floating hydrometer

[nmonks]

Oh boy, this again!

Floating hydrometers* can be perfectly adequate for brackish water tanks. I can see no advantages to using a refractometer on a brackish water aquarium, since all brackish water fishes can tolerate changes in salinity way beyond the margin of error of a hydrometer.

But hydrometers do need to be used properly. As Boxermom mentions, you need to put them in a flask or jar of completely still water. This is how they're used in labs. Putting a hydrometer into an aquarium with all its water currents won't work. Next, you need to use the hydrometer at its calibrated temperature. The more you're off that temperature, the more off the reading will be. This is because the density of the instrument has a 1.000 level only in freshwater at its calibrated temperature. If the water is too warm, the hydrometer will act as if it is too heavy, and it will underestimate SG (I hope I have the right way round!). Anyway, most aquarium ones are calibrated for 25C.

We've had this thread before, right down to links to other forums where people discuss the errors in refractometers (which can be wild). All consumer-grade instruments are compromised to some degree, and all depend on the user using them properly.

Cheers, Neale

*I'm personally not wild about the plastic swing arm models. The hinge seems to be another level of compromising, and over time salt and lime build-ups interfere with them. Simple floating hydrometers just have "fewer things to go wrong" but I'm sure a properly made, well maintained swing-arm model should be accurate enough for a brackish water tank, too.

[tanhql]

OK, I've just cave in and bought a refractometer off eBay.
http://cgi.ebay.com/Salinity-Refractome ... dZViewItem
does brand/price matter?

[nmonks]

Yes, price/brand does matter. As monsterpuffer said:

Lab grade hand-held refractometers can run up to $1K. Counter top refractometers are usually in the $5-6K range. Both are used on many different liquid types.

Obviously, if $50 refractometers were completely accurate, then scientists wouldn't be buying instruments costing hundreds of times more. So a cheap refractometers is a compromise between cost and accuracy. Simply because a refractometer has more numbers on the scale doesn't mean it is any more accurate than a hydrometer.

Also be sure and check out this thread pufferpunk found. But, as I've said before, for brackish water fish the level of accuracy doesn't matter. A GSP or mono or scat can adapt from fresh to salt water within an hour, and water change that makes an SG change of 0.02 or 0.03 on the SG scale couldn't make the least difference.

Cheers, Neale

[Phaedrus]

Be that as it may, I still wouldn't trade my refractometer for a swing arm or floating hydrometer based on convenience alone.

[nmonks]

Which is fine. I'm all for people using whichever one they want. Refractometers are certainly fun toys to play with, no question!

I just think it is unhelpful to automatically tell aquarists to ditch the $5 hydrometer in favour of a $40 refractometer when keeping brackish water fish. I'd sooner they spent the money on something more useful, like a copy of Baensch's Aquarium Atlas or the Aqualog pufferfish book. Often saving money is important: In the UK, refractometers are less widely sold and more expensive, typically around £40-50 (~$70-90). Hydrometers though are as cheap here as anywhere else.

Using a floating hydrometer is perfectly adequate for brackish water fish, and very easy to use properly (and much easier to use than a refractometer). I just think it's important that people are told the choices.

Cheers, Neale

[Boxermom]

Do floating hydrometers come with good instructions on proper use (i.e., calibrated to the correct temp, etc.)? Or is that something we need a sticky on?

[nmonks]

I have no idea. Mine didn't come with any instructions at all, just a carboard box to keep it in. It does have its calibrated temperature -- 75F -- printed on the stem though. That's really the only thing a buyer should look out for, what the calibrated temperature is. Oh, and it's still in use after 20+ years, and still sits in its cardboard box between uses so it doesn't get damaged. If you clean and protect these things, they will literally last a lifetime.

Cheers, Neale

[Boxermom]

That's what mine came in, too, just a thin box with no instructions. We've never used it though, I went and bought refractometers. Simple minds need simple toys.

[tanhql]

a few days ago, i used the hydrometer and the sg was 1.006
(calibrated with distilled water, showed 1.000). but i broke it after that reading. today, i received the refractometer, calibrated to 1.000 with distilled water, and the sg was also 1.006.

but, i have some difficulty reading the refractometer. when i read it against very strong white light bulb, and it showed a 'rainbow' at the boundary between the blue and white. when i decreased the light intensity, the rainbow became blue, affecting the reading with a 0.001 difference. which reading should i take? (i took the rainbow-less reading when calibrating and when taking sg of the tank)

[Boxermom]

Mine says read the blue.

[Peregrine]

Just be consistant...1.001 either way wont matter...
BW by its nature fluctuates depending on the season..I doubt such a minor difference would cause any problems

[Phaedrus]

did your refractometer come with any instructions?

[tanhql]

did your refractometer come with any instructions?

it does, but it just says the normal procedures for using the refract. i've used refracts in my school lab before and there wasn't such a problem (then again, they were those $5000 type)

i've decided to change the light source to a not-so-bright one in my house, which doesn't show the rainbow.

[TheSmJ]

Am I the only one with a swing arm type hydrometer that’s 100% accurate against both tap water and my LFS's $200 refractometer?

It's an Instant Ocean brand if anyone is curious, and about 2 years old.

[Peregrine]

Am I the only one with a swing arm type hydrometer that’s 100% accurate against both tap water and my LFS's $200 refractometer?

It's an Instant Ocean brand if anyone is curious, and about 2 years old.

I have the same one as well...I always found bubbles to be a constant concern...I prefer my refractometer...but that is a personal choice...I just find it easier to read, easier to clean, easier to calibrate...in essence easier, it is just what I have gotten used to.
Whether it is more accurate...that is a whole other argument...my thinking is more inline with Neil's statements.