Salinity - Wikipedia, the free encyclopedia
From Wikipedia, the free encyclopedia
Salinity is the saltiness or dissolved salt content of a body of water. Salinity in Australian English and North American English may refer to salt in soil
(see soil salination).
1 Definition
2 Systems of classification of water bodies based upon salinity
3 Environmental considerations
4 See also
5 References
Page 1 of 3
Annual mean sea surface
salinity for the World Ocean.
Data from the World Ocean
Atlas 2001
(http://www.nodc.noaa.gov/OC5/WOA01/).
Water salinity based on percentage of dissolved salts
Fresh water Brackish water Saline water Brine
< 0.05 % 0.05 - 3 % 3 - 5 % > 5 %
The salt content of most natural lakes, rivers, and streams is so small that these waters are termed fresh or even sweet water. The actual amount of salt in fresh water is, by definition, less than 0.05%. Otherwise, the water is regarded as brackish, or defined as saline if it contains 3 to 5% salt by weight. At well over 5% it is considered brine. The ocean is naturally saline at approximately 3.5% salt (see sea water). Some inland salt lakes or seas are even saltier. The Dead Sea, for example, has a surface water salt content of around 30%
[1]
.
The technical term for saltiness in the ocean is halinity, from the fact that halides—chloride specifically—are the most abundant anion in the mix of dissolved elements. In oceanography, it has been traditional to express halinity not as percent, but as parts per thousand (ppt or ‰), which is approximately grams of salt per liter of solution. Prior to 1978, salinity or halinity was expressed as ‰ usually based on the electrical conductivity ratio of the sample to "Copenhagen water", an artificial sea water manufactured to serve as a world "standard"
[2]
. In 1978, oceanographers redefined salinity in Practical Salinity Units (psu): the conductivity ratio of a sea water sample to a standard KCl solution have no units, so it is not the case that 35 psu exactly equals 35 grams of salt per litre of solution
[5]
.
[3][4]
. Ratios
These seemingly esoteric approaches to measuring and reporting salt concentrations may appear to obscure their practical use; but it must be remembered that salinity is the sum weight of many different elements within a given volume of water. It has always been the case that to get a precise salinity as a concentration and convert this to an amount of substance (sodium chloride, for instance) required knowing much more about the sample and the measurement than just the weight of the solids upon evaporation (one method of determining "salinity"). For example, volume is influenced by water temperature; and the composition of the salts is not a constant (although generally very much the same throughout the world ocean). Saline waters from inland seas can have a composition that differs from that of the ocean.
For the latter reason, these waters are termed saline as differentiated from ocean waters, where the term haline applies
(although is not universally used).
Marine waters are those of the ocean, another term for which is euhaline seas.
The salinity range for euhaline seas is 30 to 35 ‰. Brackish seas or waters have salinity in the range of 0.5 to 29‰ and metahaline seas from 36 to 40‰. http://en.wikipedia.org/wiki/Salinity
>300‰
THALASSIC SERIES
--------------------
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Salinity - Wikipedia, the free encyclopedia
These waters are all regarded as thalassic because their salinity is derived from the ocean and defined as homoiohaline if salinity does not vary much over time (essentially invariant). The table on the right, modified from Por (1972)
[6]
, follows the "Venice system" (1959)
[7]
.
In contrast to homoiohaline environments are certain poikilohaline environments (which may also be thallassic) in which the salinity variation is biologically significant
[8]
. Poikilohaline waters may range anywhere from
0.5‰ to greater than 300‰. The important characteristic is that these waters tend to vary in salinity over some biologically meaningful range seasonally or on some other roughly comparable time scale. Put simply, these are bodies of water with quite variable salinity.
Highly saline water, from which salts crystallize (or are about to), is referred to as brine.
60 - 80‰
40‰
30‰
18‰
5‰
0.5‰
Page 2 of 3 hyperhaline
-------------------metahaline
-------------------mixoeuhaline
-------------------polyhaline
-------------------mesohaline
-------------------oligohaline
--------------------
Salinity is an ecological factor of considerable importance, influencing the types of organisms that live in a body of water. As well, salinity influences the kinds of plants that will grow either in a water body, or on land fed by a water (or by a groundwater). A plant adapted to saline conditions is called a halophyte. Organisms (mostly bacteria) that can live in very salty conditions are classified as extremophiles, halophiles specifically. An organism that can withstand a wide range of salinities is euryhaline.
