10^{8}
footpound. There is no symbol for an erg. The word is always
spelled out in full. A calculator for various energyrelated
units can be found here.
F
See farad.
fc
See footcandle.
Fd
See faraday.
Fahrenheit
See degree Fahrenheit. A calculator
for various temperaturerelated units can be found here.
farad (F)
The farad is the SI unit of electric capacitance. A pair
of conductors separated by an insulator can store a substantial
amount of electrical energy, with one being negatively charged
and the other being positively charged. The better the insulator,
the larger the charge that the conductors can hold. This property
is referred to as capacitance, and is measured in farads.
One farad is the ability to store one coulomb of charge per
volt of potential difference between the two conductors. In
many electrical circuits, capacitance is measured in microfarads,
nanofarads, or picofarads. The unit is named for the British
physicist Michael Faraday (17911867), who did a lot of pioneering
work in electricity and electrochemistry.
faraday (Fd)
The faraday is a unit of electric charge. The faraday is
equal to 96.5 kilocoulombs or 26.8 ampere hours. It is commonly
used in electrochemistry, where one faraday is equal to the
charge on one mole of electrons, which is the product of Avogadro's
number (6.02 x 10^{23}) and the charge on a single
electron.
fluid Dram (fl dr)
The fluid dram is a unit of volume in the apothecary system.
One fluid dram equals 1/8 fluid ounce. This unit is usually
called the fluid dram or fluidram to avoid confusion with
the weight dram. The U. S. fluid dram contains about 0.23
cubic inches or 3.7 milliliters. In the British imperial system,
the fluid dram is about 0.22 cubic inches or 3.6 milliliters.
A calculator for various volumerelated units can be found
here.
fluid ounce (fl oz)
The fluid ounce is a unit of liquid volume, called the fluid
ounce to avoid confusion with the weight ounce. In the U.S.
there are 16 fluid ounces in a pint, so each fluid ounce represents
1.8 cubic inches or 29.6 milliliters. In the British Imperial
system there are 20 fluid ounces in an imperial pint, so each
fluid ounce represents about 1.7 cubic inches or 28.4 milliliters.
A U.S. fluid ounce of water weighs just a bit more than one
ounce avoirdupois; a British fluid ounce weighs exactly one
ounce at a specified. A calculator for various volumerelated
units can be found here.
flux density
See mE
(microeinstein).
ft (foot; ′ ; international foot)
The foot is a traditional unit of distance. The actual length
and definition of the “foot” have changed many times. In 1959,
the U.S. National Bureau of Standards redefined the foot to
equal exactly 30.48 centimeters. This definition was also
adopted in Britain by the Weights and Measures Act of 1963,
so the foot of 30.48 centimeters is now called the international
foot. A calculator for various lengthrelated units can be
found here.
ft^{2} (square
foot; sq ft)
A square foot is a unit of area equal in size to a square
1 ft x 1 ft, although it can be any shape of equal area. 1
ft^{2} = 144 in^{2}
= 929 cm^{2} = 0.093 m^{2}.
A calculator for various arearelated units can be found here.
ft^{3} (cubic
foot; cft)
A cubic foot is a unit of volume equal in size to a cube
1 ft x 1 ft x 1 ft, although it can be any shape of equal
volume. 1 ft^{3} = 1728 in^{3},
105.4 cm^{3}, 0.028 m^{3},
28.3 liters, 7.5 U.S. liquid gallons or 6.2 British Imperial
gallons. A calculator for various volumerelated units can
be found here.
footcandle (fc or ftc)
The footcandle is an older unit of illuminance or illumination,
defined as the illuminance received by a surface at a distance
of one foot from a light source of intensity one international
candle. The "international candle" was the predecessor
of the candela
as the standard unit of light intensity. Illuminance is now
measured in lux; one footcandle equals 10.76 lux or about
1.076 milliphot.
The unit is also sometimes spelled footcandle or foot candle.
A calculator for various lightrelated units can be found
here.
foot of head (ft hd)
Foot of head is a unit of water pressure used in plumbing.
One foot of head is equivalent to a pressure of 0.43 lb/in^{2},
2.99 kilopascals (kPa), 29.9 millibar (mb) or 0.88 inches
of mercury (in Hg). It is frequently used in aquarium piping
and pumping applications, and takes into account backpressure
against pumps generated from both increases in elevation and
frictional losses. A calculator for various pressurerelated
units can be found here.
foot per minute (ft/min or fpm)
Foot per minute is a unit of velocity or flow rate. One foot
per minute equals 30.48 cm/min, 5.08 mm/s, or 0.018 miles
per hour. A calculator for various speedrelated units can
be found here.
foot per second (ft/s or ft/sec or fps)
Foot per second is a unit of velocity equal to exactly 15/22
mile per hour or 1.1 kilometers per hour. Aquarists sometimes
quote flow rates in their aquaria using this unit. A calculator
for various speedrelated units can be found here.
ftc
See footcandle.
ft hd
See foot of head.
ft/min or fpm
See foot per minute.
ft/s or ft/sec
See foot per second.
g
See gram.
gallon (gal)
The gallon is a unit of liquid volume. Its name derives
from the Roman galeta, which originally meant a pailfull.
Gallons of various sizes have been used throughout history.
In the United States, the liquid gallon is now defined as
231 cubic inches and holds 4 liquid quarts or 3.79 liters.
A U.S. gallon of fresh water weighs about 8.33 pounds, and
a gallon of salt water weighs about 8.55 pounds. The British
(Imperial) gallon has a volume of 277.4 cubic inches or 4.55
liters. A calculator for various volumerelated units can
be found here.
g/cm^{3}
See gram per milliliter.
general hardness
see GH.
GEW
See gram equivalent weight.
g/g
See weight percent.
g/kg
See gram per kilogram.
g/L
See gram per liter.
g/mL
See gram per milliliter.
gpg
See grain per gallon.
gph (gallons per hour)
Gallons per hour is a unit of liquid flow that is frequently
used for aquarium pumps such as powerheads. The flow of such
pumps varies with the backpressure against which the pump
is pushing the water. A calculator for various flow raterelated
units can be found here.
gpm (gallons per minute)
Gallons per minute is a unit of liquid flow that is frequently
used for aquarium pumps. The flow of such pumps varies with
the backpressure against which the pump is pushing the water.
A calculator for various flow raterelated units can be found
here.
grain
(gr)
The grain is a unit of weight equal to 1/7000th of a pound
or 0.0648 gram. A calculator for various weightrelated units
can be found here.
grain/gallon (gpg)
The gpg is an expression of a material's concentration in
solution. One grain per gallon is equivalent to 17.1 parts
per million. Aquarists will most often encounter this unit
as a measure of hardness (magnesium plus calcium), and as
such, it is used as grains per gallon of calcium carbonate
equivalents. Nevertheless, both calcium and magnesium are
counted as if they are calcium ions. Here is one way to think
of this odd unit. The total molar concentration (moles/L)
of magnesium and calcium present would be determined with
a kit that didn’t distinguish between the two metals. This
combination molar value (moles/L) is then multiplied by the
molecular weight of calcium carbonate (100 grams/mole) to
give the hardness in g/L of calcium carbonate equivalents. To
get to a unit of ppm (~mg/L) calcium carbonate equivalents,
this value is multiplied by 1000. To convert to grains per
gallon, divide ppm calcium carbonate equivalents by 17.1.
gram (g)
The gram is a unit of mass in the metric system originally
defined to be the mass of one cubic centimeter of pure water,
but to provide precise standards it was necessary to construct
physical objects of specified mass. One gram is now defined
to be 1/1000 of the mass of the standard kilogram, a platinumiridium
bar retained by the International Bureau of Weights and Measures
in Paris. The kilogram, 1000 grams, is considered the base
unit of mass in the SI system. The gram is equal to about
15.4 grains or 0.035 ounce. The only correct symbol for the
gram is g. The abbreviations gm and gr should never be used. A
calculator for various weightrelated units can be found here.
gram per kilogram (g/kg)
Gram per kilogram is a unit of mass concentration equal to
1 per mill (1 part per thousand). One g/kg also equals 0.1%
g/g or 0.1% w/w.
gram/liter (g/L)
Gram per liter (g/L) is a concentration unit based on the
number of grams of a substance dissolved in a liquid, usually
water. When the fluid is fresh water, a g/L is approximately
equal to 1 ppt (1 part per thousand). Seawater contains approximately
35 grams per liter of total salts.
gram per milliliter (g/mL; g/cm^{3})
Gram per milliliter is a unit of density of a substance.
It is the same as grams per centimeter cubed (g/cm^{3}).
Seawater has a density of about 1.023 g/mL at 80ºF (note that
the specific gravity at this temperature is higher, about
1.0264). A calculator for various densityrelated units can
be found here.
GH (general hardness)
Hardness is a characteristic of water due to the presence
of dissolved calcium and magnesium. Water hardness is responsible
for most scale formation in pipes and water heaters (calcium
and magnesium carbonates, typically), and forms insoluble
solids when it reacts with soaps. Hardness is often expressed
in grains per gallon, parts per million or milligrams per
liter, all as calcium carbonate equivalents. It is a measure
often used in freshwater aquarium systems, but not often in
marine systems, where its values are very large. Seawater
has a total (general) hardness of about 6.3 g/L (6,300 ppm)
of calcium carbonate equivalents. These linked articles detail
