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.

[Robert Fogt]

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.

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

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

[amorLarkswota]

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

[JohnS]

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 µS/cm, 0.046 psu salinity
pubs.water.usgs.gov/tm1d3

[alfred]

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!

[JohnS]

Originally Posted by alfred

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.

[sensordev]

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 µS)

and/or

(2) Conductivity to Salinity (µS to PSU)

Bob Hunt

[Ben]

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.