Did you observe this change from 6500 Hz to 7000Hz just once or does it happen consistently? What do you have to do to get the circuit to go back to 6500Hz? Does it just go back to 6500Hz on its own if left until the night time? What kind of caps are you using for the AC coupling to the probes? Could there be leakage current through these which is presenting a small DC voltage between the probes, leading to an electrochemical reaction? I would think that, even in the absence of a DC bias, the surface of the probes would change chemically after a few hours in the salt water, or even pure water. This change might sequester some of the salt out of the solution and change the conductivity. It also might change the conductivity by insulating the probes partially or changing the ion mobility/permeability. On Wed, Sep 10, 2014 at 12:57 PM, Ravi wrote: > John, > > This is not sea water. I have used cooking salt and also sea salt in > different experiments. > > I will come to a conclusion in a day or two. > > Stirring also does not change the behavior. > > IMHO, it is cooking salt that varies in layers and not sea salt as per > one chemist's opinion. Sea salt is homogenous when dissolved. > > Cheers > > Ravi > > --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .