Two Little Fishies AccuraSea Seawater Reference and Calibration Solution AccuraSea� Seawater Reference Solution is the only calibration solution of its kind made from real seawater. It�s pure natural seawater micron filtered and UV sterilised then adjusted with reverse osmosis water to match the standard S 35 or 53 ms/cm at 25�C (77�F). Use it to accurately calibrate seawater refractometers and to check the accuracy of swing-arm hydrometers.