Refractometer vs Neptune Conductivity probe


New member
Hey guys,

I am sure this has come up before. I have read that these two will sometimes have slightly different readings but I am still perplexed. I set up my neptune controller last week and calibrated the conductivity probe with the enclosed solution. Currently it's reading my salt at a little over 36 to 37. I calibrated my refractometer using calibration fluid to 35 ppt. Since my Neptune was intermittently alarming I took out about a 0.25 gallon of tank water and let my ATO fill it back up with RODI water. Now my refractometer is reading about 32 ppt. My controller says that the salt is 36.

It seems like these are way off. I would like my salinity to be 35ppt but if I do that using my refractometer then I would imagine my controller would say that my conductivity is 40.

What is going on? Is it my refractometer? (It is about 10 years old) Do I need to recalibrate the Neptune?

Thanks for the help.


Obligate Feeder Obsessed
i would tend to trust a calibrated refractometer over the salinity probe. i actually was never really able to get good or consistent readings from the apex salinity probe, so i stopped using it.

even after calibration it was off from what both my refractometers were telling me, and it tended to drift up rather quickly.


New Old School
Premium Member
My probe and refractometer wouldn't agree even directly after calibration. I just use the probe as a way to keep an eye on stability, and it even saved my butt once. The cable for my break out box pulled out slightly, resulting in my ATO no longer functioning. On the first day of a 15 day road trip. What clued me in was my high salinity alarm. I tweaked my ATO code to run off of my salinity and all was well when I got home.


Team RC
You are measuring two different entities when you look at conductivity and salinity. You can have two samples of water with the exact same salinity that have different conductivity readings due to the chemical make up if the water, some elements like magnesium will conduct electricity at different rates. I would trust your refractometer reading and use the conductivity as a general ballpark estimate and be more concerned if the conductivity readings rise or fall rather than their relationship to the waters salinity.


New member
I think Neptune recommends letting the probe soak for a week or 2 in the tank before really paying attention to it. Also be sure there are no air bubbles trapped in the tip, they can cause faulty readings.

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