Refractometer Calibration: the Myth and Risks of using RO/DI Water

[Pufferpunk]

http://blog.captive-aquatics.com/cap...sing-rodi.html

[jherrin215]

Good info. I'm fixing to order some calibration fluid

[lucidheights]

this is true, i was keeping my water at 31ppt for a long time because of rodi calibration.

[jmchzn123]

Originally Posted by Pufferpunk

http://blog.captive-aquatics.com/cap...sing-rodi.html

Just bought this one and it seems to solve the issues. I also emailed DD and they confirm this refractometer is different than the industry standards and is specifically designed for Seawater. ALL I KNOW IS PINPOINT MONITORS ARE AWFUL AND ALMOST DESTROYED MY 10 YR OLD REEF...I'm just saying

http://www.glassreef.com/review_d-d_refractometer.php

[dwd5813]

additional reading:

http://reefkeeping.com/issues/2006-12/rhf/index.php

[phaedrus1]

Hmmm... second article recommends DI water.
I have always used DI water to claibrate, so this has me wondering. Any one else have experiences to share?

[organism]

Originally Posted by phaedrus1

Hmmm... second article recommends DI water.
I have always used DI water to claibrate, so this has me wondering. Any one else have experiences to share?

Actually it recommends the opposite.

"Despite the fact that many refractometers sold to aquarists recommend calibration in pure water, such a calibration alone will not ensure accuracy for the reasons described above. So my recommendation for calibration is as follows:

1. First calibrate the refractometer in pure freshwater.

2. The second step in calibration should be performed at least once before relying on a refractometer to accurately measure the salinity of a reef aquarium. This step involves testing it in a solution matching the refractive index of 35 ppt seawater"

It was RHF's posts on here that made me decide to check the refractometer, if I'd gone by the "general wisdom" of ro/di calibration my tank would've ended up nuked for sure. So far every refractometer I've tested with the solution is .001 to .003 high, if you haven't calibrated with calibration fluid I can pretty much guarantee that you're reading high.

It's funny how if enough people say something it becomes common knowledge, that can lead to issues when experts like RHF get drowned out by bs.

[The Punisher]

I've always been amazed at the lengths people will go to in order to save $10. There's enough evidence against using DI water alone as a calibration fluid that I don't see any reason someone would argue for it's use, other than they just don't want to spend the $10, silly. I've yet to hear a bad story about someone messing up their tank because of a refractometer being off because they used a calibration fluid.

[fcmatt]

Originally Posted by jmchzn123

Just bought this one and it seems to solve the issues. I also emailed DD and they confirm this refractometer is different than the industry standards and is specifically designed for Seawater. ALL I KNOW IS PINPOINT MONITORS ARE AWFUL AND ALMOST DESTROYED MY 10 YR OLD REEF...I'm just saying

http://www.glassreef.com/review_d-d_refractometer.php

that looks just like a sybon refractometer with a whole lot more marketing tossed
in. if i had to guess sybon made it for them to sell. a couple of small changes but
boy they look alike.

[snorvich]

Originally Posted by Pufferpunk

http://blog.captive-aquatics.com/cap...sing-rodi.html

Amen. Do NOT use RO/DI water to calibrate.

[organism]

Originally Posted by The Punisher

There's enough evidence against using DI water alone as a calibration fluid that I don't see any reason someone would argue for it's use, other than they just don't want to spend the $10, silly.

I think it's mainly due to 9 out of 10 people saying ro/di is ok because "that's what I use on mine and it's fine". If the majority of people tell you what you want to hear, and RHF and other qualified people are way in the minority, then it just doesn't go over very well

What I don't get is why the instruction for the refractometer said to specifically use ro/di and that it was ok!

[fcmatt]

Originally Posted by organism

I think it's mainly due to 9 out of 10 people saying ro/di is ok because "that's what I use on mine and it's fine". If the majority of people tell you what you want to hear, and RHF and other qualified people are way in the minority, then it just doesn't go over very well

What I don't get is why the instruction for the refractometer said to specifically use ro/di and that it was ok!

without knowing which brand he bought it is difficult to guess why. maybe it
was a refractometer designed to measure plain salt/brine water.. and not one
made for sea water?

sybon, when buying their kit, gives you a small bottle of 35 ppt.

and to take things a bit farther.. i feel a person should buy two different brands
of 35 ppt test solution to calibrate their kit to be sure. Only using one, that
was poorly stored or very old, could lead to being off just the same. I tend
not to trust just one point of information....

[organism]

Originally Posted by fcmatt

without knowing which brand he bought it is difficult to guess why.

I forget the brand, but it was one of your standard blue saltwater aquarium refractometers from an LFS. Do they tend to come with ro/di instructions, or do some recommend calibration fluid?

[jeff@zina.com]

My refractometer says to use RO/DI or distilled water to calibrate. Of course, I can't read instructions, so I've been using claibration fluid all along.

Jeff

[fishome25]

a little off topic, but does anyone understand why we need to recalibrate so often? it seems like mine is always off.

