Thank you mates, those are some really good suggestions. I will try implementing some.
Have you also tried measuring right in the middle and at other parts of the tank, including at different depths? Can you eliminate the choppy conditions and test if that improves the precision? Could you try a sub-sample of the seawater in a bucket out of the wind? Long shot but- could the tank be reacting with the seawater?
Yes in fact a few days ago I went there, and had already done some of what you suggest,. The resulsts befuddled me a little bit. So:
- Different depths in the tank do not show a relevant change in variation.
- The water had some CO2 injected previously. I thought it could be from a poorly homogeneized mixture, but tried in another tank w/o CO2 and that's not the case.
- Tried in low wind conditions, still no relevant improvement (although it's a liiiittle better).
- Now comes the interesting part. I took a sample into half of a plastic water bottle, and the results are 100% accurate. Even if I moved the bottle/sensor around, so I guessed it was something else and not the wind. Then I thought of immersing the bottle with the sample into the tank, HOWEVER as soon as I touch the water surface (just the exterior of the bottle, the waters don't mix), I pick up that variation again.
So I am left guessing this has to do with the larger body of water picking some AC noise, or EM reflection at the water surface. Do you guys reckon it too?
Upon further inspection, turns out I took my 100 nF decoupling cap from the sensor entrance out of the design a few months ago. It was "eating" my sensor output, and the arduino was registering a lower voltage than supposed (maybe because of poor shielding at the time and it was picking power grid 50 Hz noise, so I had a quasi-AC wave), so I took it out. I bet this is causing me problems now, isn't it? Is the 100 nF cap adquate for this? And will some tin foil around the BNC connection help?
Also SparkysWidgets, my design is my own. I based some things in some designs, especially pHduino, but at the end of the day decided to go with an instrumentation amplifier, which as far as I remember none of the projects I saw used.
Thank you very much.