yup, I fully agree with naesco. While hydrometers can be far from accurate, they are almost always very precise; That is, they are off by the same amount, in the same direction, every time. For example, on a sample of known salinity of 1.025, my hydrometer reads low at 1.018; so I just aim for 1.018 all the time. I have had my hydrometer for three years and recalibrate it every few months but it has yet to change. You also have to keep the things stored in freshwater; I keep mine in a tote that I store topoff water in.
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