You can calibrate it against a refractometer... I usually recheck mine every few months, and they are still 'dead on' for about a year...been doing this for many years. I give them a quick rinse with fresh water after each use...much easier and quicker to use than refractoYou cannot calibrate a hydro. If crud builds upon the float its registering as less. I think they are useless and completely unreliable.