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.
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.