Salt is difficult to remove from water, and salt content is an important factor in water use (such as potability).
Desalination
Fresh water
Seawater
Soil salination
1. ^ Goetz, P. W. (ed.): "The New Encyclopaedia Britannica (15th edn)", Vol. 3, p.937, Encyclopaedia Britannica
Inc., Chicago, 1986.
2. ^ Lewis, E.L. (1980). The Practical Salinity Scale 1978 and its antecedents. IEEE J. Ocean. Eng., OE-5(1): 3-8.
3. ^ Unesco (1981a). The Practical Salinity Scale 1978 and the International Equation of State of Seawater 1980.
Tech. Pap. Mar. Sci., 36: 25 pp.
4. ^ Unesco (1981b). Background papers and supporting data on the Practical Salinity Scale 1978. Tech. Pap. Mar.
Sci., 37: 144 pp.
5. ^ Unesco (1985). The International System of Units (SI) in Oceanography. Tech. Pap. Mar. Sci., 45: 124 pp.
6. ^ Por, F. D. (1972). Hydrobiological notes on the high-salinity waters of the Sinai Peninsula. Mar. Biol., 14(2):
111–119.
7. ^ Venice system (1959). Final resolution of the symposium on the classification of brackish waters. Archo
Oceanogr. Limnol., 11 (suppl): 243–248.
8. ^ Dahl, E. (1956). Ecological salinity boundaries in poikilohaline waters. Oikos, 7(I): 1–21.
Mantyla, A.W. 1987. Standard Seawater Comparisons updated. J. Phys. Ocean., 17: 543-548.
Retrieved from "http://en.wikipedia.org/wiki/Salinity" http://en.wikipedia.org/wiki/Salinity 3/7/2007

Seawater - Wikipedia, the free encyclopedia
From Wikipedia, the free encyclopedia
Seawater is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of ~3.5%, or 35 parts per thousand. This means that every 1 kg of seawater has approximately 35 grams of salts (mostly, but not entirely, sodium chloride) dissolved in it. The average density of seawater at the surface of the ocean is 1.025 g/mL; seawater is denser than fresh water
(which reaches a maximum density of 1.000 g/mL at a temperature of 4°C) because of the added weight of the salts and electrostriction
[1]
.
1 Explanation
2 Compositional differences from fresh water
3 Geochemical explanations
4 Potability
5 Temporary/emergency potability
6 Seawater for flushing toilet
7 See also
8 References:
Page 1 of 4
Annual mean sea surface salinity for the World Ocean.
Data from the World Ocean
Atlas 2001
(http://www.nodc.noaa.gov/OC5/WOA01/).
Although the vast majority of seawater has a salinity of between 3.1 and 3.8%, seawater is not uniformly saline throughout the world. Where mixing occurs with fresh water runoff from river mouths or near melting glaciers, seawater can be substantially less saline. The most saline open sea is the Red Sea, where high rates of evaporation, low precipitation and river inflow, and confined circulation result in the formation of unusually salty seawater. The salinity in isolated seas and salt-water lakes (for example, the Dead Sea) can be considerably greater.
The density of surface seawater ranges from about 1020 to 1029 kg·m
-3
, depending on the temperature and salinity. Deep in the ocean, under high pressure, seawater can reach a density of 1050 kg·m
-3
or higher. Seawater pH is limited to the range 7.5 to 8.4. The speed of sound in seawater is about 1500 m·s
-1
, and varies with water temperature and pressure.
Seawater is more enriched in dissolved ions of all types than fresh water.
[2]
However, the ratios of various solutes differ dramatically. For instance, although seawater is ~2.8 times more enriched with bicarbonate than river water based on molarity, the percentage of bicarbonate in seawater as a ratio of all dissolved ions is far lower than in river water; bicarbonate ions constitute 48% of river water solutes, but only 0.41% of all seawater ions.
