Refractometer calibration

[Fourthwind]

I bought some calibration fluid for 35 ppt. my atc unit was just a smidge off on the high side. I adjusted, and cleaned it with RODI. then sampled RODI. It is reading below zero. This normal? I re checked with the calibration fluid and it is dead on at 35. Concerned this unit has gone funky on me

 

[Irishman]

I think it all depends on various circumstances. How do you know that it's 35 ppt? Although they say it is, is it really?

I just zero out with RODI and haven't had any issues or concerns.


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[jda123]

It should be pretty close to zero even when calibrated with 35 - maybe not perfect, but close. Did you really shake/mix up the calibration fluid and make sure that the surface was really clean?

 

[Ghxst]

Calibrate your equipment using a reagent close to what you are testing. If you calibrate using RODI, 0, you will get better results when testing items at or around 0. If calibrating using 35ppt, you will more accurately test water that is at or around 35 ppt. For reefs use calibration standards but get one you can trust or make your own. I just received my degree in Hydrologic Science and have been doing this for the last 4 years in the field. Testing the platte river we found that our results were off because after water treatment plants the salt levels are much higher but all of our equipment was calibrated for 0 because no one thought of it. Hope that helps. Randy Homes Farley, Reef Chemist prefers Pin-Point brand, or make your own.

 

[Fourthwind]

I cleaned the surface multiple times with RODI before testing with "Aqua craft calibration solution 35ppt". I had been just checking for zero with the RODI in the past, but was concerned when I saw some reports of that not working in other groups. I will get another brand so that I can reference it.

 

[Ghxst]

I just ordered the same brand 2 days ago to reference my home made one using 1.91g of Red Sea coral pro to 50ml of water. When it gets here I will let you know the result!

 

[Fourthwind]

just for a comparison, I found my old floating hydrometer, and it exactly agrees with the refractor after I calibrated it. Look forward to your comparison with the calibration fluid Ghxst.

 

[Bdk1786]

If you calibrate using RODI, 0, you will get better results when testing items at or around 0. If calibrating using 35ppt, you will more accurately test water that is at or around 35 ppt.

This is my general practice for calibrating as well. In the past, I have had issues with calibrating to 0. I will also be much more likely to listen to the word of a Hydraulic Scientist

 

[spinycheek]

There are different calibration fluids for different types of salinity measurement. If it's 35 PPT for a conductivity meter, then it won't be correct for a refractometer. But you probably have a refractometer solution.

Apparently seawater also has different refraction properties than regular saltwater, so a 33.3 ppt solution made with table salt (NaCl) will read as 35 ppt on a refractometer used for seawater. So, if you have a calibration solution that is made with a single salt constituent like NaCl or KCl, then after adjusting your refractometer to 35 ppt, it will read below zero.

The Solution (If I'm thinking correctly): Pretend your 35 PPT calibration solution is actually 33.3 PPT and adjust it to read 33.3
or
Just adjust it with DI water.

 

[Fourthwind]

There are different calibration fluids for different types of salinity measurement. If it's 35 PPT for a conductivity meter, then it won't be correct for a refractometer. But you probably have a refractometer solution.

Apparently seawater also has different refraction properties than regular saltwater, so a 33.3 ppt solution made with table salt (NaCl) will read as 35 ppt on a refractometer used for seawater. So, if you have a calibration solution that is made with a single salt constituent like NaCl or KCl, then after adjusting your refractometer to 35 ppt, it will read below zero.

The Solution (If I'm thinking correctly): Pretend your 35 PPT calibration solution is actually 33.3 PPT and adjust it to read 33.3
or
Just adjust it with DI water.

It's refractometer solution

 

[timnem70]

I bought some calibration fluid for 35 ppt. my atc unit was just a smidge off on the high side. I adjusted, and cleaned it with RODI. then sampled RODI. It is reading below zero. This normal? I re checked with the calibration fluid and it is dead on at 35. Concerned this unit has gone funky on me

Did I read your wrong, it should read zero ppm with RODI. As some have mentioned. Best way is to use the fluid, I also used RODI forever and it's so negligible it'll be fine. Never had issues.

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[Ghxst]

I tested the Aqua craft calibration solution on 2 refractometers. First I calibrated them by adding 5 drops to each, closing the window and waiting 45 seconds as it says to do for ATC's. Once moving the blue line to 35 I rinsed with clean water and wiped using microscope wipes. I tested the calibration fluid again and one read 34 and one 36. I then repeated the rinse and wipe and tested the homemade solution, same numbers for each refract. I calibrated again and tested both and got the same results. I cannot claim that Aqua craft calibration solution is correct but can say it is at least consistent with my solution and if I averaged the 2 refract results I would get 35.

I miss my lab on campus, I could have done several test to get an exact measurement.