Originally Posted by severum mama
Happy to hear it... we also have a Sybon. You're talking like 20 miles over my head so I'm satisfied that you're satisfied. :p
Not sure what part is confusing, but let's see if I can say this different...
Calibrating with pure RO
water will make sure your refractometer reads 1.000 when the water is at 1.000. But refractometers - even though they're far more accurate than most hydrometers - can still be off. In a perfect world, if you calibrated your refractometer to any solution with a known salinity, it would read perfect for all other salinities.
But the world ain't perfect, and there's nothing that says a refractometer has to read linear - in other words, as you move away from that calibration point, the reading may be higher or lower than reality.
Since the water we're testing isn't at 1.000 specific gravity, it's better to calibrate with a solution that is closer to what we keep our tanks at. That way, any linearity error in the refractometer is minimized. If you calibrate your unit with a 1.026 solution (the 53.0 mS solution I mentioned - can be bought at most places that sell the Pinpoint brand conductivity/salinity meters), and your water in the tank is 1.026, then you're pretty much guaranteed no error.
Like I said, I'm happy with the Sybon... but even with it, when I calibrate at 1.026, and then check 1.000 with RO
water, I'm about .001 high.