Sg = Ec?
- Thread starter Rovert
- Start date
Yes there is a relationship between salinity and sg and conductivity.
Salinity is the wt of dissolved solids per liter of seawater. Historically, they used to evaporate seawater and weigh the remaining solids then make up corrections for some ions lost in the evaporation process. They subsequently discovered for the major ions in the ocean that they were found in constant proportions in open ocean. So if you measured Chloride in any ocean seawater, with the constant proportion rule, you could then calculate salinity from the measured chlorinity.
Internationally, conductivity is the standard for salinity measurements. Conductivity is an indirect measure of salinity. The more ions the more electrical conductivity. Chlorinity measurement is accurate, but not as accurate or precise as conductivity and is much more labor intensive than conductivity measures. A conductivity of 53 ms corresponds to a salinity of 35....other values of salinity are looked up on a mapping table of salinity and conductivity. Conductivity must be corrected for temperature as it is exquisitely sensitive to temperature variations. Luckily, most conductivity probes will correct for temperature.
Specific Gravity, sg, is the ratio of density of a water sample compared to pure water at a particular temperature. The " pure water at a particular temperature" caveat is because the density of water changes with temperature. SG may or may not accurately reflect salinity. But practical purposes for aquarists it does. The only way to directly measure SG is to weigh equal volumes of pure water and sample at a particular temperature...then calculate the ratio sample mass/pure water mass. It also can be measured with a glass hydrometer of known density ant a particular temperature. What is commonly referred to sg from a refractometer is actually a measure of refractive index in a sample of water, hopefully automatically corrected for temperature. A sample of water with higher sg or salinity will bend light stronger than a more dilute sample. The refractometer has a built in scale which essentially maps the refractive index with sg and/or salinity. The one caveat here is that solutions of the same sg may have different refractive indices, and therefore a different indirectly measured sg or salinity. For example two solutions with identical sg 1.0264, one potassium chloride and the other a standard seawater solution, will have very different refractive indices. The reason here is that the ions that are in the solution interact with light differently. This is why it is very important to calibrate your refractometer with appropriate solution such as PinPoint 53 mS fluid.
Short answer, yes conductivity,refractive index, and sg are indirect measures of salinity. Use of a good refractometer with automatic temperature correction and/or a conductivity probe will give you a measure of salinity.
http://www.fishchannel.com/fish-magazines/aquarium-fish-international/default.aspx
http://reefkeeping.com/issues/2006-12/rhf/index.php
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Rovert
Premium Member
Wow!
Thanks for the detailed reply. That helps sort things out. I just got ahold of one of these, and wasn't sure if I could use the EC component to test for salinity.
Where can I find that conversion chart you speak of?
http://hannainst.com/usa/prods2.cfm?id=002003&ProdCode=HI 98130&test=1
Thanks for the detailed reply. That helps sort things out. I just got ahold of one of these, and wasn't sure if I could use the EC component to test for salinity.
Where can I find that conversion chart you speak of?
http://hannainst.com/usa/prods2.cfm?id=002003&ProdCode=HI 98130&test=1
I don't know if your conductivity probe will help you. The conductivity for the probe you mentioned only goes to 20 mS. Saltwater aquariums in general 48-55 mS.
http://www.americanmarineusa.com/salinityconversion.html
http://www.americanmarineusa.com/salinityconversion.html
Rovert
Premium Member
That's wierd. The unit I have is the 98130, which is supposed to be their high-range EC model, but their web content says it has a range of only up to 2000.
Now, on the other hand the link to the LOW range EC unit is listed with a range of up to 3999:
http://hannainst.com/usa/prods2.cfm?id=002003&ProdCode=HI 98129&test=1
What's even stranger is that neither of these are in a range to be useful for marine aquarium testing. We'd need something that goes up to 6000 or slightly more, depending on your preferred salinity.
Now, on the other hand the link to the LOW range EC unit is listed with a range of up to 3999:
http://hannainst.com/usa/prods2.cfm?id=002003&ProdCode=HI 98129&test=1
What's even stranger is that neither of these are in a range to be useful for marine aquarium testing. We'd need something that goes up to 6000 or slightly more, depending on your preferred salinity.
Range pH 0.00 to 14.00 pH
Range EC 0.00 to 20.00 mS/cm
Range TDS 0.00 to 10.00 ppt
Range Temperature 0.0 to 60.0°C / 32 to 140.0°F
These are the numbers on your first link for HI 98130. May want to ask them if there is one for seawater conductivity.
BTW the low range meter is 3999 micro siemens....3.999 milli siemens.
Range EC 0.00 to 20.00 mS/cm
Range TDS 0.00 to 10.00 ppt
Range Temperature 0.0 to 60.0°C / 32 to 140.0°F
These are the numbers on your first link for HI 98130. May want to ask them if there is one for seawater conductivity.
BTW the low range meter is 3999 micro siemens....3.999 milli siemens.
http://www.advancedaquarist.com/2006/10/review/view
The above Hach instrument will do the trick.
