measuring specific gravity

[CoralKing]

I've tested my specific gravity with a refractometer, a swing-arm hydrometer, and a floating glass hydrometer, and have different readings from each. The refractometer indicates the s.g. of the tank is around 1.023, while the swing-arm indicates it's around 1.019! The floating glass hydrometer indicates an s.g. of about 1.020. Does anyone know of a way I could determine the true specific gravity of the water?

Thanks

[lux_06]

trust the refractometer i say, swing arm ones unreliable IMO, i use a hydrometer.

very interesting to get such varied results too... scary cos i was just about to drop my tank to hypo levels without a refractometer so maybe thats a real bad idea rading this.

[seattlerob]

The refractometer should be the most accurate, but you have to make sure it's calibrated, or accuracy is out the window!

[DrBDC]

You can 0 out the refractometer with ro/di but it still can be off at 35 ppt. You need to get a hold of some pinpoint salinity monitor calibration solution to really get it the most accurate. Even w/o a good 35 ppt calibration solution, I'd trust the refractometer only calibrated with ro/di before I'd trust either of the other 2. FWIW I was off by .002 when I 0'd it with the ro/di so my 1.026 was 1.028.

[Pat_13]

Go with the refractometer, it is the most accurate of all three. Make sure it is calibrated.

[Dr.Maybe]

I agree I had a hydrometer that was crap. Trust the calibrated refracto.

[manderx]

You need to get a hold of some pinpoint salinity monitor calibration solution to really get it the most accurate.

FWIW I was off by .002 when I 0'd it with the ro/di so my 1.026 was 1.028.

is the calibration fluid actually 35ppt or is it maybe just something that has the same conductivity as 35ppt seawater? unless it says you can calibrate a refractometer with it, i wouldn't assume you could. or do you mean you calibrated the pinpoint and measured the tankwater, then tested the tankwater with the refractometer to compare?

[RichConley]

<a href=showthread.php?s=&postid=7802106#post7802106 target=_blank>Originally posted</a> by seattlerob
The refractometer should be the most accurate, but you have to make sure it's calibrated, or accuracy is out the window!

Completely disagree.

If the floating bulb hydrometer is a decent one, it'll be much more accurate than the refractometer(a $50 floating bulb hydrometer is MUCH more accurate than a $50 refractometer). Swingarms ARE crap though.


What make/model is the float?

[DrBDC]

<a href=showthread.php?s=&postid=7803145#post7803145 target=_blank>Originally posted</a> by manderx
is the calibration fluid actually 35ppt or is it maybe just something that has the same conductivity as 35ppt seawater? unless it says you can calibrate a refractometer with it, i wouldn't assume you could. or do you mean you calibrated the pinpoint and measured the tankwater, then tested the tankwater with the refractometer to compare?

You know it may be just the same conductivity but I did calibrate backward from the tank water so I'm safe. Didn't think about that. I'll have to go drop some on the refractometer and see.