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How off is your hydrometer!


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I have been maintaining

my nano reef at 1.023sg

75 gal FO 1.019

And my sons pico fowlr at 1.019

all with a hydrometer. I have 2 of them and they always give the same reults. One of them is only 1 month old. I always rinse them after testing and boy are they off.


Today i bought a refractometer at my LFS. They were showing me how off these really are right in the store. They calibrated it and showed me how it works. I came home and tested all 3 of my tanks.


nano 1.026

75 FO 1.021

pico fowlR 1.021


Im really glad I bought this.



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I know my FOWLR is sitting at 1.024 and i maintain that with my hydrometer that consistently reads 1.018. it works well enough even if i cant shoot for a diff SG.

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mine reads 1.022 so i added more salt came out to 1.023ish, had a guy measure it with refractometer ? or was it chemical, he said it was a bit high, 1.026 so he said my meter was off by 4...


oh ya mine is the deep 6 by coralife


oh ya again, i heard you should measure at 80*F?..

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IIRC you have to use the hydrometer at the correct temperature.




I believe that to be incorrect. the two that I have are Seatest and instant ocean swing arm hyrometers. These are no supposed to require temp correction.


However they do suck at measuring sg.


I don't think that the standard corrections would apply to these 2 hydrometers.

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Although temp DOES matter, due to thermal expansion, which affects density (Specific Gravity) but the effect is so miniscule, it doesn't matter for our purposes.

Volume after Expansion= (Coefficient of expansion)(Volume original)( Delta T)

So if you have a given volume of say, 100, and the hydrometer is meant to read at 75 and your tank is 81, then the percent error is... .126%... so the hydrometer would read roughly 1.026 instead of 1.025. But Hydrometers are so inaccurate due to other reasons (miss manufacturing, bubbles, etc) temp differences are negligable.

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