aspects of magnesium
and calcium
in seawater.
gram equivalent weight (GEW)
The gram equivalent weight is the mass in grams of a substance
that would provide one mole of reactivity in a particular
reaction defined by the user of this unit (see “equivalent”).
Also known as gram equivalent (gEq) or gram equivalent mass
(GEM)
gtt
See drop.
h
See hour.
horsepower (hp)
Horsepower is a unit of power used
in the United States representing the power exerted by a single
horse in pulling a load. The unit horsepower was defined by
James Watt (17361819), the inventor of the steam engine,
who experimentally determined that a typical horse is capable
of generating 550 footpounds per second. A horse harnessed
to an appropriate machine, for example, can lift 550 pounds
at the rate of 1 foot per second. The SI unit of power is
named for Watt, and one horsepower is equal to approximately
746 watts. Aquarists most often encounter horsepower ratings
on motors used to drive large pumps or chillers. A calculator
for various energyrelated units can be found here.
hour (h; hr)
The hour is a unit of time equal to 60 minutes, 3600 seconds
or 1/24 day.
hp
See horsepower.
hr
See hour.
Hz (Hertz)
The hertz is the SI unit of frequency equal to one cycle
per second (cps). Frequencies of radio and television waves
are measured in kilohertz (kHz), megahertz (MHz) or gigahertz
(GHz), and the frequencies of light waves are measured in
terahertz (THz). The unit is named for the German physicist
Heinrich Rudolf Hertz (18571894), who proved in 1887 that
energy is transmitted through a vacuum by electromagnetic
waves. A calculator for various frequencyrelated units can
be found here.
imperial gallon (gal; IG)
The Imperial gallon is a unit of volume equal to 1.2 U.S.
liquid gallons or 4.55 liters. A calculator for various volumerelated
units can be found here.
in (inch; ′′)
The inch is a unit of distance equal to 1/12 foot or 2.54
centimeters. A calculator for various lengthrelated units
can be found here.
in^{2} (square
inch)
A square inch is a unit of area equal in size to a square
1 inch x 1 inch, although it can be any shape of equal area.
1 in^{2} = 6.5 cm^{2}
= 645 mm^{2}. A calculator
for various arearelated units can be found here.
in^{3} (cubic
inch)
The cubic inch (in^{3}) a
unit of volume equal in size to a cube 1 in x 1 in x 1 in,
although it can be any shape of equal volume. It is a traditional
unit of volume in Englishspeaking countries. One cubic inch
equals 1/1728 = 0.000058 ft^{3},
16.4 cm^{3}, 16.4 milliliters,
0.55 U.S. fluid ounces or 0.58 British Imperial fluid ounces.
A calculator for various volumerelated units can be found
here.
inch of mercury (in Hg)
The inch (in) of mercury (Hg) is a unit of atmospheric pressure.
In the United States, atmospheric pressure is often expressed
as the height of a column of mercury exerting the same pressure
as the atmosphere. In a mercury barometer, this height is
read directly as the height of the mercury column. One inch
of mercury is equivalent to 0.49 pounds per square inch (psi)
or 3.39 kilopascals (33.9 millibars). A calculator for various
pressurerelated units can be found here.
iodine number
A measure of activated carbon's ability to adsorb substances
with low molecular weights (that is, small molecules). It
is usually reported in mg/g, which equals the milligrams of
iodine (I_{2}) that can be adsorbed onto one gram
of activated carbon.
ionic strength (mole/kg = molal or mole/L = molar
= M)
The ionic strength is used in interpreting a variety of physical
phenomena in aqueous solutions such as seawater. For example,
to what extent the ions in an aqueous solution interfere with
the interactions of charged species with one another is related
to the ionic strength. The ionic strength is calculated by
multiplying the concentration of each charged species by the
square of its charge, summing the values obtained for each
species, and dividing by two. For example, 1 molar (M) sodium
chloride would have an ionic strength of (1x1 + 1x1)/2 = 1
M. Likewise, 1 M calcium chloride (CaCl_{2}) would
have an ionic strength of (1 x 4 + 2 x 1)/2 = 3 M.
joule (J)
The joule is the SI unit of energy. It is defined to be the
work done by a force of one newton acting to move an object
through a distance of one meter. One joule is also the kinetic
energy of a mass of two kilograms moving at a velocity of
1 m/s, or 10^{7} ergs, 0.74
footpounds, 9.5 x 10^{4}
Btu, 0.24 (small) calories
or 2.8 x 10^{4} watt hour.
The joule is named for the British physicist James Prescott
Joule (18181889), who demonstrated that mechanical and thermal
energy were equivalent in 1843. The unit is usually pronounced
"jool." A calculator for various energyrelated
units can be found here.
Kelvin (K)
Kelvin is the SI unit of temperature, previously called the
degree Kelvin (°K), but that term is no longer used. One
Kelvin represents the same temperature difference as one degree
Celsius. In 1967 the General Conference on Weights and Measures
defined the temperature of the triple point of water (the
temperature at which water exists simultaneously in the gaseous,
liquid, and solid states) to be exactly 273.16 Kelvins. Since
this temperature is also equal to 0.01°C, the temperature
in Kelvins is always equal to 273.15 plus the temperature
in degrees Celsius. The Kelvin equals exactly 1.8 degrees
Fahrenheit. A calculator for various temperaturerelated units
can be found here.
K (Kelvin color temperature)
Color temperature is a measure of the overall "color"
of a light source, such as a lamp used over an aquarium.
The measurement is obtained by comparing the spectrum that
it emits (i.e., the wavelengths of light) to the spectrum
of a perfect emitter at a specific temperature. CCT (correlated
color temperature) is expressed in Kelvin (K). Lower temperatures
indicate more red and yellow light, higher temperatures more
blue. A calculator for various lightrelated units can be
found here. An
article with detailed information on such lighting units and
their application to reef aquaria can be found here.
kcal (kilocalorie; Calorie)
The kcal is an ambiguous unit of energy. The ambiguity comes
about because there are two "calories" in common
use. These are the calorie (the small calorie equal to 4.19
kilojoules) and the Calorie (the large calorie equal to 4.19
megajoules). The term kilocalorie means 1000 calories, which
is the same as 1 Calorie. Kilocalorie is the correct term
for the unit known in human nutrition as the "calorie."
A calculator for various energyrelated units can be found
here.
kDa
See kiloDalton.
kg (kilogram)
The kilogram is the unit of mass in the SI and MKS versions
of the metric system. The kilogram is defined as the mass
of the standard kilogram, a platinumiridium bar held by the
International Bureau of Weights and Measures (BIPM) near Paris,
France. Copies of this bar are kept by the standards agencies
of all the major industrial nations, including the U.S. National
Institute of Standards and Technology (NIST). One kilogram
equals 1000 grams or 2.2 pounds. The kilogram was designed
to be approximately the mass of a liter of water. A calculator
for various weightrelated units can be found here.
kilodalton (kDa)
The kilodalton is a unit of mass equal to 1000 atomic mass
units, or daltons.
kilogram
See kg.
kilopascal (kPa)
The kilopascal is a unit of pressure equal to 1000 pascals
(Pa), 10 millibars (mb), 0.145 pounds per square inch (lbf/in^{2}
or psi), 20.9 pounds per square foot, 7.5 millimeters of mercury
(mm Hg), 0.30 inches of mercury (in Hg), 4.02 inches of water
column (in WC) or 0.335 foot of head (ft hd). A calculator
for various pressurerelated units can be found here.
kilowatt (kW)
The kilowatt is a unit of power, equivalent to 1000 watts,
1.34 horsepower or 738 footpounds per second.
kWh
See kilowatt hour.
kPa
See kilopascal.
kW
See kilowatt.
kWh (kilowatt hour)
The kilowatt hour is the commercial unit of electric energy
in the United States. One kilowatt hour represents the amount
of energy delivered at a rate of 1000 watts over a period
of one hour, or 100 watts over 10 hours. The kilowatt hour
is equivalent to 3.6 megajoules, 3412 BTU, 860 (kilogram)
Calories or 2.66 million foot pounds. A calculator for various
energyrelated units can be found here.
L (l; liter; litre)
The liter is the common metric unit of volume. It was originally
defined to be the volume occupied by a kilogram of water,
although it was given a series of more precise definitions
over the years. The liter is 61.0 cubic inches, 1.056 U. S.
liquid quarts, 33.814 fluid ounces, 0.908 U. S. dry quarts,
or 0.88 British imperial quarts. Both the lower case letter
l and the upper case L are accepted symbols for the liter,
but the U.S. Department of Commerce specifies that L be used,
to avoid confusion with the numeral 1. In scientific prose,
chemists are more inclined to use L while biologists often
use l. The unit is spelled liter in the U.S. and litre in
Britain. A calculator for various volumerelated units can
be found here.
lb (lbm; #)
The pound (lb) is a unit of weight that has evolved over
the years. The unit now used in the United States is the avoirdupois
pound. The avoirdupois pound is divided into 16 ounces. One
avoirdupois pound is equal to 453.6 grams. A calculator for
various weightrelated units can be found here.
lethal dose (LD)
The LD is a measure used in toxicology to express the percentage
of a population killed by a dose of the substance being studied.
The percentage killed is often given as a subscript. For example,
the toxicity of a chemical compound is commonly expressed
by stating the LD_{50}. The
LD_{50} is the amount of the
substance that kills 50% of the test population. Aquarists
often encounter it in studies of the toxicity to aquarium
inhabitants of compounds such as copper. It can have any of
a variety of normal concentration units, such as ppm, ppb,
etc.
lbs/cu ft (pounds per cubic foot)
Pounds per cubic foot is a measure of density, which aquarists
are most likely to encounter in relation to the bulk density
of solids such as calcium hydroxide, calcium oxide or calcium
carbonate powders. A calculator for various densityrelated
units can be found here.
lbs/sq ft (pounds per square foot)
Pounds per square foot is a unit of force or pressure. It
is often used in specifying the maximum loading of floors
(which might be important to aquarists planning large aquaria).
A calculator for various pressurerelated units can be found
here.
ld
See lethal dose.
Liter
See L.
liter per hour
see L/h.
lm
See lumen.
lm/m^{2}
See lux.
lph (liter per hour)
Liters per hour is a unit of liquid flow that is frequently
used for aquarium pumps such as powerheads. The flow of such
pumps varies with the backpressure against which the pump
is pushing the water. A calculator for various flow raterelated
units can be found here.
lumen (lm)
The lumen is the SI unit for measuring the flux of light
through a unit portion of a sphere. Light's intensity is measured
in candelas. One lumen represents the total flux of light
emitted, equal to the intensity in candelas multiplied by
the solid angle in steradians (1/(4·p)
of a sphere) through which the light passes. The total flux
of a onecandela light source, if the light is emitted uniformly
in all directions, is 4·p
lumens. A calculator for various lightrelated units can be
found here. An
article with detailed information on such lighting units and
their application for reef aquarists can be found here.
lumen/watt (lm/w)
Lumen per watt is a way of gauging the efficiency of a light
source at producing light using electrical energy. The higher
the lm/w, the less electricity is required to produce a fixed
amount of light. A calculator for various lightrelated units
can be found here. An
article with detailed information on such lighting units and
their application to reef aquaria can be found here.
lux (lx; lm/m^{2})
Lux is the SI unit for measuring the illumination of a surface.
One lux is defined as an illumination of one lumen per square
meter. The intensity of the light source is measured in candelas;
the total light flux in transit is measured in lumens (1 lumen
= 1 candela·steradian); and the amount of light received per
unit of surface area is measured in lux (1 lux = 1 lumen/square
meter). One lux is equal to approximately 0.09290 foot candle.
A calculator for various lightrelated units can be found
here.
An article with detailed information on such lighting units
and their application for reef aquarists can be found here.
m (meter; metre)
The meter is the metric and SI unit of distance. Originally,
the meter was designed to be one tenmillionth of the distance
between the Equator and the North Pole. Over the years it
has become better defined, and is now defined in terms of
the speed of light and the specific fraction of the length
that light travels in a second in vacuum. One meter equals
1.09 yards, 3.28 feet or 39.4 inches. The unit is spelled
meter in the U.S. and metre in Britain. A calculator for various
lengthrelated units can be found here.
M (molar; moles/L)
Molar is a unit used by scientists to describe the concentration
of chemical solutions. A one molar solution contains one mole
(6.02 x 10^{23} molecules) of the chemical dissolved
in 1 L of solution. To convert molar concentration into ppt
(parts per thousand), multiply the molar concentration by
the molecular weight of the chemical. To convert into ppm
(parts per million), multiply the molar concentration by 1000
and by the molecular weight.
m^{2} (square meter)
A square meter is the SI unit of area equal in size to a
square 1 m x 1 m, although it can be any shape of equal area.
1 m^{2} = 10^{4}
cm^{2}= 10.8 ft^{2}=1.2
square yards. A calculator for various arearelated units
can be found here.
m^{3} (cubic meter)
The cubic meter (m^{3}) is
the SI unit of volume equal in size to a cube 1 m x 1 m x
1 m, although it can be any shape of equal volume. It equals
10^{6} cm^{3},
1000 liters, 35.3 ft^{3}, 1.31
yd^{3}, 264 U.S. liquid gallons,
or 220 British Imperial gallons. A calculator for various
volumerelated units can be found here.
mb
The millibar (mb) is a unit of atmospheric pressure equal
to 0.001 bar, 100 pascals, 1000 dynes/cm^{2},
0.0295 inches (0.75 millimeters) of mercury or 0.0145 lb/in^{2}.
A millibar is equal to a hectopascal(hPa) and is often used
by meteorologists. A calculator for various pressurerelated
units can be found here.
mcd
A millicandela (mcd) is a unit of light intensity equal to
0.001 candela. The intensity of lightemitting diodes (LEDs)
is often stated in millicandelas. A calculator for various
lightrelated units can be found here.
An article with detailed information on such lighting units
and their application for reef aquarists can be found here.
mEq or meq
A milliequivalent (mEq) is a unit of the relative amount
of a substance or a collection of related substances with
similar reactivity. One mEq equals 0.001 equivalent.
meq/L
A milliequivalent per liter (meq/L) is a unit of the concentration
of a substance or collection of substances. It is equal to
one milliequivalent dissolved in 1 L of fluid, or 0.001 equivalents
dissolved in 1 L (eq/L). Aquarists most often encounter it
when measuring alkalinity,
which is a little unusual in that alkalinity is not a measure
of a single substance in solution, but rather the aggregate
of several reactive species. Alkalinity is a measure of the
amount of acid necessary to reduce the pH of water to the
point where all carbonate and bicarbonate have been converted
into carbonic acid (about pH 4.2 in seawater). In seawater,
nearly all of the alkalinity is provided by bicarbonate and
carbonate, although some is contributed by other ions such
as borate. Consequently, alkalinity can be used by reef aquarists
as a surrogate measure for bicarbonate and carbonate which
are taken up by calcifying organisms. The units of alkalinity
can be meq/L (milliequivalents per liter), dKH (degrees of
carbonate hardness) or ppm (meaning ppm of calcium carbonate
equivalents). 1 meq/L = 2.8 dKH = 50 ppm CaCO_{3}
equivalents. Seawater has an alkalinity of about 2.5 meq/L
= 7 dKH = 125 ppm CaCO_{3}
equivalents. An article explaining in detail what alkalinity
is and what its units mean is online here.
A calculator for converting between different alkalinity units
is online here. A calculator for determining
how much of different supplements to add to boost alkalinity,
as well as calcium and magnesium, is online here.
meq/L/pH unit (buffering capacity; b)
The buffering capacity (meq/L/pH unit), often labeled b,
is a measure of how strongly buffered an aqueous solution
is. The buffering capacity of solutions such as seawater changes
with pH. The units can be understood as the amount of acid
(or base) in meq/L necessary to change the pH by 1 pH unit.
n seawater at a given pH, the higher the alkalinity,
the higher the buffering capacity. Details on the buffering
capacity definition and what values it has at different pH
values in seawater due to borate, carbonate and bicarbonate
is online here.
meter
See m.
mg
The milligram (mg) is a unit of mass equal to 0.001 gram,
1000 micrograms (µg), 0.0154 grains or 35.3 x 10^{6}
ounce. A calculator for various weightrelated units can
be found here.
mg/kg(milligram per kilogram; mpk)
mg/kg is the symbol for milligram per kilogram, a unit often
used in medicine to measure dosage rates. 1 mg/kg is equivalent
to 10^{6} g/g or 1 part per million. For chemical
concentrations in seawater, where 1L weighs approximately
1.023 kg, 1 mg/kg is approximately 1.023 mg/L. mg/kg and ppm
are units of measure often used for chemicals in aquaria.
mho
mho is an old name for the unit of electrical conductivity,
the siemen. It is the reciprocal of the ohm (mho is ohm spelled
backwards).
mE
(microEinstein)
A microeinstein is one millionth of an Einstein, or 10^{6}
einstein. The microeinstein is a unit of light energy. One
microeinstein is the energy carried by 6.02 x 10^{17}
photons (10^{6} moles of photons).
Light has frequency v (in hertz) and wavelength l
(in meters). One microeinstein is then equal to (4.0 x 10^{16})v
or (1.2 x 10^{7})/l
joules per mole (J/mol). The unit is named for the physicist
Albert Einstein (18791955). A calculator for various lightrelated
units can be found here. An
article with detailed information on such lighting units and
their application to reef aquaria can be found here.
mE/s/m^{2}
(microEinstein/m^{2}/second;
PAR; PPFD)
A microeinstein per second per square meter (mE/s/m^{2}
or mE/s*/m^{2})
is a unit of light energy per second landing on (or passing
through) a given area (Figure 2). Photosynthetically active
radiation (that is, light in the 400 nm – 700 nm wavelength
range; PAR) can be given in energy terms as PAR irradiance,
which is the total energy in the 400700 nm range. In that
case, the units of PAR are watt/m^{2}.
It can also be measured in terms of photons. In that case,
PAR is called Photosynthetic Photon Flux Density (PPFD).
PPFD is a measure of the actual number of photons in the 400
nm to 700 nm wavelength range that are incident per time unit
on a surface unit. When expressed in PPFD, all photons are
equivalent regardless of their energy and consequently, PPFD
is measured in microEinstein/m^{2}/second.
A calculator for various lightrelated units can be found
here.
An article with detailed information on such lighting units
and their application for reef aquarists can be found here.
Figure 2. These green star polyps have lost their
color due to inadequate mE/s/m^{2}.