[3]
,
[4]
Differences like these are due to the varying residence times of seawater solutes; sodium and chlorine have very long residence times, while calcium (vital for carbonate formation) tends to precipitate out much more quickly.
[5]
Scientific theories behind the origins of sea salt started with Sir Edmond Halley in 1715, who proposed that salt and other minerals were carried into the sea by rivers, having been leached out of the ground by rainfall runoff. Upon reaching the ocean, these salts would be retained and concentrated as the http://en.wikipedia.org/wiki/Seawater
Elemental composition of Earth's ocean water (by mass)
Element Percent Element Percent
Oxygen 85.84
Sulfur 0.091
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Seawater - Wikipedia, the free encyclopedia Page 2 of 4 process of evaporation (see Hydrologic cycle) removed the water. Halley noted
Hydrogen 10.82
Calcium 0.04
that of the small number of lakes in the world without ocean outlets (such as the
Dead Sea and the Caspian Sea, see
Chlorine 1.94
Sodium 1.08
Potassium 0.04
Bromine 0.0067
Magnesium 0.1292
Carbon 0.0028
endorheic basin), most have high salt content. Halley termed this process
"continental weathering".
Chemical composition of sea salt
Halley's theory is partly correct. In addition, sodium was leached out of the ocean floor when the oceans first formed. The presence of the other dominant ion of salt, chloride, results from "outgassing" of chloride (as hydrochloric acid) with other gases from Earth's interior via volcanos and hydrothermal vents. The sodium and chloride ions subsequently became the most abundant constituents of sea salt.
Ocean salinity has been stable for billions of years, most likely as a consequence of a chemical/tectonic system which removes as much salt as is deposited; for instance, sodium and chloride sinks include evaporite deposits, pore water burial, and reactions with seafloor basalts
[6]
Since the ocean's creation, sodium is no longer leached out of the ocean floor, but instead is captured in sedimentary layers covering the bed of the ocean. One theory is that plate tectonics result in salt being forced under the continental land masses, where it is again slowly leached to the surface.
Even on a ship or island in the middle of the ocean, there can be a "shortage of water" meaning, of course, a shortage of fresh water. This is described famously by a line from Samuel Taylor
Coleridge's The Rime of the Ancient Mariner:
"Water, water, every where
Nor any drop to drink."
Seawater can be turned into drinkable (potable) water by one of a number of desalination processes, or by diluting it with fresh water to reduce the salinity. Almost all ocean-going vessels create fresh water from salt with reverse osmosis.
Otherwise, seawater should not be drunk because of its high dissolved mineral content. In the long run, more water must be expended to eliminate these minerals (through excretion in urine) than is gained from drinking the seawater itself.
C
T
Br
-
B
T
Sr
2+
F
-
Total Molar Composition of Seawater (Salinity =
35)[1]
(http://cdiac.esd.ornl.gov/ftp/cdiac74/chapter5.pdf)
Component Concentration (mol . kg
-1
)
H
2
O
Cl
-
Na
+
Mg
2+
SO
4
2-
Ca
2+
K
+
53.6
0.546
0.469
0.0528
0.0283
0.0103
0.0102
0.00206
0.000844
0.000416
0.000091
0.000068
Some information in this article or section has not been verified and may not be reliable.
Please check for inaccuracies, and modify and cite sources as needed.
In extreme emergencies at sea, one is better off drinking urine, even untreated urine, to conserve fresh water in situations where one may run out. Any detrimental effects of drinking urine are much longer term and some authorities claim it is actually beneficial. http://en.wikipedia.org/wiki/Seawater 3/7/2007

Seawater - Wikipedia, the free encyclopedia Page 3 of 4
However, with extreme precautions taken to avoid perspiration and water loss and to retain minerals temporarily in the system, experiments have shown that even exhausting physical activity can continue for extended periods while drinking just over a litre (a quart) of seawater per day (sipped in small amounts) as one's only drinking water supply. Ocean rowing adventurers report fairly reliably (http://www.theoceans.net/news.php?id=832|) that they plan to do this or actually do this as routine practice, but that kidney damage will result from drinking more than that amount. Silver citrate and rain catch can be used to assist in the dilution and fixing of minerals to make it less dangerous, but in general, it requires significant medical expertise to determine the effects of ingestion.
