Refractometer?

[an411]

Do they need to be calibrated and if so how is this done?

 

[Randy Holmes-Farley]

Yes. While manufacturers often claim that pure fresh water is adequate, the ones sold to the hobby can be so cheaply made that that is not always true, and calibration with a seawater standard is better. This article has more:

Refractometers and Salinity Measurement
http://reefkeeping.com/issues/2006-12/rhf/index.php