Which salinity test to trust?!? Calibration concerns.

[T-J]

I'm having a situation and I hope R2R can help guide me....

Yesterday, my calibration solution came in for my refractometer. Prior to it being calibrated with solution, it was calibrated with RODI water at 0. I keep my tank at 35ppt (1.026). At least I thought it was at 35...

I calibrate my refractometer with the solution and of course it's off. Basically it was reading higher than it actually was. After calibrating, I threw some RODI water on it to see if it read zero. Nope. It is below the zero scale.

Ok, let's crack out the old swing arm and see what it says. RODI water comes in exactly at 0. Cool. Let's throw some tank water in there and on the refractometer to see how they compare.

Swing arm says about 1.025+ a little extra.
Refractometer reads at about 1.025 or 33.188ppt.

My questions I guess are this: Is it ok for the refractometer to be off on a zero read, but spot on with the 35ppt calibration fluid? I know that 1.025 is still fine, but I prefer my tank to be a bit higher, is raising it to 1.026 in one shot too much, or is that ok?

Thanks R2R!

 

[Gareth elliott]

Randy has an article explaining drift errors that will explain this better. but the further from your desired point you wish to test the more errors there can be. Ie If the drift is .02ppt off per .1 sg calbrating at 0, by the time you get to .26sg you are off by .05. Again the article explains better lol.
If not sure what to trust bring a sample to the lfs they will test this for free. Then can draw new lines on the hydrometer so both your testing meters are accurate

 

[amps]

I'm not an expert but I've always understood that a refractometer calibrated at 35ppt won't read 0ppt with RODI or distilled. The scale isn't perfectly linear and it functions most accurately in a range around it's calibration point. Ph probes are the same, if you calibrate at 10ph it's not going to read 4ph accurately and vice versa.

I would trust the refractometer.

 

[Crabs McJones]

From my understanding you want to calibrate to a known refractive index and not RO/DI water. Imperfections and inexpensive manufacturing can skew results when calibrating to 0.
@Randy Holmes-Farley has an article dedicated to this, it's a very good read through.

http://reefkeeping.com/issues/2006-12/rhf/index.php#13

 

[T-J]

Thanks everyone. Seems what I’m experiencing is normal and that I should trust the calibrated refractometer.

 

[saltyhog]

Thanks everyone. Seems what I’m experiencing is normal and that I should trust the calibrated refractometer.

Yes sir! Any laboratory equipment should be calibrated in the range you expect to test in for the best accuracy. There are refractometers that are made to be calibrated with RO/DI water but I don't commonly see them offered for sale on salt water aquarium suppliers.

 

[Randy Holmes-Farley Verified Community Expert]

I'm not an expert but I've always understood that a refractometer calibrated at 35ppt won't read 0ppt with RODI or distilled. The scale isn't perfectly linear and it functions most accurately in a range around it's calibration point. Ph probes are the same, if you calibrate at 10ph it's not going to read 4ph accurately and vice versa.

I would trust the refractometer.

Correct, a brine refractometer (as many in the hobby are) calibrated to read 35 ppt correctly SHOULD NOT read ro/di correctly.