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There are different kinds of refratometers, Seawater, Saltwater and clinical.
But a note. There is no such thing as a hand-held refract for Seawater. The closest is salt water. There has only been one of any kind of refract made for only seawater, a digital fiber optic by Misco and it was discontinued and it was ~$600. There are also many other kinds of refracts, sugar, Alcohol, antifrezze etc. Some companies say for seawater but they are still salt water refracts. There are some very expensive digital refracts that can test for almost anything you can use a refract for to include seawater.