Refractometer calibration: RODI/Distilled or saline? Can't use both.

[acroholic]

Most refractometer manufacturers tell you to use distilled/RODI water to calibrate their Refractometers. Places like Bulk Reef supply have a warning telling you that you must use a reference solution, like the American Marine Pinpoint Reference Solution (SG 1.026) to calibrate refractometers, and specifically state on their website that if you use distilled water to calibrate you will get erroneous readings.
http://www.bulkreefsupply.com/Salt-Mixes/c122/p708/Refractometer-for-reading-salinity-w/-Free-Calibration-fluid/product_info.html">http://www.bulkreefsupply.com/Salt-Mixes/c122/p708/Refractometer-for-reading-salinity-w/-Free-Calibration-fluid/product_info.html</a>

I used distilled, but Ares was kind enough to let me calibrate using his Pinpoint solution last night, and it turns out my 1.025 reef water was actually 1.022.

My Refractometer now reads below zero with distilled water in it.

Both methods of calibration cannot be right, so which is it? Is it what the manufacturers say, or is it what Bulk Reef Supply says?

 

[ralph atl]

pinpoint 100%. that's what Randy says, so that's what I do. It has to do with the imperfect graduation rate from 0 to 35. So, if you use 0, and the refractor is off a bit, by the time you get to 35, it's off. If you start at 35, then it's 35.

 

[glxtrix]

Pinpoint. There was a big thread somewhere on RC about it. Ralph pretty much summed it up though

 

[lilrobb]

when you calibrate something, the further you get away from the reading which was used to calibrate it, the more the refractometer's margin of error causes a deviation from that mark.

Ergo, if a refractometer has a deviation or margin of error of 1% per .001, that means that if you calibrate it to 1.000 with distilled water, that reading 1.025 might be as much as 25% off. Granted, this is a highly exaggerated situation for the purposes of explanation.

Therefore, what you want to do is calibrate the device using a reading that is as close to what you will usually be reading with the device, so if you are usually reading 1.025, then you should calibrate with a 1.025 solution, so if you read water that is 1.024, even with the ridiculous margin of error described above, you will only be 1% off at most....

Of course, the problem is that in order to do that, you need to prepare a calibration solution that has been tested using a "reference" refractometer. It's similar to how you sometimes set a clock according to a reference clock, such as an atomic clock. You have to have a trusted source.. .clock or refractometer, that you know to be exactly correct. The problem is that most people don't have access to such a refractometer.

So, in the end, just calibrate it with rodi water. None of them are as inaccurate as i described above, and the bottom line is that if your refractometer reads 1.025 and your seawater is 1.024 or 1.026, it's not going to hurt anything. The important thing is that your refractometer reads the same thing every time, because it's more important that you maintain seawater at a precise certain salinity than what exactly that certain salinity is.

amen

 

[kernelpanic]

pinpoint. You likely have a salt(NaCl) rather than seawater refractometer, as do most of us.
From Randy
"Fortunately for aquarists, the differences between a salt refractometer and a seawater refractometer are not too large. A 35 ppt sodium chloride solution (3.5 weight percent sodium chloride in water) http://reefkeeping.com/issues/2004-06/rhf/index.php">has the same refractive index</a> as a 33.3 ppt seawater solution, so the error in using a perfectly calibrated salt refractometer is about 1.7 ppt, or 5% of the total salinity. This error is significant, in my opinion, but not usually enough to cause a reef aquarium to fail, assuming the aquarist has targeted an appropriate salinity in the first place."

and

"
It turns out that this is a slope miscalibration in the sense that a perfectly made sodium chloride refractometer necessarily has a different relationship between refractive index and salinity than does seawater. This type of problem with a refractometer [B]IS NOT[/B] at all corrected by calibrating it with pure freshwater. If you have this type of refractometer, and it was perfectly made and calibrated in freshwater, it will ALWAYS read seawater to be higher in salinity than it actually is (misreporting an actual 33.3 ppt to be 35 ppt). "

Whole article:
[IMG]http://reefkeeping.com/issues/2006-12/rhf/index.php">http://reefkeeping.com/issues/2006-12/rhf/index.php</a>

I have offered to bring my pinpoint solution to meetings in the past. I can do so again if there is interest.

 

[kernelpanic]

Of course, if my tank looked like yours, I wouldn't care.

 

[atlorange]

Does anyone know if any LFS's sell the pinpoint calibration solution?

 

[acroholic]

I don't think anyone has it on the shelf, but I think Tim at Keen can order it for you thru Premium Aquatics. I called around for it a few days ago myself.
Dave

 

[acroholic]

KernelPanic;350524 wrote: Of course, if my tank looked like yours, I wouldn't care.

Hi Kevin,
Thanks for the kind words. I have had a bit of tissue recession on the base of a couple of my acroporas, and I am eliminating causes. No Redbugs, AEFW, normal CA, KH, Mag, low NO3, no phosphates, and I restarted using GAC, and I do regular water changes. I don't know if hyposalinity could affect them, but now that is corrected as well.

I think it was either low salinity, not using GAC, the corals themselves shading their own lower parts, or I have an unknown predator like a crab.
Dave

 

[jmaneyapanda]

I was so steadfastly opposed to this principle, until I did the research myself. Man, it sucks that the refractometer we buy are not made for testing seawater. The slope and variation is enough to cause issues like you ahve just seen. I bought a VERY pricey refarctometer years ago, and either Im lucky, or they amde them different back then, but it reads very very close to zero with distilled after I calibrate it with pinpoint. Regardless, it has made me reconsider the ways I calibrate all my measuring devices (not only for the hobby, but also at the workplace). Remarkable, many of the tools I was using alos had problematic slopes to their functions, and required more attention to calibration.

 

[bobz]

Jeez! I was unaware of these issues! Thanks for bringing this to our attention. Looks like I will need to re-calibrate as well. But, at least my water is consistently off, just a question of how much now.
Bob

 

[acroholic]

jmaneyapanda;351110 wrote: I was so steadfastly opposed to this principle, until I did the research myself. Man, it sucks that the refractometer we buy are not made for testing seawater. The slope and variation is enough to cause issues like you ahve just seen. I bought a VERY pricey refarctometer years ago, and either Im lucky, or they amde them different back then, but it reads very very close to zero with distilled after I calibrate it with pinpoint. Regardless, it has made me reconsider the ways I calibrate all my measuring devices (not only for the hobby, but also at the workplace). Remarkable, many of the tools I was using alos had problematic slopes to their functions, and required more attention to calibration.

I guess this is one of those cases where calibration close to the value you need works best. So is a refractometer slope linear or non-linear? I had always assumed it was linear, unlike pH meters/controllers, which are non-linear.

 

[jmaneyapanda]

It is linear, but it may not be at the proper inclination. That is why when zeoed with RODI, the reading is off at 35 PPT, and vice versa.