Josef,


> I am not sure, what your setup is. Is your conductivity probe still
> connected one wire to the Vdd and the other to some PIC input and thru
> capacitor to the ground ?

Right now I have:
                           ---      +--------------------+
                          |   |     |                    |
                          |  \ /    |    8.2k            |
RA0   ------------ PROBE ---/\/\/\--+--/\/\/\--+        ---
                             1.6k   |          |        --- .47uF
                                    |          |         |
RA3   -----------------------------------------+         |
                                    |                  -----
TMR0   -----------------------------+                   ---
                                                         -

The program switches +5 and gnd between RA0 and RA3, at aprox 270Hz,
which gives enough time for the cap to charge and TMR0 to charge in one
half cycle. (while RA0 is high)

It does the cycle only once, before allowing ample discharge
time and taking another measurment.


> Keep in mind, that there is always some voltage limit, above which you will
> make changes to the solution, possibly irreversible, and that will increase
> you problems with polarization. That is why the peak (not average) voltage
> across the probe should not exceed this limit. For most solutions, the limit
> is 1V - 2V, so 5 V is for sure too much, several hundred mV is generally
> considered safe.

Right. That's probably what is happening now. A question tho:  Is it volts
that changes the solution, or amps? What I've seen suggests it's amps, so
if I can limit the current below 1mA then no changes should occur. I
think?


> For your precission and resolution requirements, you can
> certainly use square-wave instead of ideal sinus-wave, but the no DC rule is
> still valid. I cannot come up with any simple way like the resistor +
> capacitor for DC measurement.

Maybe I could use a RMS measuring technique to convert the square wave to
a DC value, and get a PIC with an ADC on it to do the conversion...?

> What I have in the simplest instrument is one
> TL064 + one 4053 (+ ICL7136), no PICs. Maybe you could try RC oscilator and
> convert R to frequency. I would opt for 2 OZ type with modification - two
> antiparallel diodes will limit the voltage on the probe to about 650 mV:

I'm gonna keep that in mind for the future. Right now tho, I'm trying to
do it with very little modifications to the original circuit; because we
already have over 250 boards printed and populated.


Thanks everyone, for your help. With a little luck this project should be
done real soon! :)


-Shane.