In the only known experiment of ocean rowing across the Atlantic using only plain untreated seawater as the drinking water supply, the subject and experimenter was a British doctor. The experiment did however prove that many people had died of thirst at sea unnecessarily.
Some information in this article or section has not been verified and may not be reliable.
Please check for inaccuracies, and modify and cite sources as needed.
Hong Kong has an extensive use of seawater for flushing toilets territory-wide. More than 90% of toilets in the territory are flushed by seawater as a means of conserving fresh water resources. The development of this approach was started in the 1960s and 1970s when water shortages became a severe problem as the population of the then British colony grew.
This is unusual because saline water cannot be treated (in a waste water treatment plant) by the usual methods. The principal problem with treating marine waste water is the high dissolved sulfate concentration. Within anoxic (oxygen free) environments found in some stages of conventional wastewater treatment sulfate is reduced to the very smelly and toxic gas hydrogen sulfide.
Fresh water
Salinity
Sea ice
Water
1. ^ http://duedall.fit.edu/ocn1010eng/jan27sp.htm
2. ^ http://www.waterencyclopedia.com/Mi-Oc/Ocean-Chemical-Processes.html Thomson Gale, "Ocean Chemical
Processes". Retrieved 12/2/06.
3. ^ Gale.
4. ^ Paul R. Pinet, Invitation to Oceanography, (St. Paul: West Publishing Company, 1996), pp. 126, 134.
5. ^ Pinet, p. 135.
6. ^ Pinet, 133.
Retrieved from "http://en.wikipedia.org/wiki/Seawater"
Categories: Wikipedia articles needing factual verification | Aquatic ecology | Chemical oceanography | Liquid water |
Physical oceanography http://en.wikipedia.org/wiki/Seawater
This page was last modified 09:29, 2 March 2007.
All text is available under the terms of the GNU Free Documentation License.
(See Copyrights for details.)
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Introduction to Physical Oceanography : Chapter 6 - Temperature, Salinity, and Density - Definition of Sal...
Page 1 of 4
(6.1) Definition of Salinity
(6.2) Definition of Temperature
(6.3) Geographical Distribution of Surface
Temperature and Salinity
(6.4) The Oceanic Mixed Layer and Thermocline
(6.5) Density, Potential Temperature, and Neutral
Density
(6.6) Measurement of Temperature
(6.7) Measurement of Conductivity
(6.8) Measurement of Pressure
(6.9) Measurement of Temperature and Salinity with
Depth
(6.10) Light in the Ocean and Absorption of Light
(6.11) Important Concepts
Heat fluxes, evaporation, rain, river in flow, and freezing and melting of sea ice all influence the distribution of temperature and salinity at the ocean's surface. Changes in temperature and salinity can increase or decrease the density of water at the surface, which can lead to convection. If water from the surface sinks into the deeper ocean, it retains a distinctive relationship between temperature and salinity which helps oceanographers track the movement of deep water. In addition, temperature, salinity, and pressure are used to calculate density. The distribution of density inside the ocean is directly related to the distribution of horizontal pressure gradients and ocean currents. For all these reasons, we need to know the distribution of temperature, salinity, and density in the ocean.
Before discussing the distribution of temperature and salinity, let's first define what we mean by the terms, especially salinity.
At the simplest level, salinity is the total amount of dissolved material in grams in one kilogram of sea water. Thus salinity is a dimensionless quantity. It has no units. The variability of dissolved salt is very small, and we must be very careful to define salinity in ways that are accurate and practical. To better understand the need for accuracy, look at
Figure 6.1. Notice that the range of salinity for most of the ocean's water is from 34.60 to 34.80 parts per thousand, which is 200 parts per million. The variability in the deep North Pacific is even smaller, about 20 parts per million. If we want to classify water with different salinity, we need definitions and instruments accurate to about one part per million. Notice that the range of temperature is much larger, about 1°C, and temperature is easier to measure.
Writing a practical definition of salinity that has useful accuracy is difficult (see Lewis, 1980, for the details), and various definitions have been used.
Figure 6.1 Histogram of temperature and salinity of cold water in the oceans. Height is proportional to volume.