µg (microgram; mcg)
µg is the symbol for the microgram, which is a unit of mass
equal to 0.001 milligram (mg) or one millionth of a gram.
A calculator for various weightrelated units can be found
here.
µL
(microliter; rarely mcl or mcL)
µL is the symbol for the microliter, which is a unit of volume
equal to 0.001 milliliter or 1 cubic millimeter (mm^{3}).
Chemists often measure very small volumes in microliters. A
calculator for various volumerelated units can be found here.
µm
(micrometer; micron)
µm is a symbol for the micrometer, which is a unit of distance
equal to 0.001 millimeter. The name micron is also used for
this unit. A calculator for various lengthrelated units can
be found here.
µM (micromolar, micromole/L)
Micromolar is a unit of concentration equal to 1 micromole
dissolved in 1 L of fluid. It is also equal to 10^{6}
molar. It is a very common unit of concentration in chemistry,
chemical oceanography and biochemistry.
micron
See µm.
µS/cm (microSiemen per centimeter)
Microsiemens per cm (mS/cm) is a unit of conductivity
equal to 10^{6} siemens/cm
or 10^{3} mS/cm. It is often
used in aquaria to measure the “purity” of fresh water via
conductivity. It is especially useful in determining whether
reverse osmosis (RO) and deionizing (DI) water purification
systems are functioning properly. In this case, an RO unit
ought to drop the conductivity by a factor of 10 to 20 or
more, and a DI unit ought to drop the conductivity to 01
µS/cm. A solution's conductivity is normally quoted at 25ºC;
most conductivity meters make this correction automatically. This
linked article
describes the various conductivity scales, how it is measured,
and how to use it for fresh water purification systems such
as RO/DI.
mil
A mil is a unit of distance equal to 0.001 inch, or a "milliinch."
Mils are used primarily in the United States for small tolerances
in engineering work. One mil is 25.4 microns. A calculator
for various lengthrelated units can be found here.
millibar
See mb.
millicandela
See mcd.
milliequivalent
See meq.
milliequivalents per liter
See meq/L.
milliequivalents per L per pH unit
See meq/L/pH unit.
milligram
See mg.
milligram per kilogram
see mg/kg.
milligram per liter
See mg/L.
milliliter
See mL.
millilux (mlx)
Millilux is a unit of illumination equal to 0.001 lux. Typical
illumination outside at night is measured in millilux. A calculator
for various lightrelated units can be found here.
An article with detailed information on such lighting units
and their application for reef aquarists can be found here.
millimeter
See mm. A calculator for various lengthrelated
units can be found here.
millimeter of mercury
See mm Hg.
mg/L (milligram per liter)
Milligram per L is a unit of concentration often used for
chemicals in aquaria. For chemical concentrations in freshwater,
1 mg/L is approximately 1 ppm (= 1 mg/kg). For chemical concentrations
in seawater, where 1 L weighs approximately 1.023 kg, 1 mg/L
is approximately 0.978 ppm (= 0.978 mg/kg).
mpk
See mg/kg.
mL (milliliter; ml)
The milliliter is a metric unit of volume. One milliliter
equals 0.001 liter, one cubic centimeter (cm^{3}
or cc) or 0.061 cubic inch. The milliliter is normally used
for measuring the volumes of liquids. Solids are normally
measured in cubic centimeters.
mlx
See millilux.
mM (millimolar; mmol/L; mmol/l)
Millimolar is the SI unit used frequently in science
for concentrations of chemicals in water. 1 mM = 0.001 M.
To convert mM to ppm, multiply the concentration in mM by
the molecular weight of the chemical. For calcium, for example,
multiply the concentration in mM by 40 to get ppm.
minute (min; m; ' )
The minute is a unit of time equal to 60 seconds or to 1/60
hour. The SI system specifies min as the symbol for the time
unit and ' as the symbol for the arcminute.
mm
The millimeter (mm) is a unit of distance equal to 0.001
meter, 0.1 centimeter, 0.0394 inch, or 39.4 mils. A calculator
for various lengthrelated units can be found here.
mm Hg
A millimeter of mercury (mm Hg) is a unit of pressure equal
to the pressure exerted by a column of mercury 1 millimeter
high. When a mercury barometer is used, the pressure is read
directly as the height of the mercury column in millimeters.
One millimeter of pressure is equivalent to 0.0394 in Hg,
0.0193 lbf/in^{2}, 1.33 millibars
or 133 pascals. In engineering, the millimeter of mercury
is sometimes replaced by the torr, the two units being nearly
equal. A calculator for various pressurerelated units can
be found here.
mpk
See mg/kg.
molal
See mole/kg.
molar
See M.
mole (mol)
The mole is the SI unit for the amount of a substance. Moles
measure the actual number of atoms or molecules in an object,
rather than the weight that is used by other sorts of units,
such as ppm. It is sometimes called the gram molecular weight,
because one mole of a chemical compound contains the same
number of grams as the molecular weight of that molecule.
The number of atoms or molecules in a mole is called Avogadro's
number after the Italian chemist and physicist Amedeo Avogadro
(17761856). It is approximately 6.02 x 10^{23}.
mole fraction
The mole fraction is a unitless value that represents the
fraction of the total number of moles that is represented
by a single species. The total of all mole fractions in a
single solution must add up to 1.00. For example, a solution
containing one mole of ethanol and nine moles of water is
said to have a 0.1 mole fraction of ethanol and 0.9 of water.
mole/kg
Molal is a concentration unit defined as one mole per kg
of solution. Since 1 L of water weighs close to 1 kg, molal
(mole/kg) and molar (mole/L) are close to the same.
mole/L
See M.
mS/cm (milliSiemen per centimeter)
Millisiemens per cm (mS/cm) is a unit of conductivity equal
to 0.001 siemens/cm. It is often used in aquaria to measure
salinity via the conductivity. The conductivity of a solution
is normally quoted at 25ºC and most conductivity meters make
this correction automatically. Seawater (with a salinity of
35 ppt) has a conductivity of approximately 53 mS/cm. The
concentration of other materials can also be determined this
way, assuming that they are the only substances in solution. Saturated
limewater (calcium hydroxide in fresh water) has a conductivity
of about 10.3 mS/cm at 25ºC. Finally, conductivity is also
very useful in determining whether reverse osmosis and deionizing
water purification systems are functioning properly. This
linked
article details how conductivity can be used to measure
salinity. This linked article describes
in detail the relationship between conductivity and seawater
salinity, and how to make standards.
mV
The millivolt (mV) is a unit of electric potential equal
to 0.001 volt. It is often used in electrical circuits and
in measuring ORP. This linked article
details what ORP is and how it is used in reef aquaria.
N (normal)
Normal is a unit of relative concentration used in chemistry
to describe a solution having a concentration of 1 equivalent
per liter. See “equivalent”.
nanomole
See nmole.
nanomolar (nM; nmole/L)
See nM.
nm
A nanometer (nm) is a unit of distance equal to 10^{9}
meter. The nanometer replaced the millimicron in 1951. One
nanometer equals 0.001 micrometer or 10 angstroms. Individual
molecules are nanometer in scale. A calculator for various
lengthrelated units can be found here.
nmole
A nanomole (nmole) is a unit of amount of substance equal
to 10^{9} mole, or 6.02 x
10^{14} individual molecules
or ions.
nM (nanomolar; nmole/L)
Nanomolar is a unit of concentration equal to 10^{9}
mole per liter.
ohm (W)
The ohm is the SI unit of electric resistance (as well as
other properties such as impedance for alternating currents).
When a conductor connects two locations having different electric
potentials (voltages), a current flows through the conductor.
The amount of the current will depend on the potential difference
(the voltage) and also on the resistance of the conductor.
One ohm is the resistance that requires a potential difference
of one volt to cause one ampere of current to flow. The unit
honors the German physicist Georg Simon Ohm (17871854).
ORP (mV)
ORP stands for the oxidation reduction potential. It is a
measure of water's relative oxidizing and reducing power. In
the aquarium hobby it is normally measured in mV relative
to the standard silver/silver electrode. In other scientific
fields, it can be measured in volts, and may use a different
