Randy Holmes-Farley on refractometer calibration in Reefkeeping Online
Homemade Calibration Standards stated:
"It is widely believed that only pure water is required to calibrate refractometers. That fact is true of many refractometers, and is certainly appropriate for routine calibration, but it assumes that they were manufactured correctly and have not been damaged since manufacturing. As refractometers used by aquarists become less and less expensive (with some now selling for less than $30), there is every reason to believe that at some point they will no longer be accurate enough.
The only way to be sure that a given refractometer gives useful information is to check its accuracy in a solution similar to aquarium water. I believe that all refractometers should be checked in this fashion when first purchased, and again any time there is a reason to be concerned. For example, an aquarist might be concerned if an aquarium that had been running for years at a salinity of 35 ppt suddenly reads 39 ppt."
Also from Randy, from this:
Refractometers and Salinity Measurement comes this.
"For many refractometers used by reef aquarists, the manufacturer calls for pure freshwater to be used for calibration. With a perfectly made refractometer (that hasn't changed since its manufacture), that single point calibration at the end of the range (Figure 5) would be adequate, albeit not perfect. A better single point calibration might be performed in the middle of the range being used, and for higher accuracy, more than one calibrating solution would be used."