Height of highest peak corresponds to a volume of 26 million cubic kilometers per bivariate class of 0.1° C and
0.01psu. From Worthington (1981).
http://oceanworld.tamu.edu/resources/ocng_textbook/chapter06/chapter06_01.htm
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Introduction to Physical Oceanography : Chapter 6 - Temperature, Salinity, and Density - Definition of Sal...
Page 2 of 4
A Simple Definition
Originally salinity was defined to be the "Total amount of dissolved material in grams in one kilogram of sea water."
This is not useful because the dissolved material is almost impossible to measure in practice. For example, how do we measure volatile material like gasses? Nor can we evaporate sea-water to dryness because chlorides are lost in the last stages of drying (Sverdrup, Johnson, and Fleming, 1942: 50).
A More Complete Definition
To avoid these difficulties, the International Council for the Exploration of the Sea set up a commission in 1889 which recommended that salinity be defined as the "Total amount of solid materials in grams dissolved in one kilogram of sea water when all the carbonate has been converted to oxide, the bromine and iodine replaced by chlorine and all organic matter completely oxidized." The definition was published in 1902. This is useful but difficult to use routinely.
Salinity Based on Chlorinity
Because the above definition was difficult to implement in practice, because salinity is directly proportional to the amount of chlorine in sea water, and because chlorine can be measured accurately by a simple chemical analysis, salinity S was redefined using chlorinity:
S = 0.03 + 1.805
Cl (6.1) where chlorinity Cl is defined as "the mass of silver required to precipitate completely the halogens in 0.328 523 4kg of the sea-water sample."
As more and more accurate measurements were made, (6.1) turned out to be too inaccurate. In 1964 UNESCO and other international organizations appointed a Joint Panel on Oceanographic Tables and Standards to produce a more accurate definition. The Joint Panel recommended in 1966 (Wooster, Lee, and Dietrich, 1969) that salinity and chlorinity be related using:
S = 1.80655 Cl (6.2)
This is the same as (6.1) for S = 35.
Salinity Based on Conductivity
At the same time (6.2) was adopted, oceanographers had began using conductivity meters to measure salinity. The meters were very precise and relatively easy to use compared with the chemical techniques used to measure chlorinity. As a result, the Joint Panel also recommended that salinity be related to conductivity of sea water using:
S = -0.08996 + 28.2929729
R
15
+ 12.80832 R
2
15
-10.67869
R
3
15
+ 5.98624
R
4
15
- 1.32311
R
5
15
(6.3a) http://oceanworld.tamu.edu/resources/ocng_textbook/chapter06/chapter06_01.htm
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Introduction to Physical Oceanography : Chapter 6 - Temperature, Salinity, and Density - Definition of Sal...
Page 3 of 4
R
15
=
C ( s ,15,0)/ C (35,15,0) (6.3b) where C ( S , 15 , 0) is the conductivity of the sea-water sample at 15°C and atmospheric pressure, having a salinity S derived from (6.4), and C (35 , 15 , 0) is the conductivity of standard "Copenhagen" sea water. Millero (1996) points out that (6.3) is not a new definition of salinity, it merely gives chlorinity as a function of conductivity of seawater relative to standard seawater.
Practical Salinity Scale of 1978
By the early 1970s, accurate conductivity meters could be deployed from ships to measure conductivity at depth. The need to reevaluate the salinity scale led the Joint Panel to recommend in 1978 that salinity be defined using only conductivity, breaking the link with chlorinity. All water samples with the same conductivity ratio have the same salinity.
The Practical Salinity Scale of 1978 is now the official definition:
S psu
= 0.0080 - 0.1692 R
1/2
15
+ 25.3851 R t
+ 14.0941
R
3/2 t
-7.0261 R
2 t
+ 2.7081 R
5/2 t
+
?
S
R t
=
C ( S ,t,0) / C ( KCl ,t,0)
?
