Calibrating refractometer - SG vs. salinity

[RobertK]

So I got some new Pinpoint refractometer calibrating solution, the bottle says it is SG 1.026 and 35 ppm salinity. I noticed that the scale on my refractometer (which I bought at least 10 years ago) reads 1.025 SG as 35 PPM, and 1.026 SG as slightly over 36 ppm. Which number should I calibrate it at? Or do I need a new one? Thanks!
(I tried taking a picture through the eyepiece of the refractometer but it didn't work.)

 

[KrisReef]

Pick one and keep it stable.

 

[Randy Holmes-Farley Verified Community Expert]

That’s because it is not a seawater refractometer. What you note is the tell tale sign. Use the standard and then set the refractometer to either 35 ppt or sg = 1.0264, depending on which scale you want to use, and then use whichever scale you set it to.

 

[RobertK]

Thanks for the replies! I had no idea that not all salinity refractometers were intended for seawater use. Is that because there are other elements besides NaCl in seawater?

I bought mine from what I thought was a reputable marine aquarium equipment supplier. I will calibrate it to 35 ppm and do my best to keep it stable.

 

[Randy Holmes-Farley Verified Community Expert]

Yes, that is why. It will work fine when calibrated to 35 ppt, but not ro/di. :)

 

[RobertK]

Yes, that is why. It will work fine when calibrated to 35 ppt, but not ro/di. :)

Good to know. Thanks Randy!

 

[Randy Holmes-Farley Verified Community Expert]

You’re welcome.

Happy Reefing!