Refractometer at Zero can still be out of calibration at higher-end readings.

If a refractometer is tested with distilled water and consistently gives 32 degrees as the freeze point, would a certified calibration still be necessary? I am trying to understand how the refractometer could be outside of its specifications and require a certified calibration if it is consistently giving the expected reading with a control fluid such as distilled water. Also, how often does MISCO recommend that the digital refractometer receive certified calibration?

It is possible for a refractometer to read correctly with respect to a distilled water value but be out of calibration on the higher-end (span) of the refractometer. You can tell a better quality refractometer since it has a method of zero-setting to distilled water and also calibrating the span of the instrument using NIST traceable calibration standards. Calibration is the best way to maximize the accuracy of a refractometer. We recommend zero-setting the refractometer at least every day and checking/or setting the span about every month. We also recommend that a refractometer be sent in for an annual certified calibration.

See our Importance of Refractometer Calibration page for more information.