S = [(t - 15) / (1 + 0.0162(t - 15))] + 0.005 - 0.0056 R
1/2 t
- 0.0066 R t
- 0.0375
R
3/2 t
+ 0.636
R
2 t
- 0.0144 R
5/2 t
2
=
S
=
42
(6.4a)
(6.4b)
(6.4c) where C ( S , t, 0) is the conductivity of the sea-water sample at temperature t and standard atmospheric pressure, and
C ( KCl , t , 0) is the conductivity of the standard potassium chloride ( KCl ) solution at temperature t and standard atmospheric pressure. The standard KCl solution contains a mass of 32.435 6 grams of KCl in a mass of 1.000 000kg of solution. An extension of (6.4) gives salinity at any pressure (see Millero 1996: 72).
Table 6.1 Major Constituents of Sea Water
Ion Atoms
55.3%
30.8%
Chlorine
Sodium
55.3%
30.8%
Chlorine
Sodium
7.7% Sulfate
3.7% Magnesium
1.2% Calcium
1.1% Potassium
3.7%
2.6%
1.2%
1.1%
Magnesium
Sulfur
Calcium
Potassium
Comments
The various definitions of salinity work well because the ratios of the various ions in sea water are nearly independent of salinity and location in the ocean (Table 6.1). Only very fresh waters, such as are found in estuaries, have significantly different ratios. The result is based on Dittmar's (1884) chemical analysis of 77 samples of sea water collected by the Challenger Expedition and further studies by Carritt and Carpenter (1958).
" The importance of this result cannot be over emphasized, as upon it depends the validity of the chlorinity: salinity: density relationships and, hence, the accuracy of all conclusions based on the distribution of density where the latter is determined by chemical or indirect physical methods such as electrical conductivity...
" Sverdrup, Johnson, Fleming (1942).
The relationship between conductivity and salinity has an accuracy of around ± 0.003 in salinity. The very small error http://oceanworld.tamu.edu/resources/ocng_textbook/chapter06/chapter06_01.htm
3/7/2007

Introduction to Physical Oceanography : Chapter 6 - Temperature, Salinity, and Density - Definition of Sal...
Page 4 of 4 is caused by variations in constituents such as SiO
2
which cause small changes in density but no change in conductivity.
Instruments for measuring salinity are calibrated using Normal Standard Seawater (P-series). The standard water is made from large samples of water from the north Atlantic, carefully diluted to S = 35, which is distributed in 275 ml sealed glass ampoules. Each is labeled for its conductivity ratio and salinity according to the Practical Salinity Scale
1978 and distributed worldwide by Ocean Scientific International in England since 1989. Each sample is carefully calibrated using the standard KCl solution. chapter contents
Department of Oceanography, Texas A&M University
Robert H. Stewart, stewart@ocean.tamu.edu
All contents copyright © 2005 Robert H. Stewart,
All rights reserved
Updated on September 15, 2006 http://oceanworld.tamu.edu/resources/ocng_textbook/chapter06/chapter06_01.htm
3/7/2007

Introduction to Physical Oceanography : Chapter 6 - Temperature, Salinity, and Density - Measurement of...
Page 1 of 2
(6.1) Definition of Salinity
(6.2) Definition of Temperature
(6.3) Geographical Distribution of Surface
Temperature and Salinity
(6.4) The Oceanic Mixed Layer and Thermocline
(6.5) Density, Potential Temperature, and Neutral
Density
(6.6) Measurement of Temperature
(6.7) Measurement of Conductivity
(6.8) Measurement of Pressure
(6.9) Measurement of Temperature and Salinity with
Depth
(6.10) Light in the Ocean and Absorption of Light
(6.11) Important Concepts
Conductivity is measured by placing platinum electrodes in seawater and measuring the current that flows when there is a known voltage between the electrodes. The current depends on conductivity, voltage, and volume of sea water in the path between electrodes. If the elecrodes are in a tube of non-conducting glass, the volume of water is accurately known, and the current is independent of other objects near the conductivity cell (Figure 6.13). The best measurements of salinity from conductivity give salinity with an accuracy of ±0.005.
Figure 6.13 A conductivity cell. Current flows through the seawater between platinum electrodes in a cylinder of borosilicate glass 191 mm long with an inside diameter between the electrodes of 4 mm. The electric field lines (solid lines) are confined to the interior of the cell in this design making the measured conductivity (and instrument calibration) independent of objects near the cell. This is the cell used to measure conductivity and salinity shown in Figure 6.15. From Sea-Bird
Electronics.
