I have tried that but the fit I want is a reciprocal fit which is giving me a bad fit. This is the main reason why I want to use time vs voltage. Additionally, when running the experiement the voltage was set and teh time was measured. So the voltage measurements are discrete. I agree if I could have a voltage vs time fit (bottom graph) it would make my analysis easier. Maybe the fit I am using is wrong when I do a voltage vs time..