[allsps40]

I agree with the article on this. I calibrated my refractometer with RO wand had issues. Got Pin Point calibration fluid and my was salinity was 1.030 in my tank.

[Rybren]

Originally Posted by fishome25

a little off topic, but does anyone understand why we need to recalibrate so often? it seems like mine is always off.

Mine was like that for the longest time, then I realised that I was exposing it to temperature extremes. I now keep mine inside the tank stand (at a constant temp) and the calibration holds for months.

[reefinmo]

This is all interesting to me. I'm no chemist (very far from it in fact), but does it really matter what kind of refractometer you use? I know every measuring tool like this can be off, but once you know how much its off, can't you just adjust for it?

I'm asking because because I'm using a different kind of refractometer that reads brix. It's used for measuring the specific gravity or wort (unfermented beer). Making beer is one of my other hobbies and I already had this refractometer. I always make sure by brix reading is 6, or just a little over, which translates to a 1.024-1.025 sg. Is there a reason to buy one for measuring saltwater? I didn't think it would make a difference as long as I knew the conversion to use?

[Mike31154]

Interesting info about the brix refractometer. Wonder if this works the other way around, can I use my SG refractometer to measure brix of my wine must using a conversion table?

Back to subject at hand. So if I zero my refractometer with RODI, measure my tank water at 1.025, then purposely recalibrate up one division above zero using RODI and again measure tank water, if it now reads 1.026, is it not logical to assume that the scale on my refracto is linear and the tank water reading is ok? Yes, I'm too cheap to buy calibration fluid but my system is doing fine using RODI to calibrate. I've also compared refracto reading to 3 other hydrometers, including a glass floating one, which IMO is still one of the best ways to measure SG provided you compensate for temperature.

[InvaderJim]

This has really got me thinking. Who knows how off my refractormeter is.

[organism]

bump

[disc1]

Originally Posted by organism

What I don't get is why the instruction for the refractometer said to specifically use ro/di and that it was ok!

Because it is the best they can do with the instructions. The manufacturer sells you an instrument that measures refractive index, but they don't know what purpose you might be using it for.

For example, you buy a refrac and use it for a brackish tank running at 1.010 SG. Now, if the scale is not linear to seawater then using a 35ppt solution to calibrate would be just as bad as using RODI. So since there are so many different things a person might be doing with their refrac, they give the most general instruction that they can, which is to zero the instrument with pure water. They know that will get a person going and it will leave an two of their same brand of instruments comparable.

But, we can do better because we DO know what we will be measuring with it. For example if we want to measure 35ppt seawater, then we calibrate to 35ppt. If you want to use it for beer or wine making, then calibrate it to something that has a similar refractive index to the number you are looking for. Note I said the same refractive index, NOT the same specific gravity. For example you can calibrate a refractometer for 35ppt seawater using a 36.5ppt solution of table salt. It has a different concentration, and a different specific gravity, but the refractive index which is after all what a refractometer measures will be the same as 35ppt seawater.

If you want to use more than just one number on the refrac, then calibrate at two points. Since you only have one single adjustment on a refractometer, you can't do this manually. You're going to have to do some math. (I know scary huh?) But if your refractometer measures 35ppt as 35ppt and with the same calibration setting also measures pure water at zero, then congratulations your scale is linear to seawater and you can use that refrac at any salinity number and you can calibrate it all day long by zeroing it out to RODI water or using any other known value. But ONLY after you have confirmed that it reads both accurately at the same calibration setting.


If you want to be cheap and not buy the solutions, or if you are like me and like to measure stuff yourself and generally don't trust the calibrants to remain true over their shelf life, then you can calibrate a refractometer using a solution made with 3.65g of table salt (iodized is fine) and 96.35g of pure DI water.

HTH

[nrbelk]

Here is a related question. Awhile ago I bought a refractometer and it came with a calibration solution in a drip bottle (like what you get with the API test kits). I calibrated it and all was dandy until I looked for the calibration bottle again a few hours (maybe even the next day....?) and it was still uncapped.....

Can enough liquid escape the small opening at the top of the bottle to change the results enough to worry about?

[disc1]

Originally Posted by nrbelk

Here is a related question. Awhile ago I bought a refractometer and it came with a calibration solution in a drip bottle (like what you get with the API test kits). I calibrated it and all was dandy until I looked for the calibration bottle again a few hours (maybe even the next day....?) and it was still uncapped.....

Can enough liquid escape the small opening at the top of the bottle to change the results enough to worry about?

Yes it can. Mostly depends on temperature and humidity. It can even happen from a closed bottle given enough time. That's why I make my own.

[AcroporAddict]

For those not wanting to mess with an ocular refractometer, here is one of the best things you can buy for your reef tank: the MI Digital Seawater Refractometer. Takes human error out of the equation. I've had mine for a couple years. One of the most useful pieces of equipment I own. About $110 online. Hanna makes one as well, but it looks like a rebranded MI or vice versa, but the Hanna is about $160. Go figure. Name premium maybe.



http://www.milwaukeetesters.com/MA887.html