Consider calibrating Your refractometer !

cracker

Well-Known Member
Hello everybody, I learned a good lesson today.It's about paying attention to little details for a successful Marine aquarium. I always used RODI water to calibrate my refractometer. It's close enough I told myself !
Well ,I finally ordered some calibration solution. What I thought was close enough was 4 point off ! Thought my salinity was 1.025. It was 1.021. I guess the fish & corals didn't mind too much but it really screwed up test results. Now I have more work to do !
 

Pat24601

Well-Known Member
Hello everybody, I learned a good lesson today.It's about paying attention to little details for a successful Marine aquarium. I always used RODI water to calibrate my refractometer. It's close enough I told myself !
Well ,I finally ordered some calibration solution. What I thought was close enough was 4 point off ! Thought my salinity was 1.025. It was 1.021. I guess the fish & corals didn't mind too much but it really screwed up test results. Now I have more work to do !

@DaveK mentioned this in a thread the other day.

I actually always calibrated mine using both calibration solution and distilled water (as I guess you are supposed to do), but really it didn't ever seem to matter. I never felt justified spending the small fortune they charge for the "calibration solution" even though I did it.

But, I guess my experiences mislead me. It clearly can really matter as you just noted.

Also, someone once told me to use distilled water NOT RO water. I don't remember why, but that's what I did.
 

Desmond

Well-Known Member
You have me worried now :) I have just ordered myself a milwaukee tester
Also waiting on ICP test so i suppose that will give me an indication of how accuarate my refractometer is .
 

cracker

Well-Known Member
Hello Desmond, I would think a brand new meter would be fine for testing. I know now meters do wonder so need to be set right every once in awhile.
 

chipmunkofdoom2

Well-Known Member
@Pat24601 calibration fluid isn't that expensive. I'd recommend the one that BRS carries, it only costs $7 with free shipping. That may seem like a lot for a few fluid ounces of saltwater, but it lasts a while. You only use a few drops each time you test.

I've actually started using Randy Holmes-Farley's recipe for a DIY calibration standard. I calibrate my refractometer with the stock solution, then measure the salinity of the DIY solution. It usually is around 33ppt or 34ppt, depending on the table salt brand. I then calibrate with the DIY solution and verify with the commercial solution every few weeks. It's nice because the DIY solution is so cheap and easy to make that there's no reason to not calibrate as often as you like.

I've found that most hobby refractometers that I've tried can't hold calibration for more than a few months after purchase. Instead of throwing out a "bad" refractometer, I just calibrate before every use to ensure accuracy.

EDIT: Also, check out RHF's points on slope miscalibration here. This is a good reason why you should not calibrate with RO/DI or distilled water. It doesn't matter how accurately your refractometer measures salinity at 0ppt. We're not trying to measure salinity at 0ppt. We're trying to measure at 35ppt. The second graph at that link actually shows that a properly calibrated refractometer, when calibrated at 35ppt salinity, may not read zero when reading RO/DI water, and that this is perfectly okay.
 
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chipmunkofdoom2

Well-Known Member
Yeah that solution is designed to calibrate a conductivity salinity meter, the American Marine PINPOINT Salinity Monitor. It's a lot easier to make a solution read 35ppt on a refractometer, why fluid for refractometer calibration is a bit cheaper.

For a refractometer, just throw enough salt in the water and it'll eventually affect light enough to make a refractometer read 35ppt. Conductivity meters are a bit more tricky. They are very precise, and work by testing how well electricity moves through water. Because of their precision, I believe it's harder to get the electrical conductivity of a solution to be precise, which could explain why the price is higher.
 

Pat24601

Well-Known Member
Yeah that solution is designed to calibrate a conductivity salinity meter, the American Marine PINPOINT Salinity Monitor. It's a lot easier to make a solution read 35ppt on a refractometer, why fluid for refractometer calibration is a bit cheaper.

For a refractometer, just throw enough salt in the water and it'll eventually affect light enough to make a refractometer read 35ppt. Conductivity meters are a bit more tricky. They are very precise, and work by testing how well electricity moves through water. Because of their precision, I believe it's harder to get the electrical conductivity of a solution to be precise, which could explain why the price is higher.

Great info! Liked your diy tip too
 

cracker

Well-Known Member
so today I wanted to retest my refractometer. The adjust screw(tiny with a wimpy straight slot) disintegrated ! It was so rusted that the slot is no longer. so now I need new refractometer !
LOL !
 

Pat24601

Well-Known Member
so today I wanted to retest my refractometer. The adjust screw(tiny with a wimpy straight slot) disintegrated ! It was so rusted that the slot is no longer. so now I need new refractometer !
LOL !

Well, I'm not sure what the moral to this story is. I'm going to say it's don't bother to test your salinity at all. It's more trouble than it's worth. :)
 

cracker

Well-Known Member
LOL No moral really, I just wish I would pay attention to little details so it doesn't cost $$$ & more work in the future .
 
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