[Solved] How to calculate the resistance measured between probes in water

Brett Asks: How to calculate the resistance measured between probes in water
I have read already quiet a lot about the conductivity of water/ fluids but I cannot find a formula to calculate the resistance between probes in a fluid.

For example:


In the answer Peter Bennett measured about 50.000kOhms.

I would like to know how I can calculate it and what the parameters are.

Parameters I guess so far:

  • average distance of the probes
  • copper surface exposed to the water
  • conductivity of the water (amount of salt for example)
  • volume of the water ??
  • critical voltage or linear??

I will be doing some home experiments with copper/metal in water and measuring the resistance with a small AC/DC current through the water, but I want to calculate it first and then see if I can reproduce the result.


I did some small tests with the answers below in mind. Especially the EDLC component was clearly measurable. I did a small test in about 100ml /3.5g salt solution. My findings:

AC: With 5cm /0.2mm probes I got about 70 ohm at 5cm. The amount of probe in the liquid did alter the value. The distance was a key factor, my measurement was not accurate enough to give me a function. Over all very consistent results.

DC: The same probes I first tried with around 1.5V and did notice electrolysis at that point the resistance was low but increased rapidly because of the corroded probes. If I lowered the voltage over the probes to 0.5V the resistance was about 1kiloohm, which was higher than I expected and no visible electrolysis did occur. Please be careful, DC in a salt solution will generate chlorine gas do it in a well ventilated place.

I did the measurements over a voltage divider circuit, will do these again later with a more scientific method and logging, but I did them now to get an overview of what to expect.

I’m still interested in formulas to calculate before I do more tests.

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