Also consider PinPoint Salinity probe...
The above Hach instrument will do the trick.
Also consider PinPoint Salinity probe...
Rovert I think that is the way to go. Make sure to calibrate the refractometer with the correct fluid...for example PinPoint 53 mS calibration fluid...not DO/DI water...or your refractometer will give inaccurate rusults.
Correction for my first post...in case one of the smarties out there will correct it. I said...
More correctly
i.e. it is solid material in one kg of water not liter.
Correction for my first post...in case one of the smarties out there will correct it. I said...
More correctly
http://www.osil.co.uk/web/osil/osil.nsf/a/5789D6CD960998AA80256D2C0036BC0C!opendocument
i.e. it is solid material in one kg of water not liter.
Jon
I put that in about wt/kg water is a historical reference.
But even with the current modern standard of conductivity as a measure of salinity, conductivity for seawater still refers back to the original definition. For example a standard KCL reference cell with a conductivity of exactly 35 would not have the same salinity (wt/kg seawater) as normal seawater with a conductivity of exactly 35, despite the definition of salinity in terms of conductivity ratio. Kinda circular......
Mike
I put that in about wt/kg water is a historical reference.
But even with the current modern standard of conductivity as a measure of salinity, conductivity for seawater still refers back to the original definition. For example a standard KCL reference cell with a conductivity of exactly 35 would not have the same salinity (wt/kg seawater) as normal seawater with a conductivity of exactly 35, despite the definition of salinity in terms of conductivity ratio. Kinda circular......
Mike
Jon
I put that in about wt/kg water is a historical reference.
But even with the current modern standard of conductivity as a measure of salinity, conductivity for seawater still refers back to the original definition. For example a standard KCL reference cell with a conductivity of exactly 53 mS would not have the same salinity (wt/kg seawater) as normal seawater with a conductivity of exactly 53 mS, despite the definition of salinity in terms of conductivity ratio. Kinda circular......
Mike
I put that in about wt/kg water is a historical reference.
But even with the current modern standard of conductivity as a measure of salinity, conductivity for seawater still refers back to the original definition. For example a standard KCL reference cell with a conductivity of exactly 53 mS would not have the same salinity (wt/kg seawater) as normal seawater with a conductivity of exactly 53 mS, despite the definition of salinity in terms of conductivity ratio. Kinda circular......
http://www.osil.co.uk/web/osil/osil.nsf/a/5789D6CD960998AA80256D2C0036BC0C
Mike
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For example a standard KCL reference cell with a conductivity of exactly 35 ***would not have the same salinity (wt/kg seawater) as normal seawater with a conductivity of exactly 35***, despite the definition of salinity in terms of conductivity ratio.
Mike is this a typo ? Or am I misreading you ?
Yes it will that is what the std is all about.
53 ms @25 C KCl (32.4356 grams of Potassium Chloride ) = 35 ppt NSW @25 C, actually 53.065 mS = 35 ppt NSW, temp corrected, as long as the K-15 ratio is 1. And a K-15 ratio of 1 = 32.4356 grams of Potassium Chloride = 53.065 ms or 53,065 uS = 35 ppt NS
http://gaea.es.flinders.edu.au/~mattom/Utilities/salcon.html
and
http://gaea.es.flinders.edu.au/~mattom/Utilities/salinity.html
Salinity Equation of State
http://oceanworld.tamu.edu/resources/ocng_textbook/chapter06/chapter06_01.htm
Mike is this a typo ? Or am I misreading you ?
Yes it will that is what the std is all about.
53 ms @25 C KCl (32.4356 grams of Potassium Chloride ) = 35 ppt NSW @25 C, actually 53.065 mS = 35 ppt NSW, temp corrected, as long as the K-15 ratio is 1. And a K-15 ratio of 1 = 32.4356 grams of Potassium Chloride = 53.065 ms or 53,065 uS = 35 ppt NS
http://gaea.es.flinders.edu.au/~mattom/Utilities/salcon.html
and
http://gaea.es.flinders.edu.au/~mattom/Utilities/salinity.html
Salinity Equation of State
http://oceanworld.tamu.edu/resources/ocng_textbook/chapter06/chapter06_01.htm
Boomer
Your equation is strictly not true:
Both sides of the equation use different units with one side in mS the other ppt....you are actually making the same conceptual jump that I am making i.e. relating practical salinity with real or absolute salinity (ppt). What is really meant is that if the conductivity of a seawater sample is equal to the conductivity of a standard KCL cell (each at STP), then the salinity of the sample is 35 by definition of the practical salinity scale. The conductivity of ratio of one is mapped to the number 35 in order for the practical salinity scale to be consistent with the prior chlorinity scale. When the definition of the practical salinity scale was made, a sample of standard seawater with exactly 35 ppt of dissolved salt (calculated from chlorinity) was used to standardize the needed KCL mass to create a conductivity ratio of one.