zero point standard, such as the standard hydrogen electrode
or calomel electrode. This linked article
details what ORP is and how it is used in reef aquaria.
osmole (Osm; osmolarity)
The osmole (Osm) is a unit of osmotic pressure (see below).
One osmole is the osmotic pressure of a one molar solution
(that is, a solution with a concentration of one mole per
liter of solvent) of a substance that does not dissociate,
such as sugar in water. Osmotic pressure depends on the total
number of dissolved particles, so for a substance that dissociates
into two ions, such as sodium chloride, a one molar solution
has an osmotic pressure of 2 osmoles. Likewise, a 1 M solution
of calcium chloride (CaCl2) has an osmotic pressure of 3 osmoles.
Typical values range from 0.02 Osm for fresh water to 1.01
Osm for seawater.
osmotic Pressure
The osmotic pressure is the pressure difference that exists
between solutions on either side of a semipermeable membrane.
Osmotic pressure is caused by water's tendency to flow from
higher salt concentrations to lower salt concentrations. Every
100 ppm (mg/L) of TDS (total dissolved solids) produces about
one pound per square inch (psi) of osmotic pressure. Osmotic
pressure is overcome by tap or pumped water pressure in the
reverse osmosis process.
ounce (oz; oz av)
The ounce commonly used as a unit of weight in the United
States is the avoirdupois ounce. It is 1/16 pound, 28.35 grams, 0.91
troy ounce or 437.5 grains. A calculator for various weightrelated
units can be found here.
ounce (oz or fl oz)
The fluid ounce (fl oz) is a unit of liquid volume. See fluid
ounce. A calculator for various volumerelated units can be
found here.
ounce per gallon (oz/gal)
The ounce per gallon is a unit of mass concentration. One
ounce per U.S. gallon equals 7.49 grams per liter (g/L). In
Britain, 1 ounce per imperial gallon is equal to 6.24 grams
per liter (g/L).
Pa (pascal)
The pascal (Pa) is the unit of pressure in the SI and MKS
systems. It is equal to one newton per square meter or one
kilogram per meter per second per second. It also equals 0.000
145 pounds per square inch, 0.021 lbf/ft^{2}
or 0.007 5 mm Hg. Pressure is commonly measured in kilopascals
(kPa), with 1 kPa = 0.145 lbf/in^{2}.
A calculator for various pressurerelated units can be found
here.
PAR
See mE/s/m^{2}
(microEinstein/m^{2}/second).
part
The "part" is a unit used in statements of proportion
or in recipes for mixtures. The fraction of an ingredient
present is the number of parts of that ingredient divided
by the total number of parts present. For example, hydrochloric
acid diluted to clean calcium carbonate from pump impellers
can be made by mixing one part of acid into 10 parts of water.
In this instance, the implication is that the volume of acid
used is one tenth the volume of water, but that the users
can mix the ingredients on any convenient scale. The unit
"part" can apply to weight (mass) or volume. It
can be ambiguous if it isn't made clear whether the part refers
to weight or volume. For example, "Mix 3 parts water
with 1 part gasoline" is ambiguous, and can mean either
by weight or volume. Typically, the difference is small unless
the densities are substantially different.
parts per billion
See ppb.
parts per million
See ppm.
parts per thousand
See ppt.
parts per trillion (pptr, ppt)
Parts per trillion (ppt or pptr) is a unit of proportion
equal to 10^{12}. It is equal
to 10^{9} g/kg, and is close
to 10^{9} g/L (in fresh water;
in seawater, it equals about 1.023 x 10^{9}
g/L). It is also equal to 0.001 ppb. It is sometimes used
to measure the concentrations of trace elements in reef aquaria.
A calculator for various weightrelated units can be found
here.
percent relative humidity
The relative humidity is a measure of the amount of gaseous
water in the air as a percentage of total moisture that the
air can hold at a given temperature. It can vary from 0% to
100%.
percent solution
A “percent solution” refers to a solution made by dissolving
the given weight percentage of a chemical into a solution
(usually water). For example, a ten percent sodium chloride
solution is made by dissolving 10 grams of sodium chloride
into 90 grams of water.
pH
pH is a logarithmic, hence unitless measure used to quantify
how acidic or basic a solution is. Acidic solutions have more
H^{+} (hydrogen ions), and
pH is a measure of these ions. Specifically, the pH of a solution
is defined to be the negative logarithm of the concentration
(or even more precisely, the activity) of H^{+},
measured in moles per liter. Lower pH readings correspond
to greater acidity. The scale is open ended, but typically
runs from about 1 to 15. A neutral solution, such as pure
water, has a pH of 7. Lowering the pH by 1 unit means multiplying
the hydrogen ion concentration by a factor of 10. Seawater
with a pH of 8.2 has just under one tenth the amount of H^{+}
in solution than does pure fresh water. This linked
article describes how to measure pH with a meter, and
this linked article describes
how to buy and use pH calibration solutions. This linked
article describes the relationship between alkalinity
and pH, and these describe how to deal with excessively high and low pH in reef aquaria.
pint (pt)
The pint is a unit of volume equal to 1/2 quart. The U. S.
liquid pint equals 28.9 cubic inches, 16 fluid ounces, or
473.2 milliliters. The U. S. dry pint equals 33.6 cubic inches,
or 550.6 milliliters. The British Imperial pint equals 20
British fluid ounces, 34.7 cubic inches or 568 milliliters.
A calculator for various volumerelated units can be found
here.
pound
See lb.
pounds per cubic foot
See lbs/cu ft.
pounds per square foot
See lbs/sq ft.
power factor
The power factor is the ratio of an electrical device’s actual
power used in watts to the apparent power measured in voltamps
(VA). For example, a 400 W appliance with a power factor of
0.8 would require a power source of 500 VA to drive it properly.
Since the power factor is a ratio, it does not have units.
The power factor ranges from 0 to 1.
ppb (parts per billion)
ppb is a unit of proportion equal to 10^{9}.
It is equal to 10^{6} g/kg,
and is close to 10^{6} g/L
(in fresh water; in seawater, it equals about 1.023 x 10^{6}
g/L). It is also equal to 0.001 ppm. It is often used to measure
the concentrations of minor or trace elements in reef aquaria. A
calculator for various weightrelated units can be found here.
PPFD
See mE/s*m^{2}.
ppm (part per million)
ppm is a unit of proportion equal to 10^{6}.
It is equal to 10^{3} g/kg
and 1 mg/kg, and is close to 10^{3}
g/L or 1 mg/L (in fresh water; in seawater, it equals about
1.023 x 10^{3} g/L). It is
often used to measure the concentrations of different species
in reef aquaria. A calculator for various weightrelated units
can be found here.
ppm calcium carbonate equivalents
ppm calcium carbonate (CaCO_{3})
equivalents is an ambiguous unit used for a variety of measures
by reef aquarists, including alkalinity,
calcium,
magnesium
and total hardness. In the case of alkalinity and calcium,
the unit refers to the amount (in ppm) of calcium carbonate
that would have to dissolve into pure water to give the same
calcium concentration or alkalinity (even if that would be
impossible to accomplish). For calcium, 1000 ppm calcium carbonate
equivalents equals 400 ppm calcium ion. For alkalinity, 100
ppm calcium carbonate equivalents equals 2 meq/L or 5.6 dKH. In
the case of magnesium, the concentration refers to the amount
of calcium carbonate that would have to dissolve to provide
the same number of calcium ions as magnesium ions are present. For
magnesium, 1000 ppm calcium carbonate equivalents equals 243
ppm magnesium ion. In the case of total hardness, the unit
refers to the amount of calcium carbonate that would have
to dissolve to provide the same number of calcium ions as
the total of calcium and magnesium in solution. Using ppm
calcium carbonate equivalents for magnesium and total hardness
is poor practice as they are hard for most aquarists to understand,
but they are used for historical reasons by certain testing
companies (e.g., Hach, Figure 3). A calculator for converting
between different alkalinity units is online here. A
calculator for determining how much of different supplements
to add to boost alkalinity, as well as calcium and magnesium,
is online here.
Figure 3. This Hach "Total Hardness and
Calcium" test kits gives magnesium in the ridiculous
units of ppm calcium carbonate equivalents.

