I just finished assembling the new version of the SpiroBoard.  I hope to post testing/debugging updates over the next few weeks.

The new design is much different than the original.  It doesn't even use the peak detect circuit that I described earlier.  Instead, it uses the pulse generation routines in the pigpio library on the RPi to generate a square wave and the associated sample-and-hold signals to measure the conductivity after the rising edge of the square wave.  Note that the square wave is 50% duty cycle, so there is no DC component.

I will be posting a new schematic and circuit description when debugging is complete.