Before conductivity measurements were widely used, salinity was measured using chemical titration of the water sample with silver salts. The best measurements of salinity from titration give salinity with an accuracy of ±0.02.
Individual salinity measurements are calibrated using standard seawater. Long-term studies of accuracy use data from measurements of deep water masses of known, stable, salinity. For example, Saunders (1986) noted that temperature is very accurately related to salinity for a large volume of water contained in the deep basin of the northwest Atlantic under the Mediterranean out flow. He used the consistency of measurements of temperature and salinity made at many hydrographic stations in the area to estimate the accuracy of temperature, salinity and oxygen measurements.
He concluded that the most careful measurements made since 1970 have an accuracy of 0.005 for salinity and 0.005°
C for temperature. The largest source of salinity error was the error in determination of the standard water used for calibrating the salinity measurements.
Gouretski and Jancke (1995) estimated accuracy of salinity measurements as a function of time. Using high quality measurements from 16,000 hydrographic stations in the south Atlantic from 1912 to 1991, they estimated accuracy by plotting salinity as a function of temperature using all data collected below 1500m in twelve regions for each decade from 1920 to 1990. A plot of accuracy as a function of time since 1920 shows consistent improvement in accuracy http://oceanworld.tamu.edu/resources/ocng_textbook/chapter06/chapter06_07.htm
3/7/2007

Introduction to Physical Oceanography : Chapter 6 - Temperature, Salinity, and Density - Measurement of...
Page 2 of 2 since 1950 (Figure 6.14). Recent measurements of salinity are the most accurate. The standard deviation of modern salinity data collected from all areas in the South Atlantic from 1970 to 1993 adjusted as described by Gouretski and
Jancke (1995) was 0.0033. More recent instruments such as the Sea-Bird Electronics Model 911 Plus have an accuracy of better than 0.005psu without adjustments. A careful comparison of salinity measured at 43° 10'N, 14° 4.5'W by the
911 Plus with historic data collected by Saunders (1986) gives an accuracy of 0.002 (Figure 6.15).
Figure 6.14. Standard deviation of salinity measurements at depths below 1500 m in the South Atlantic from 1920 to
1993. Each point is the average of data collected for the decade centered on the point. The value for 1995 is an estimate of the accuracy of recent measurements. From
Table 1 of Gouretski and Jancke (1995).
Figure 6.15. Results from a test of the Sea-Bird
Electronics 911 Plus CTD in the North Atlantic Deep
Water in 1992. Data were collected at 43.17°N and
14.08°W from the R/V
Electronics (1992).
Poseidon . From Sea-Bird chapter contents
Department of Oceanography, Texas A&M University
Robert H. Stewart, stewart@ocean.tamu.edu
All contents copyright © 2005 Robert H. Stewart,
All rights reserved
Updated on September 15, 2006 http://oceanworld.tamu.edu/resources/ocng_textbook/chapter06/chapter06_07.htm
3/7/2007

All aqueous solutions conduct electricity to some degree. The measure of a solution’s ability to conduct electricity is called
“conductance” and is the reciprocal of resistivity (resistance).
Adding electrolytes such as salts, acids or bases to pure water increases conductance (and decreases resistivity).
SIEMENS/ cm@20
°
08
C
A conductivity system measures conductance by means of electronics connected to a sensor immersed in a solution. The
07
06
05
HCI analyzer circuitry impresses an alternating voltage on the sensor and measures the size of the resulting signal, which is linearly related to the conductivity.
Because conductivity has a large temperature coefficient (as much as 4% per °C–see Fig. 1), an
04
03
02
01
0
0 10
KCI
20 30
KOH
H
2
SO
4
NaCH
HNO
3
40 50 60 70 80 90 100
Figure 1
Conductivity vs. Concentration integral temperature sensor incorporated into its circuitry adjusts the reading to a standard temperature, usually 25°C (77°F).
Historically, the unit of conductivity measurement has been the “mho/cm” (a mho is the multiplicative inverse of an ohm).
