Refractometer, not sure how to use?

[midgetwaiter]

It's not the right kind of refractometer, the scale is way to big. Seawater averages 35 parts per thousand which would be 3.5%. That is equivalent to 1.0265 SG.

I'm not sure if that will measure the refractive index of seawater correctly though. It should be close enough if you keep it in between 3% and 4% but keep in mind that 3% is about 1.023 and 4% is about 1.030 so you may be better off with something that is more precise.

[Tigerarmy]

The refracto I got has lines in between 3 and 4.Is there a mathmatical equation I can use to find out SG from ppt?

[midgetwaiter]

Originally Posted by Tigerarmy

The refracto I got has lines in between 3 and 4.Is there a mathmatical equation I can use to find out SG from ppt?

Not an easy one, there is no linear relationship between specific gravity and salinity. Given that you aren't even measuring the salinity directly but instead are reading the refractive index of the sample and then use that to determine the salinity it would never be reliable. I can give you a guess but it's predicated on a big assumption. If the refractometer you have is designed to measure the refractive index of sodium chloride solutions then this should be close. If not it's not then this might not be anywhere near correct.

The refractive index of 35ppt seawater is the same as a 36.5ppt solution of sodium chloride. You can get a seawater equivalent reading by multiplying your measurement by .96

Take the number you get and refer to table 3 in this article:
http://www.reefkeeping.com/issues/2004-06/rhf/index.php