SG and Oceanic Calculator

[henn]

Randy,

In your article Reef Aquarium Salinity: Homemade Calibration Standards (http://www.reefkeeping.com/issues/2004-06/rhf/index.php) in Table 3 "Density and specific gravity as a function of salinity of seawater" you have a note (the footnote 3) that the table is based on values calculated by an Oceanic Calculator from Oceanteacher website. That site is reorganized now, so the original link does not work any more, but some calculator could be found here (http://ioc.unesco.org/Oceanteacher/OceanTeacher2/02_InfTchSciCmm/01_CmpTch/05_OcSoft/01_Toolbox/OcCalc/OcCalc.htm). I'm not sure if that one would be the same you used for calculations, but anyway, that calculator gives rather different values for SG.

When looking closer how the SG is calculated there, so as written in calculator's Read Me, the SG is calculated from Specific Volume (SV), ie. SG=1/SV. As much I know the SV is the inverse of density, not SG. http://www.oc.nps.navy.mil/nom/day1/parta.html

May be the following formula used for calculation SV could give some hint? Here a-temperature, b-salinity and c-pressure
SV=.702 + 100 * (17.5273 + .1101 * a - .000639 * a*a - .039986 * b - .000107 * b*a) / (c+1 + 5880.9 + 37.592 *a - .34395 *a*a + 2.2524 *b)
The referenced source is
WILSON, W. and BRADLEY, D., 1968, Specific Volume of Sea Water as a function of Temperature, Pressure, and Salinity: Deep-Sea Research, v. 15, p. 355-363

[Randy Holmes-Farley]

Specific volume is the inverse of density, and the link you provide shows that:

"Specific volume, , is the inverse of density; . It has
dimensions of volume/mass, units of m3/kg, cm3/g, or
centiliters per metric ton (cliter/ton) are usually used, where
1 cliter/ton=10-5 cm3/g."

So it would not be correct to get specific gravity by taking the inverse of the specific volume.

[henn]

OK, then what's the origin of SG values in the mentioned Table 3 in your article?

[Randy Holmes-Farley]

I don't recall if those values were directly sg from the reference, or density that I divided by the density of water, but they are specific gravity, and are accurate as far as I know.

[henn]

I think I found the answer to my question.;) There is another calculator (http://ioc.unesco.org/Oceanteacher/oceanteacher2/01_GlobOcToday/02_CollDta/02_OcDtaFunda/02_OcMeasUnits/SWequationofstatecalculator.htm) based on the UNESCO International Equation of State (IES 80) as described in Fofonoff, JGR, Vol 90 No. C2, pp 3332-3342, March 20, 1985.

You found densities for each salinity and divided these values to the density of zero salinity. Right?

[Randy Holmes-Farley]

Probably, unless I actually used a sg table. :D