A standard solution of KCL (32.4356 grams of KCL) would have a real or absolute salinity of 32.4356 ppt....but a conductivity ratio of 1 with another standard KCL cell. I am not saying that a seawater sample with a practical salinity of 35 has exactly 35 ppt....but it will be very, very close because of constant composition of seawater.
Salinity is defined as the ratio of the mass of dissolved material in sea water to the mass of sea water (IAPSO Publication Scientifique, no. 32, 1985). The 'practical salinity' (S) of a seawater sample is defined as the ratio of the electrical conductivity of the sample (at 15 °C, and one standard atmospheric pressure) to that of a standard solution of potassium Chloride (KCl). A ratio of 1 is equivalent to a 'practical salinity' of 35 (UNESCO technical papers in marine science, no. 45, 1985).
Salinity is an important variable in the International Equation of State (EOS-90 ), the officially recognized equation used by oceanographers to calculate the density of seawater. Without the context of you are dealing with seawater and seawater's constant composition assumption, measurement of conductivity as a reflection of real or absolute salinity, the EOS-90 is meaningless.
The current definition of salinity by IAPSO assumes you are dealing with seawater.....not a bucket of resistors with a conductivity ratio of 1.
What I should have said was:
For example a standard KCL reference cell with a conductivity of exactly 35 ***would not have the same real or absolute salinity (wt/kg seawater) as normal seawater with a conductivity of exactly 35***, despite the definition of salinity in terms of conductivity ratio.
__________________________________________________
Other quotes and crap to further pad this excessively long post on a very esoteric subject and semantic argument.
Your equation is strictly not true:
.
Both sides of the equation use different units with one side in mS the other ppt....you are actually making the same conceptual jump that I am making i.e. relating practical salinity with real or absolute salinity (ppt). What is really meant is that if the conductivity of a seawater sample is equal to the conductivity of a standard KCL cell (each at STP), then the salinity of the sample is 35 by definition of the practical salinity scale. The conductivity of ratio of one is mapped to the number 35 in order for the practical salinity scale to be consistent with the prior chlorinity scale. When the definition of the practical salinity scale was made, a sample of standard seawater with exactly 35 ppt of dissolved salt (calculated from chlorinity) was used to standardize the needed KCL mass to create a conductivity ratio of one.
A standard solution of KCL (32.4356 grams of KCL) would have a real or absolute salinity of 32.4356 ppt....but a conductivity ratio of 1 with another standard KCL cell. I am not saying that a seawater sample with a practical salinity of 35 has exactly 35 ppt....but it will be very, very close because of constant composition of seawater.
Salinity is defined as the ratio of the mass of dissolved material in sea water to the mass of sea water (IAPSO Publication Scientifique, no. 32, 1985). The 'practical salinity' (S) of a seawater sample is defined as the ratio of the electrical conductivity of the sample (at 15 °C, and one standard atmospheric pressure) to that of a standard solution of potassium Chloride (KCl). A ratio of 1 is equivalent to a 'practical salinity' of 35 (UNESCO technical papers in marine science, no. 45, 1985).
Salinity is an important variable in the International Equation of State (EOS-90 ), the officially recognized equation used by oceanographers to calculate the density of seawater. Without the context of you are dealing with seawater and seawater's constant composition assumption, measurement of conductivity as a reflection of real or absolute salinity, the EOS-90 is meaningless.
The current definition of salinity by IAPSO assumes you are dealing with seawater.....not a bucket of resistors with a conductivity ratio of 1.
What I should have said was:
For example a standard KCL reference cell with a conductivity of exactly 35 ***would not have the same real or absolute salinity (wt/kg seawater) as normal seawater with a conductivity of exactly 35***, despite the definition of salinity in terms of conductivity ratio.
__________________________________________________
Other quotes and crap to further pad this excessively long post on a very esoteric subject and semantic argument.
complete chemical analysis of seawater is the only reliable way to determine the true or absolute salinity of seawater (S t in parts per thousand). This method, however, is too time consuming for routine studies.
Millero Chemical Oceanography 3rd Ed pg 63
That for sure I agree with is the only way of absolute means
Salinity is defined as the ratio of the mass of dissolved material in sea water to the mass of sea water
And yes also
35 ***would not have the same real or absolute salinity (wt/kg seawater) as normal seawater with a conductivity of exactly 35***,
and yes also
But for practical purposes this is more than we need. Not only that but NSW follows the law of equal proportions pretty much.
http://gaea.es.flinders.edu.au/~mattom/Utilities/salcon.html
Millero Chemical Oceanography 3rd Ed pg 63
That for sure I agree with is the only way of absolute means
Salinity is defined as the ratio of the mass of dissolved material in sea water to the mass of sea water
And yes also
35 ***would not have the same real or absolute salinity (wt/kg seawater) as normal seawater with a conductivity of exactly 35***,
and yes also
But for practical purposes this is more than we need. Not only that but NSW follows the law of equal proportions pretty much.
http://gaea.es.flinders.edu.au/~mattom/Utilities/salcon.html
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