I have read already quiet a lot about the conductivity of water/ fluids but i can not 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 measure 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 test 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 at 5cm about 70Ohm. 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. Overal very consistent results.
DC: 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 1kOhm what 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.