Current to voltage circuit
The preferred method used by good RLC meters is to inject a voltage limited constant current sweep and then with a 5 decade resistance but only 3 decades of voltage measurement use a switch to change R scaled constant AC current.
But I can look later when time permits to see how to improve your calculations with a scalable voltage amplifier with AGC then the Vagc records the resistance on a log scale with such an IC.
Galvanic corrosion often raises the resistance of electrodes and pressure reduction as well.