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From Reef Central

Over on the chemistry forum the consensus is that refractometers should be calibrated with a solution that has a salinity close to that of natural sea water. A lot of people are finding out that even though they calibrated their refractometer with distilled water, when they tested a calibration fluid their measurements were way off.

I'm one of those people. I've been calibrating my refractometer with distilled water since I got it. After reading those posts, I recalibrated it with a calibrating solution. My refractometer was off by 0.005. My reef's actual salinity was 1.021 when I thought it was 1.026. Quite a big difference.

That being said, Randy has written an article detailing how to make your own calibration solution using distilled water and table salt.

Pinpoint Salinity Calibration Fluid