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  • Salinity to Conductivity conversion

    Ran across a thread on this forum from August 2005 and haven't figured out how to make a reply in it yet. I also searched long for a direct relationship between Salinity and Conductivity to no avail, and finally worked out the relationship myself with less than 100% confidence in the result. 90% plus maybe, but would appreciate critical comment. An Excel spreadsheet can be downloaded (110KBytes) at:

    http://www.sensordev.com/PSU_vs_Conductivity.xls

    The bulk of the sheet is just to illustrate how the relationship was determined. There's a small calculator at the left middle that will crank out values across the range of 2 to 42 PSU (practical salinity units, a unitless number, go figure). Note also that it's based on comparison with a KCl standard solution, which is part of what bothers me. KCl and NaCl do not have the same conductivity vs. concentration curves, nor the same response to temperature changes. My unconfirmed hope is that the folks who derived the PSU scale based their results on empirical measurements of the KCl standard ratios with NaCl dominated seawater, which I believe would get around the differences.

  • #2
    Re: Salinity to Conductivity conversion

    I downloaded the above spreadsheet and attached it to this message. Just in case the download is ever removed.

    It was scanned for viruses but not verified for accuracy.

    Thanks for the file Sensordev, we appreciate the time you spent on it.

    Comment


    • #3
      Re: Salinity to Conductivity conversion

      Robert,

      Thank you, and this was something I had to do to confirm results on a sensor being developed. I have since posting the spreadsheet had occasion to use it and it appears to be quite accurate. Probably not up to the levels of a research tool but certainly good enough for commercial and industrial uses.

      Best regards,
      Bob Hunt

      Comment


      • #4
        Re: Salinity to Conductivity conversion

        I just wanted to say thanks for this chart. I have been looking everywhere for something like this.

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        • #5
          Salinity to Conductivity conversion

          its unfortunate that there isnt a standard in terms of salinity of the water to the measurments of the salt, but i dont think there will ever be a world wide standard, just a standard in every house hold aquarium :P

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          • #6
            Re: Salinity to Conductivity conversion

            Some additional resources on practical salinity scale, giving the defining equations. The first one, which is the fundamental definition fails to mention the conductivity of the standard KCl solution at 15 C is 42.914 mS/cm. With that info from another source, the algorithm is completely defined to work directly from measured conductance.

            Details on Practical Salinity Scale
            140 opage document, WAY too much background, but
            algorithm is defined on pp 136-139. Note the variable R sub-p is a pressure term, for water at or near the surface, it is unity and can be ignored.
            unesdoc.unesco.org/images/0004/000479/047932EB.pdf

            UNESCO Technical Paper 44
            Algorithms for computation of fundamental properties of seawater
            repeats the salinity algorithm, has density and other algorithms, includes FORTRAN code.
            http://www.google.com/url?url=http:/...xFxH0mfvrYrUXw

            USGS Manual on Water Quality Monitoring
            Gives a simplified salinity computation algorithm for surface water at 25 C
            Validity isn't clear but they give a table down to 100 S/cm, 0.046 psu salinity
            pubs.water.usgs.gov/tm1d3

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            • #7
              Re: Salinity to Conductivity conversion

              hey there...osum remarks....since we are in the same subject...really do need some help in the conversion of conductivity (micro s/cm) to salinity...is there like a formula of any sort where u can use in a excel sheet for numerous water levels?....

              thanx!

              Comment


              • #8
                Re: Salinity to Conductivity conversion

                Originally posted by alfred View Post
                hey there...osum remarks....since we are in the same subject...really do need some help in the conversion of conductivity (micro s/cm) to salinity...is there like a formula of any sort where u can use in a excel sheet for numerous water levels?....

                thanx!
                See the links above in post 6. The first two only consider ocean water, moderate to high salinity but include pressure and temperature terms (and are quite complex).

                The last applies only to low salinity (fresh water) at atmospheric pressure and 25 C.

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                • #9
                  Re: Salinity to Conductivity conversion

                  And vice-versa.

                  It only took me a few years to realize that I described the spreadsheet function poorly.

                  It has a small dual calculator section in the midst of all the tables that will convert both:

                  (1) Salinity to Conductivity (PSU to S)

                  and/or

                  (2) Conductivity to Salinity (S to PSU)

                  Bob Hunt

                  Comment


                  • #10
                    Re: Salinity to Conductivity conversion

                    Hi Bob, I sort of stumbled across this post when I was looking for info on salt contamination. I was just wondering if you knew how much variation it makes if you take into account that there is probably many other soluble salts in sea water, all with different conductivity/concentration ratios. Maybe there presence is insignificant compared to KCl? I was also wonder how similar the ratio was between KCl and NaCl?
                    Its a very interesting subject that seems to be misunderstood by a lot of people as far as I can tell in my limited search for information.
                    Regards,
                    Ben.

                    Comment


                    • #11
                      Re: Salinity to Conductivity conversion

                      Originally posted by Ben View Post
                      Hi Bob, I sort of stumbled across this post when I was looking for info on salt contamination. I was just wondering if you knew how much variation it makes if you take into account that there is probably many other soluble salts in sea water, all with different conductivity/concentration ratios. Maybe there presence is insignificant compared to KCl? I was also wonder how similar the ratio was between KCl and NaCl?
                      Its a very interesting subject that seems to be misunderstood by a lot of people as far as I can tell in my limited search for information.
                      Regards,
                      Ben.
                      Ben, sorry I only come around once a year or so. Check out "The Development of the Chlorinity/Salinity Concept in Oceanography" By William J. Wallace, look for excerpts in Google and start at page 161.

                      Comment


                      • #12
                        Re: Salinity to Conductivity conversion

                        Originally posted by Ben View Post
                        Hi Bob, I sort of stumbled across this post when I was looking for info on salt contamination. I was just wondering if you knew how much variation it makes if you take into account that there is probably many other soluble salts in sea water, all with different conductivity/concentration ratios. Maybe there presence is insignificant compared to KCl? I was also wonder how similar the ratio was between KCl and NaCl?
                        Its a very interesting subject that seems to be misunderstood by a lot of people as far as I can tell in my limited search for information.
                        Regards,
                        Ben.
                        which I believe would get around the differences.

                        Comment


                        • #13
                          Re: Salinity to Conductivity conversion

                          Kaitlyn & Ben

                          The formulas in post #6 apply only to "sea salt." Sea salt is dominated by NaCl, but a lot of other ions. However, the oceans are pretty well mixed and the salt composition of real sea salt is pretty similar everywhere and close to the assumed "standard sea salt." What varies is the concentration vs fresh water added by rivers, or in areas of high evaporation, stronger than normal concentrations.

                          The UNESCO formulas in post #6 are only intended for varying concentrations of normal sea salt. The extension to mineral content in fresh water by USGS is probably dubious, but better than nothing. The ions making up the mineral content in "fresh water" are much more variable depending on what the fresh water runs through.

                          There is not a lot of KCl in sea salt. KCl is only used for calibration of the appartus. (I don't know the background. Maybe it is in the 140 page UNESCO document. I only skimmed it looking for the algorithm.)

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