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Will RO/DI water work to calibrate a refractometer?


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Refractometers are rarely accurate over their entire scale. So while RO/DI will work for calibrating at 0ppt, that doesn't help much when measuring saltwater. You could be off by several ppt when measuring tank water. You want to calibrate at the value you'll be targeting. Pick up a bottle of 35ppt calibration solution, and your refractometer will always be accurate at 35ppt.


Refractometers and Salinity Measurement


Refractometer calibration fluid

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many people say no it will be off. I always calibrated mine with DI water from grocery store to zero. I also have 35ppt calibration solution to check and it has always been dead on 35 using Distilled water checking it with the calibration fluid. Its just another thing in this hobby that gets a little out of hand.

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many people say no it will be off. I always calibrated mine with DI water from grocery store to zero. I also have 35ppt calibration solution to check and it has always been dead on 35 using Distilled water checking it with the calibration fluid. Its just another thing in this hobby that gets a little out of hand.


This may work in some cases, but there are plenty of people who have found their refractometers to be off by several ppt when calibrating with RO/DI or distilled water. Without a reference, you're just guessing at the accuracy at 35ppt. For something as important as salinity, I think it's worth the $7.

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Having been bit by this particular question, I'll weigh in - as penfold2 stated pure water (RODI/distilled) is valid for getting your refractometer calibrated to read water that has no salt present, i.e. "0". After that point the accuracy when reading at ~35ppt is greatly dependent on how tight the tolerances are on your particular unit. Using a calibration standard solution allows you to compensate for this unknown by doing the calibration well within the range you're planning to be testing at.


I've had one refractometer that couldn't be trusted even with solution after a few months of use. Another I find I only need to check with DI/35ppt solution when feeling paranoid - but haven't had to actually adjust it in over 2 months after the initial calibration. Same store, same price range. Go figure. :rolleyes: Point is, your mileage may vary. A lot.


THAT SAID - I've had mixed results with Pinpoint's 35ppt solution that's commonly sold here... one vial was consistently off by 2-3ppt, another that I think I keep tightly sealed but crept up 1-2ppt over a month or two of once-a-week use. If practical in your case I suggest to just take your own refractometer in for a sanity check every once in awhile at a local store - around here that saves about $12.95 a vial.


Product idea - does any online vendor sell packs of single-use eyedroppers filled with 35ppt reference solution? That might avoid the issues I've run into.

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OP asked RO/DI vs distilled for calibration...


OP, yes it will work. people do it all the time and it's a good starting point. Is it perfect? no. No worse than using a dumb swing arm hydrometer. If you're keeping a few softies, and fish it's fine for now as long as it's consistent.


Eventually you may want to get calibration fluid or make your own. Is it necessary? Depends on you. It's a cheap investment compared to everyone else in this hobby tho.

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You can calibrate it however you want, but 35ppt calibration fluid is $7 with $3 shipping. The answer seems obvious.


If you can't wait for the fluid to arrive then another way to calibrate is to buy a 2.5 gallon container of the natural seawater (SeaPure or equivalent) and test it. My testing showed almost exactly 35ppt.


There will be errors when not using calibration fluid but the error rate depends on the type of refractometer. The BRS refractometer comes with calibration fluid and a big orange card telling you to ignore the instructions and use the fluid to calibrate.


Also make sure you have a quality refractometer that is temperature correcting, and give it 1 minute to temp acclimate before using the reading.


It's your money and tank. Everything is always fine, right up until it isn't. :)

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Glad to have brought up a topic of substance.


Thanks for the responses. Will do a mix of RO/DI, distilled, and calibration fluid and report back on the results.

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  • 1 month later...

I know this is an old post... but I would be very interested in seeing what you've uncovered regarding this.


I measure SG with my IO swing arm hydrometer and an ATC refractometer (originally calibrated with RODI) and then averaged the two readings... Hydrometer was always 1.020 and refractometer was always 1.030. That led me to believe my S.G. was around 1.025. That meshed with my salt's expected Ca value as well... using RSCP salt... my Ca was always a little low (~400-420) in freshly-mized SW.


When I made my own 35ppt calibration standards ,using Reefkeeping's article, it revealed (not to my surprise) that both of my metrologies were erroneous. My conclusion was: hydrometer was reading -4ppt and the refractometer was reading +4ppt! <_<



Well, 'repeatable' WAS the name of my game... but after making these calibration standards, I can now repeatably make 'precise' water :)


Just make 3.65% (by weight) NaCl:RODI solution for your refractometers, or a 3.71% NaCl:RODI solution for your hydrometers. Incase you wanted to try it (and didn't want to read the above article for 1+ hours)...


1.3394 Refractive Index, Refractometer calibration fluid:

Mix 3.65 grams Morton's Iodized Salt (NaCl) and 96.35 ml (or grams) RODI to make a ~1.026 S.G. calibration standard.


1.0266 Specific Gravity, Hydrometer calibration fluid:

Mix 3.71 grams Morton's Iodized Salt (NaCl) and 96.29 ml (or grams) RODI to make a ~1.026 S.G. calibration standard.


Notice the two solutions are VERYY similar. So much so that, in a pinch, I've used one to substitute for the other with no discernible differences. If you don't have access to a decent scale (resolution of 0.1g or better), then just mix 1 teaspoon of salt in 2/3 cup RODI or distilled water. 2/3 of a US cup should come out to ~157ml and the results are going to give you a great idea of how 'calibrated' your metrology is.


Good luck!


Edit: Actual Dr. Holmes-Farley/ReefKeeping's Homemade Calibration Standards Article (the first link is more refractometer-oriented).

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I want to add that, I first calibrated mine with ro/di, then checked it with ro/di solution and it was spot on.

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I calibrated mine with RO/DI. The instructions say to use distilled or RO/DI so I doubt it's that big of an issue. When I got some seawater from the beach and checked it it was 1.026 so I know it's accurate.

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Calibration fluid is super cheap and will last you a long time, and you'll know it's accurate.. A refractometer gets less accurate the farther you get away from the calibration point. So calibrate to what you want to test for..

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