refractometer calibration

[rayjay]

If you have a seawater refractometer then calibration with water should provide a good reading at normal salt water ranges. However, it is still recommended that one use calibration fluid.
If you have a salt water refractometer (NaCl) which is most common in the hobby, there will be an error at normal salt water ranges if calibrated with water.
Randy Holmes-Farley on Refractometers and Salinity