ppmC (part per million carbon)
ppmC is a way to measure total carbon atoms in solution.
It is equal to the ppm concentration of carbon atoms, regardless
of what chemical form they take. It is often used to describe
the concentration of dissolved inorganic carbon, dissolved
organic matter (DOM),
particulate organic
matter (POM), or total organic matter (TOM)
in seawater. It can also be used to quantify the concentration
of specific carboncontaining species, such as ethanol (CH_{3}CH_{2}OH),
but that usage is unusual.
ppmN (part per million nitrogen)
also ppm nitrateN, ppm nitriteN, ppmammoniaN
ppmN is a way to measure nitrogen atoms in solution. It
is equal to the ppm concentration of nitrogen atoms, and usually
specifies what form they take. It is often used, for example,
to describe the concentration of ammonia, nitrite,
or nitrate
ions in aquaria. It can also be used to describe the amount
of nitrogen in dissolved organic matter (DOM),
particulate organic
matter (POM), or total organic matter (TOM)
in seawater. In these cases, the results are reported as “nitrate
= 12 ppmN" or “12 ppm nitrateN,” which means that there
is 12 ppm of nitrogen in the solution is in the nitrate form. In
order to convert ppmN to ppm, divide the ppmN value by the
fraction of weight of nitrogen in the ion. The table below
shows these corrections:
Species

