Question regarding salinty

beachsidereefer · Jul 2, 2015

Hi Everyone,

so this is probably going to sound really amateurish and sorry in advance if this has already been answered.

I use this refractometer that I believe it compensates for temperature variation. Also I calibrate it every month using a solution that is labeled as 35 PPT and 1.026.
http://www.tequipment.net/ExtechRF2...4WNT0j1srX5v8oAlKeN4OQ_UqDmeVSxkYAhoC6b3w_wcB

When I look through the eye piece I swear the line measuring 35 PPT is not directly parallel to the line that measures 1.026 SG.

So my question is are my eyes going bad : ) are these two lines really parallel to each other and I'm just unable to see it or is 35 PPT line actually parallel to the 1.027 line which is what I believe I am seeing.

What I have been doing is just going with the PPT side and targeting my aquarium water at 35 PPT in the hopes I'm doing the right thing ; )

Thanks all in advance for the responses!

redfishbluefish · Jul 2, 2015

HERE's a nice little read from Randy. Actually 35 ppt equals 1.0264 specific gravity and maybe that's why you're seeing 1.027. Regardless of what you use (ppt or specific gravity), what is critical is consistency.

beachsidereefer · Jul 2, 2015

awesome! I'll definitely read the article.

Thanks!

Randy Holmes-Farley · Jul 2, 2015

beachsidereefer said:
Hi Everyone,

so this is probably going to sound really amateurish and sorry in advance if this has already been answered.

I use this refractometer that I believe it compensates for temperature variation. Also I calibrate it every month using a solution that is labeled as 35 PPT and 1.026.
http://www.tequipment.net/ExtechRF2...4WNT0j1srX5v8oAlKeN4OQ_UqDmeVSxkYAhoC6b3w_wcB

When I look through the eye piece I swear the line measuring 35 PPT is not directly parallel to the line that measures 1.026 SG.

So my question is are my eyes going bad : ) are these two lines really parallel to each other and I'm just unable to see it or is 35 PPT line actually parallel to the 1.027 line which is what I believe I am seeing.

What I have been doing is just going with the PPT side and targeting my aquarium water at 35 PPT in the hopes I'm doing the right thing ; )

Thanks all in advance for the responses!

Excellent question!!!

That is exactly the problem with brine refractometers. The 35 ppt and 1.0264 lines do not line up because it is made for sodium chloride in water, not seawater. A 35 ppt sodium chloride solution (3.5 weight percent sodium chloride in water) has the same refractive index as a 33.3 ppt seawater solution, so the error in using a perfectly calibrated salt refractometer is about 1.7 ppt, or 5% of the total salinity.

Looking at it differently, a 35 ppt sodium chloride solution should have a specific gravity of 1.0250, and that is hopefully close to what you observe if it is made correctly.

Without going into all the science, just pick one or the other (35 ppt or 1..0264) for calibration and use that unit of measure and ignore the other.

Calibratng with the standard is the way to go!