Refractometer calibration discrepancies

[ryshark]

I have 2-bottles of the same brand calibration solution with 2-different readings, they are 2ppt apart. Which would you trust below? I’m starting to think I may stop buying refractometer calibration solution and just use RODI.

Bottle 1 - expires 2022. When I calibrate my refractometer with RODI at 0 and then use this solution it shows dead on at 35ppt, which is where it supposed to be. 35ppt.

Bottle 2 - brand new - same brand expires 2026 shows at 33ppt when my refractometer is calibrated to 0 with RODI.

Which bottle would you trust? I’m thinking the older Bottle 1 which matches the RODI calibration dead on at 35ppt.

 

[mtfish]

Can't really answer your question for sure, but I do have more questions. Is the refractometer temperature compensating? Are you using the same pipette for all of the samples? If the standard bottles have not been contaminated and have not had any evaporation, I would trust both bottles. I would also standardize at 35 ppt and not 0 since this is what you are looking for in your tank. There is not much difference between 33 and 35 ppt in one pipette of water, especially if you might have contaminated the sample with RODI water.

 

[ryshark]

Can't really answer your question for sure, but I do have more questions. Is the refractometer temperature compensating? Are you using the same pipette for all of the samples? If the standard bottles have not been contaminated and have not had any evaporation, I would trust both bottles. I would also standardize at 35 ppt and not 0 since this is what you are looking for in your tank. There is not much difference between 33 and 35 ppt in one pipette of water, especially if you might have contaminated the sample with RODI water.

No pipettes used, the bottles have their own dropper like eye drops. The RODI came straight out of the RODI tube, it didn’t even touch a container. I did the test a few times with each and every way it was a 2ppt discrepancy. I did all the test within 10-minutes and I did let the temperature adjust on the glass prism for over 30-seconds.
The reverse way I did it was to calibrate the refractometer at 35ppt to one bottle and then Put drops on the clean glass prism from the other bottle and it was always 2ppt difference.

 

[ryshark]

I know 2ppt isn’t that much, unless you’re already at the minimum or maximum level in the display, then that discrepancy can lead to problems. Especially with acropora.

 

[Shirak]

Do you have a decent scale? You could make Randy's refractometer calibration solution. That's what I use.

 

[ryshark]

Do you have a decent scale? You could make Randy's refractometer calibration solution. That's what I use.

I do have a digital food scale. I may try that.

 

[Shirak]

Link here http://reefkeeping.com/issues/2004-06/rhf/index.htm

 

[v3v3]

+1

Do you have a decent scale? You could make Randy's refractometer calibration solution. That's what I use.

 

[ryshark]

Link here http://reefkeeping.com/issues/2004-06/rhf/index.htm

I also saw this article by @Randy Holmes-Farley saying that the first step to calibrating is to use RODI, etc. And the second step would be to use a calibration solution at 35ppt. I'm happy to pay $7 for 2-ounces of saltwater to avoid the hassle of making my own solution....if its accurate. But I feel I need a third source to verify which bottle is more accurate. I think a discrepancy of 1ppt is reasonable, as mentioned in the article. I'll take both bottles to a trusted LFS and see which one is closer to the 35ppt on their calibrated refractometer.

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

 

[Randy Holmes-Farley Verified Community Expert]

I have 2-bottles of the same brand calibration solution with 2-different readings, they are 2ppt apart. Which would you trust below? I’m starting to think I may stop buying refractometer calibration solution and just use RODI.

Bottle 1 - expires 2022. When I calibrate my refractometer with RODI at 0 and then use this solution it shows dead on at 35ppt, which is where it supposed to be. 35ppt.

Bottle 2 - brand new - same brand expires 2026 shows at 33ppt when my refractometer is calibrated to 0 with RODI.

Which bottle would you trust? I’m thinking the older Bottle 1 which matches the RODI calibration dead on at 35ppt.

RO/DI calibration is supposed to be off if you are using a brine refractometer (which are most in the hobby) rather than a true seawater refractometer.

So going with the one that matches DI isn't necessarily best.

What refractometer are we talking about, and what brand calibration fluids?

 

[ryshark]

RO/DI calibration is supposed to be off if you are using a brine refractometer (which are most in the hobby) rather than a true seawater refractometer.

So going with the one that matches DI isn't necessarily best.

What refractometer are we talking about, and what brand calibration fluids?

Hi Randy,
It’s the BRS brand refractometer and solution. I’m going to take both bottles today to a LFS which has nice reef tanks to see how they match up. If the newer bottle matches his, then it was probably an evaporation issue with the older bottle.

 

[ryshark]

I took both bottles to the LFS and they confirmed my original suspicion. Bottle 1, my old bottle which expires in 2022 is accurate and the new Bottle 2, is off. On his calibrated refractometer he read bottle 1 at 1.025 and bottle 2 at 1.023.
This also means that evaporation wasn’t the issue because the new bottle is reading too low.

 

[Randy Holmes-Farley Verified Community Expert]

I took both bottles to the LFS and they confirmed my original suspicion. Bottle 1, my old bottle which expires in 2022 is accurate and the new Bottle 2, is off. On his calibrated refractometer he read bottle 1 at 1.025 and bottle 2 at 1.023.
This also means that evaporation wasn’t the issue because the new bottle is reading too low.

Um, just a word of warning. LFS are not a paragon of testing accuracy.

If they calibrated a brine refractometer with RO/DI, as many folks mistakenly do since it is the manufacturer recommendation, then they will be wrong.

 

[ryshark]

Um, just a word of warning. LFS are not a paragon of testing accuracy.

If they calibrated a brine refractometer with RO/DI, as many folks mistakenly do since it is the manufacturer recommendation, then they will be wrong.

I agree. However he used his calibration solution in front of me. I’m not sure which brand or type refractometer he has though. I went to a LFS that has lots of really nice display tanks. After using his solution then he also put water from his main display in the same refractometer. I’ll trust the bottle I have that closely matches his solution and display tank.

 

[X-37B]

I had a similar issue.
Bought new solution and it was reading 37.
Old solution 35 on refrac, swing arm 1.026.
Tropic marin floater 1.026.
Bought a different brand of calibration soulution and it was 35.
I always have at least 2 different ways to test things.