@Randy Holmes-Farley
@JimWelsh
Curiosities
The spreadsheet to calculate thr salinity level from the Triton results is in PSU
I've noticed in some salt mixes that calculated number to be lower then the salinity I've measures in PPT via my refractometer. But sometimes very close. I think there's been one instance it was higher.
Would the variations in element ratios effect the differences of PSU calculated vs PPT via refractometer?
As I read it, there are basically two questions here:
(1) Would variations in ionic ratios affect the refractometer reading?
(2) Is there a difference between the PSU and PPT scales?
If I misunderstand what you are asking, please feel free to correct me.
Regarding #1, I've done some work to investigate the effect of various ions on the refractive index of the solution. This ended up being much more complex than I expected, but the over-simplified answer is that "yes", different solutions with the same salinity (defined as g/kg of total dissolved salts), but different ionic ratios, may very well have different refractive indexes. Admittedly, the effect is small, but it does exist. The most significant imbalance is Mg, which contributes a relatively low refractive index, and will make the sample's salinity read slightly lower when Mg is elevated, and slightly higher when Mg is lower than normal. I don't have numbers handy to quantify this effect, but it is rather small.
Regarding #2, how my spreadsheet does these calculations is described on the "Introduction" tab. In short, it calculates the estimated g/kg of the total salts in the sample. It then compares that to the total amount of salts that are in seawater with S=35 according the the PSS-78 standard, which is 35.16504 g/kg. So, if the estimated total salts is 35.16504 g/kg exactly, then the spreadsheet will call the salinity "35.00". If the total salts are higher or lower than 35.16504 g/kg, then the salinity will be shown as higher or lower, proportionally. If you are calibrating your refractometer according to Randy's article found here
http://www.reefkeeping.com/issues/2004-06/rhf/ then you are calibrating it to the PSU scale, and not PPT. Otherwise the potential in scale differences depends upon what standard you are calibrating your refractometer with. One note: The "Temperature" input of my spreadsheet is intended to be the temperature *at which Triton is taking the sample and against which the results are being reported*, and NOT the tank temperature. I didn't make that clear in the "Introduction".
I hope this helps!
