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Hydrometers are not usually calibrated, so it is anyones guess...
I have one really cheap one that reads dead on, compared to my refractometer and a Deep-Six that is WAY off... The Deep-Six however is notoriously bad.
Important elements can be greatly effected by the salinity, so why risk it? Say your salt should read 1200ppt magnesium at s.g. 1.026 35ppt, but you are actually only at 32ppt (s.g. 1.024), now your mg is actually only 1097ppm. If you are actually at 28ppt (s.g. 1.021), now you only have 960ppm mg!
With a hydrometer, you just can't be confident. a refractometer is the best investment you can make in this hobby IMO.
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Some people say, "How can you live without knowing?" I do not know what they mean. I always live without knowing. That is easy. How you get to know is what I want to know. - Richard Feynman
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