A resistivity of 100 ohms x cm is
MilliSiemens/cm equivalent to a conductivity of
1/100 mho/cm. The mho/cm unit of measurement is now being
1000
15% NaOH replaced in industry by an equal and interchangeable international unit called the “Siemen/cm.”
Conductivity is usually expressed in millionths of a Siemen, that is, in microSiemen/cm. Resistivity is
800
600
400
8% HNO
3
5% H
2
SO
4 still expressed in terms of
Megohm (M
Ω
) x cm for high purity water – usually from 0.1 to 20 M
Ω x cm.
RESISTIVITY
In high purity water, with
200
1 mol NaCl
0
0 10 20 30 40 50 60 70 80 90
°
Conductivity vs. Temperature
Figure 2 conductivity typically less than 1 microSiemen/cm, the measurement is referred to as
“resistivity” with units being M
Ω x cm. Pure water has a resistivity of about 18.3 Megohm x cm at 25°C. One consideration that must be made when measuring solutions is the temperature coefficient of the conductivity of the water itself. To compensate accurately, a second temperature sensor and compensation network must be used. Specific sensors and analyzers are recommended for measurement in high purity water.
The contacting-type sensor usually consists of two electrodes, insulated from one another. The electrodes, typically 316 stainlesssteel, titanium-palladium alloy or graphite, are specifically sized and spaced to provide a known “cell constant.” Theoretically, a cell constant of 1.0 describes two electrodes, each being one square centimeter in area and spaced one centimeter apart (Fig. 3).
In measuring the 1 microSiemen/cm solution, the cell would be configured with large electrodes spaced a small distance apart. This results in a cell resistance of approximately 10,000 ohms, which can be measured quite accurately. Using cells with different constants, the measuring instrument can operate over the same range of cell
SURFACE AREA OF
ELECTRODE A (1
SQUARE CENTIMETER)
VOLUME OF MEASURED
SOLUTION (1 CUBIC
CENTIMETER)
SURFACE AREA OF
ELECTRODE B (1 SQUARE
CENTIMETER)
Theoretical Cell Constant of 1.0
Figure 3 resistance for both ultra-pure water and high conductivity seawater.
The electrodeless type of sensor operates by inducing an alternating current in a closed loop of solution and measuring its magnitude to determine conductivity (Fig. 4). The conductivity analyzer drives Torroid A, which induces the alternating current in the solution. This ac signal flows in a closed loop through the sensor bore and surrounding solution. Torroid B senses the magnitude of the induced current, which is proportional to the conductance of the solution. This signal is
ANALYZER
ELECTRONICS processed in the analyzer to display the corresponding reading.
Since the electrodeless sensor has no electrodes, common problems facing contacting-type sensors are eliminated. Polarization, oily
TORROID A
SENSOR BORE
SENSOR HOUSING
TORROID B fouling, process coating or non-conducting electrochemical plating do not affect the performance of electrodeless sensors until gross fouling occurs.
INDUCED ELECTRIC
CURRENT IN THE
SOLUTION
Electrodeless Sensor
Figure 4
Cell constants must be matched to the analyzer for a given range of operation. For instance, if a sensor with a cell constant of 1.0 were used in pure water with a conductivity of 1 microSiemen/cm, the cell would have a resistance of 1,000,000 ohms. Conversely, the same cell in seawater might have a resistance of 30 ohms. Since the resistances are so different, it is difficult for ordinary instruments to accurately measure such extremes with only one cell constant.
E-3
TEMPERATURE COMPENSATION
Conductivity measuring system accuracy is only as good as its temperature compensation. Since common solution temperature coefficients vary on the order of 1-3% milliSiemens/cm
1000
15% NaOH per °C, measuring instruments with adjustable temperature compensation should be utilized.
Solution temperature coefficients are
800
600
10% NaOH
5% NaOH somewhat non-linear and usually vary with actual conductivity as well (Fig. 5). Thus, calibration at the actual
400
200 measuring temperature will yield the best accuracy.
®
0
20 30 40 50 60 70 80 90 100 C
Conductivity vs. Temperature
CONDUCTIVITY VS. TEMPERATURE
FOR DIFFERENT CONCENTRATIONS for Different Concentrations
Figure 5
Reproduced with permission of
Great Lakes Instruments.

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