ppmN

ppm

NH_{3}
(ammonia)

1.00

1.21

NH_{4}^{+}
(ammonium)

1.00

1.29

NO_{2}^{}
(nitrite)

1.00

3.29

NO_{3}^{}
(nitrate)

1.00

4.43


ppmP (part per million phosphorus)
ppmP is a way to measure total phosphorus
atoms in solution. It is equal to the ppm concentration of
phosphorus atoms, regardless of what chemical form they take. It
is often used to describe the concentrations of phosphorus
in dissolved inorganic phosphate, dissolved organic matter (DOM),
particulate organic
matter (POM), or total organic matter (TOM)
in seawater.
ppm TDS
(part per million total dissolved solids)
ppm total dissolved solids is a measure, in parts per million
(e.g., mg/kg), of the materials fully dissolved in water.
The values can be determined by evaporating the water and
determining the weight of what remains. This method gives
true total dissolved solids. In the aquarium hobby, total
dissolved solids is also determined by the solution's conductivity,
although aquarists may not even realize that is the method
being used. The device is typically a small electronic meter,
often simply called a TDS meter, but it always works by measuring
conductivity. In this case, only conductive ions contribute
to ppm TDS. Neutral organic or inorganic materials (such
as boric acid, B(OH)_{3}) will not be detected by
this method. ppm TDS is most frequently used by marine aquarists
to assess the proper function of their water purification
systems, such as reverse osmosis (RO) and deionization (DI)
systems.
There is no simple conversion between ppm TDS and standard
conductivity units, such as mS/cm and mS/cm.
Actually, several ppm TDS scales are used to convert from
a measured conductivity to ppm TDS, and each of these scales
relates to how much of a particular salt (in ppm) would result
in that same conductivity. The various scales include sodium
chloride, potassium chloride, and one called “442” which is
a mixture of salts designed to mimic natural fresh waters. Each
of these provides a different nonlinear relationship between
the measured conductivity and the ppm TDS reading provided
by the meter, since a salt solution's conductivity is not
linear with its concentration. Because of this nonlinear
relationship, no exact conversion exists for how many ppm
TDS equals 1 mS/cm. The
fact that there is more than one scale, and the fact that
aquarists rarely know what scale they are using, makes ppm
TDS suitable primarily as a relative, rather than absolute,
measure of conductivity. It is in this relative sense that
aquarists are best advised to use the information in assessing
the performance of an RO/DI system.
This linked article
describes the various ppm TDS scales, how TDS is measured,
and how to use it for fresh water purification systems such
as RO/DI.
ppt or ppth (parts per thousand)
ppt (parts per thousand) is a unit of proportion equal to
10^{3}. It is equal to 1 g/kg
and, and is close to 1 g/L(in fresh water; in seawater, it
equals about 1.023 g/L). It is often used to measure the salinity
of seawater, which is about 35 ppt. In that case, it is the
sum of all salts in the water, which equals about 35 grams
per kg of seawater. A calculator for various weightrelated
units can be found here.
ppt (parts per trillion)
ppt (parts per trillion) see parts per trillion.
practical salinity unit
See PSU.
prefixes
Many units can have prefixes which alter the value by a certain
power of 10. For example, a milliliter is 1/1000 (milli) of
a liter. The table below includes a complete list of such
prefixes. The top four and bottom two or three are not frequently
used.
prefix

value

value

yotta (Y)

10^{24}

1 septillion

zetta (Z)

10^{21}

1 sextillion

exa (E)

10^{18}

1 quintillion

peta (P)

10^{15}

1 quadrillion

tera (T)

10^{12}

1 trillion

giga (G)

10^{9}

1 billion

mega (M)

10^{6}

1 million

kilo (k)

10^{3}

1 thousand

hecto (h)

10^{2}

1 hundred

deka (da)**

10

1 ten

deci (d)

10^{1}

1 tenth

centi (c)

10^{2}

1 hundredth

milli (m)

10^{3}

1 thousandth

micro (µ)

10^{6}

1 millionth

nano (n)

10^{9}

1 billionth

pico (p)

10^{12}

1 trillionth

femto (f)

10^{15}

1 quadrillionth

atto (a)

10^{18}

1 quintillionth

zepto (z)

10^{21}

1 sextillionth

yocto (y)

