Almost all refractometers available to the hobby are designed for brine or saltwater. These will read approximately 1.5PPT too high, i.e if the refractometer reads 35 PPT the true reading for seawater would be around 33-33.5PPT. As a result of this, D-D produced a bespoke refractometer specifically designed to measure seawater

Click here for more information on specific gravity and its measurement.