instant ocean hydrometer...or me?

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aimsz88

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Nov 7, 2004
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hello,

i'm starting a new sw tank...so i started salting up my water and now i have a question. my tank is a 55 gallon, and i bought enough instant ocean salt for 55 gallons of water. now, given that there is sand in there, there is perhaps less than 55 gallons of water, right? so that means i don't use all of my salt.

well, i decided to be slow and conservative and probably dumped a little more than 75 percent of the salt so i could wait for the salt to dissolve and measure my SG slowly. funny thing is, this morning, i read it more than a few times, and its coming up 1.020...and i would suspect it should be way lower than that considering i didn't use nearly all of my salt. temp is steady at 78 and i swear that i can't be that retarded using the hydrometer. what's going on?

:confused:

thanks for any feedback...
 

OrionGirl

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Aug 14, 2001
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Hydrometers aren't 100% accurate. Your best bet will be to calibrate it against a known, or see if your LFS has a refractometer to calibrate it with. This will tell you how far off it is (a + or - amount) to give you the 'real' value. Being off by .004 either way wouldn't surprise me.
 

mogurnda

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Apr 29, 2003
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Two possibilities.
One: swinging arm hydrometers can be poorly calibrated. I use one, but have checked it with a refractometer to determine how far off it is.
Two: there are small bubbles on the arm, giving a spuriously high reading.

It's possible that it's correct, I suppose.
 

aimsz88

AC Members
Nov 7, 2004
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hrrrmmm

thanks to both of you for your quick advice...

nope, no bubbles. i'm a scientist of sorts and i know better than to have bubbles. :)

as for the checking against a known, i'm almost positive my local petco isn't going to have a refractometer. they didn't even know what bio-spira was last time i was there. i might be able to bring it in for a water test, though...i don't know if they check SG.

perhaps for now before i can get out to a farther lfs, should i buy a bobbing hydrometer and check against that one? i know it's like taking two variables with errors and checking them against each other..but who knows...

thanks again!
 

RTR

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Oct 5, 1998
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One on my 55s has a working volume of 42 gallons, the other 41. No 55 I've ever seen would hold even 50 gallons without any substrate or anything else. Huge difference between working and nominal volumes is pretty standard - magnified a bit in my case because I use deep substrates/plenums
 

koelling

Registered Member
Mar 29, 2005
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the bags of salt i buy (instant ocean scientific grade) are supposed to make 55 gallons of salt water. that means it will do a minimum of 55 gallons of salt water. i've noticed them doing more along the lines of 60 or so gallons at 1.022-1.023
 
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