10^{24}

1 septillionth


psi (pounds per square inch; lbf/in^{2})
PSI is a unit of pressure equal to 144 pounds per square
foot (psf), 6.9 kilopascals (kPa), 69 millibars (mb), 2.04
inches of mercury (in Hg), 27.8 inches of water (in WC) or
70.5134 centimeters of water (cm H_{2}O). See also
psig. A calculator for various pressurerelated units can
be found here.
psig (pounds per square inch gauge)
psig is a symbol for a pressure unit used in plumbing. Psi
is a symbol for pound per square inch (see above). psig is
a symbol for pound per square inch gauge; this means that
the pressure has been determined from a gauge which measures
the difference between the pressure of the fluid and the pressure
of the atmosphere. So a pipe containing 25 psig when the atmospheric
pressure is 15 psi contains an absolute pressure of 40 psi. A
calculator for various pressurerelated units can be found
here.
PSU (practical salinity unit)
PSU is a standard measure of the salinity of seawater. The
"unit" is actually a dimensionless ratio obtained
by measuring the water sample's conductivity. Seawater of
salinity 35 PSU (a typical open ocean seawater, similar to
a salinity of 35 ppt) has the same conductivity as a solution
of potassium chloride (KCl) with a concentration of 3.24356
% by weight. For example, a sample of salinity of 1 PSU would
have conductivity 1/35 of that of the standard solution. Using
this definition, it is easy for oceanographers (and reef aquarists)
to determine the salinity of seawater with a conductivity
meter. PSU is often abbreviated as just S.
qt (quart)
The quart (qt) is a unit of volume. It equals exactly 1/4
of a gallon. In the U. S., one liquid quart is 57.75 cubic
inches, 32 fluid ounces or 0.946 liters. In the U. S., one
dry quart is 67.2 cubic inches or 1.1 liters. In the British
imperial system, one quart (liquid or dry) is 69.4 cubic inches,
40 fluid ounces or 1.14 liters. In all cases, the quart equals
2 pints. A calculator for various volumerelated units can
be found here.
refractive index
The refractive index is the ratio of the speed of light in
a vacuum to the speed of light in the medium being measured.
Light is slowed by passing through solids and liquids, so
the index of refraction is an indication of this speed change. Since
the refractive index of solutions of salts in water depends
on the amount of salt present, marine aquarists often use
refractive index as a way to determine the salinity of aquarium
water. Since it is a ratio of two speeds, it does not have
units. This linked
article shows the relationship between refractive index
and salinity for seawater, and how to make standards to calibrate
refractometers.
relative humidity
See percent relative humidity.
rpm (revolution per minute; r/min)
The rpm is a unit of angular velocity. One r/min equals 0.105
radian per second. Aquarists may encounter fans or motors
with rpm ratings.
s (second; sec; " )
The second (s) is a fundamental unit of time in all standard
measuring systems. The definition of the second has evolved
through time. In 1967, scientists agreed to define the second
based on the frequency of radiation emitted by atoms of cesium133.
Salinity
The salinity of seawater is often quoted in ppt (parts per
thousand). Natural seawater has a salinity of about 35 ppt,
implying that there are about 35 grams of salt in a liter
of seawater. See also PSU, practical salinity units. This
linked
article describes in more detail the relationship between
the three main properties used to measure salinity in reef
aquaria (refractive index via a refractometer, specific gravity
via a hydrometer and conductivity via a conductivity probe),
and how to make standards for each.
S/cm (Siemen per centimeter)
Siemens per cm (S/cm) is a unit of conductivity equal to
1000 millisiemens/cm and 10^{6 }mS/cm.
It is not often used in aquaria directly as it is much larger
than typical aqueous conductivity. Seawater with a salinity
of 35 ppt, for example, has a conductivity of approximately
53 mS/cm. This linked
article details how conductivity can be used to measure
salinity. This linked article describes
in detail the relationship between conductivity and seawater
salinity, and how to make standards.
second
See s.
SI Units
The International System of Units (SI units) SI is a system
that bases all units of measure on a common set of fundamental
measures. These are:
ampere for electric current
candela for light intensity
kelvin for temperature
kilogram for mass
meter for distance
mole for amount of substance
second for time
mole for amount of substance
The SI system allows certain combination and special units
to go by their own name. These are:
angles (degree, arcminute, and arcsecond)
astronomical unit
atomic mass unit (dalton)
degree Celsius for everyday temperature measurement
electricity: the coulomb (charge), volt (potential), farad
(capacitance), ohm (resistance), electronvolt
gray and sievert for radiation dose
hertz for frequency of regular events and the becquerel
for rates of radioactivity and other random events
joule for energy and watt for power
katal for catalytic activity in biochemistry
lumen for light flux and the lux for illuminance
liter for volume
magnetism: the weber (flux), tesla (flux density), and henry
(inductance) units bel and neper (and their multiples, e.g.,
decibel)
newton for force and pascal for pressure
radian and steradian for plane and solid angles
siemens (conductance)
time: (minute, hour, day, and year)
tonne (metric ton) for large masses
specific gravity
Specific gravity is a unitless measure that aquarists often
use to measure the salinity of the water in marine aquaria.
Specific gravity is the ratio of the density of the sample
to the density of pure water at the same temperature (or some
other specified temperature). Adding salts to fresh water
increases its density, so specific gravity rises as salinity
increases. While seawater's density changes considerably with
temperature, specific gravity does not change much because
the density of fresh water is changing in a similar fashion.
The specific gravity of natural seawater with a salinity of
35 ppt is on the order of 1.0264, while the density is about
1.023 g/cm^{3}.
Specific gravity is most often measured with a hydrometer.
This linked
article details how specific gravity is determined, and
explains issues around different temperature standards. This
linked article describes
how to make temperature corrections for hydrometers.
spectral irradiance
See watt/m^{2}.
square centimeter
See cm^{2}.
square foot
See ft^{2}.
square inch
See in^{2}.
square meter
See m^{2}.
sr (steradian)
The steradian (sr) is the standard unit of measure of solid
angles. There are 4p
steradians in a sphere. One steradian equals 0.080 sphere.
There are 129 600/p
= 41, 253 square degrees in a sphere, so 1 steradian also
equals approximately 3283 square degrees.
tablespoon (tbsp, tblsp or Tsp)
The tablespoon is a unit of volume. In the U.S. the tablespoon
equals 1/2 fluid ounce, or 14.8 milliliters. In presentday
Britain, Canada, and New Zealand, the tablespoon is 15 mL.
It is 20 milliliters in Australia. The U.S. tablespoon is
equal to 3 teaspoons or 1/16 cup. A calculator for various
volumerelated units can be found here.
TDS
See ppm TDS.
teaspoon (tsp)
The teaspoon is a unit of volume. The U.S. teaspoon is equal
to 1/3 tablespoon, 1/48 cup, 1/6 fluid ounce, 0.30 cubic inches
or 4.9 milliliters. In Britain, Canada, Australia, and New
Zealand, a metric teaspoonful is 5.0 milliliters. A calculator
for various volumerelated units can be found here.
Tsp
See tablespoon. The capital 'T' can imply
tablespoon while tsp (with a lowercase 't') means teaspoon.
torr (Torr)
The torr is a unit of pressure equal to 1/760 atmosphere,
1.33 millibars, 133 pascals or 0.019 pound per square inch
(psi). The pressure of 1 atmosphere is approximately equal
to the pressure of a column of mercury 760 millimeters high.
Consequently, 1 torr is similar to 1 mm Hg. A calculator for
various pressurerelated units can be found here.
transmission
Transmission is a unit sometimes used to measure the penetration
of light through an object. Transmission is the fraction of
light that passes through the object. Transmission ranges
from zero (no light penetrates) to 1 (all incident light penetrates). Fifty
percent light penetration implies a transmission of 0.5. More
frequently, scientists use a logarithmic unit for light penetration,
which is called absorbance (see absorbance).
V (volt)
The volt (V) is the SI unit of electric potential. One volt
represents a potential of one joule per coulomb of charge.
VA (volt ampere)
The volt ampere (VA) is a unit of electrical load. The product
of the potential (in volts) and the actual current (in amperes)
is the load, in volt amperes.
W (watt)
The watt is the SI unit of power. Power is the rate at which
energy is used. One watt is equal to one joule
of energy per second, 0.0013 horsepower (hp) or 0.738footpound
per second (lbf/s). One watt is also the power produced by
a current of one ampere flowing across an electric potential
of one volt.
watt/m^{2}
Watts per square meter is a unit of energy per unit area.
It is sometimes used in lighting analysis to determine how
much light energy is landing on (or passing through) a surface.
In that context it is sometimes called spectral irradiance
or PAR irradiance. A calculator for various lightrelated
units can be found here. An
article with detailed information on such lighting units and
their application to reef aquaria can be found here.
Wh (watt hour)
The watt hour is a unit of work or energy. It is the total
energy delivered at a rate of one watt for one hour. A 100
W light bulb lit for one hour consumes 100 watt hours of energy.
One watt hour equals 3.6 kilojoules (kJ), 3.4 Btu, 0.86 large
Calories (kcal) or 2655 foot pounds. A calculator for various
energyrelated units can be found here.
wavelength
Wavelength is a unit of distance equal to the length of a
wave including water waves, light waves and radio waves.
For light waves, common units for wavelength are nanometers
or Angstroms. A calculator for various lengthrelated units
can be found here.
weight percent (w/w; weight to weight; g/g)
Weight percent and w/w (weight to weight) are descriptions
used in chemical recipes for making a solution that contains
2 percent of the total weight of a particular ingredient. For
example, a 2 weight percent salt solution contains 2 grams
of salt for every 98 grams of water. It can also be written
as g/g when using grams. A calculator for various weightrelated
units can be found here.
w/w
see weight percent. A calculator for various
weightrelated units can be found here.
