Measuring distance in a conductive liquid

Question

What is needed to measure distance between two electrodes imersed in seawater ? I have tried measuring the voltage across the path (a suposed resistance) with a constant current source (about 10uA) passing the water to GND of circuit. (the resistance is suposed proportional to distance of electrodes) The probes are 100Hz reversing switched to avoid polarization. And I am using AG/ AGCl pellets as electrodoes. But until now I could not have stable results. What may be wrong ? Could someone help ? Thanks

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The AgAgCl sinterized disks was choose because it is very used as reference electrode for seawater potential measurements (cathodic protection) with very good results and low maintenance, but I really dont know if it is the the best for my application. The suface area is about 1 cm2 then we are expecting about 20 Ohms per cm of distance in seawater. With a 10uA as (AC square wave) current source between electrodes voltage will range from about few milivolts up to 1 volt at 50m. As you see the potential is bellow of water electrolisys. What you mean "with a lot of corrosion protection" for Stainless Steel plates ? I understand that salinity, temperature etc could vary in the media but these factors are not expected to change so quickly and I also intend to use a known distance sensors for reference as well averaging the measures. I intend use this sistem in a dynamic environment where the reletive speed between sensors or the flow may be about 10 cm / sec. What you think about the method ? There are a better one ?

Answer 1

(Initial, preliminary answer, here.) There are several possible reasons for your unstable results:

Possibility 1: Wrong electrodes

Ag|AgCl pellets seem like an odd choice of electrodes for this application. What are the reasons for using them as electrodes? What is their surface area? Their complex shape means the electrical current maybe isn't uniform between the two electrodes. Also, the fact that you're drawing current through an electrode with an AgCl coating may mean the coating is building-up/breaking-down (which could mess up your measurements) ... In your current application, it seems like you'd want to use two, big, parallel stainless steel plates (with a lot of corrosion protection!) as your electrodes.

Possibility 2: Changing salinity

Depending on the local conditions, the conductivity of the seawater may be changing. For example, if you are by a river delta, the tides may be washing fresh water between your plates. Or, if the electrodes are at the water surface in really sunny location, day-time evaporation may increase salinity ... If either of those are the case, install a second set of plates with a known, fixed electrode spacing to measure salinity, and use that to modify the results from the original set of electrodes.

Possibility 3: Convection Currents

Is the seawater between the two plates moving? If so, how fast? Does the speed change over time? All of these factors could contribute to unstable electrochemical readings, too ... If so, you may add mesh or baffles to create more-consistent flow conditions.

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(Follow-up answer, here)

As far as I can tell, you're method sounds alright from an electrochemical perspective. Cathodic protection is fine for minimizing corrosion. Having "known distance sensors" will avoid problems with changing salinity, etc (as you mentioned).