calibrating refractometer

[goochesfish]

I have a little dilemma in calibration. I bought a calibration solution at 35ppt. If I calibrate the refractometer using that solution and then check it against zero tds RO/DI water, they are off by 2ppt. Which is more accurate? Which should I use to calibrate?

[allsps40]

Use the calibration fluid. Even Ro water is not 0ppt salinity. Found that out after I used Ro to calibrate my refractometer and SG was a bit high 1.028 when I thought it was 1.025.

[goochesfish]

yes, that's what happens to me too. So i will use the calibration fluid. Thanks!

[The Punisher]

I'd second going by the calibration fluid. The calibration fluid is a know variable that will give the best results.

[jflick345]

Most refractometers actually come with instructions to NEVER calibrate them to 0. Use the solution, save yourself some headaches.

[greenbean36191]

The issue isn't that your RO/DI water has a salinity of 2 ppt, but that the refractometer is off by 0.06 units per ppt. That inaccuracy adds up the further you get from the calibration point. If you calibrate it to 0 and then try to read at 32 ppt it will be off by almost 2 ppt. If you calibrate it to 35 ppt and try to read it at 32 ppt it will only be off by less than 0.2 ppt. A good rule of thumb is that if you're only using 1 calibration point, always choose the one closest to the values you want to measure.

[sneeyatch]

A good rule of thumb is that if you're only using 1 calibration point, always choose the one closest to the values you want to measure.

Agreed - calibration solution.