yamanoor sairam wrote:

>> As others have pointed out, this is normal operation for an ORP
>> probe. It is extremely high impedance when out of the solution.  It
>> then is susceptible to picking up electrical noise (line frequency,
>> etc.) or simply floating to one rail or the other.  When you connect
>> your DVM to the input, you are pulling the input to ground.  But
>> with no DVM on the input, the circuit will produce indeterminate
>> signals.
> 
> In order to avoid noises, I have used an RC filter combination at the
> PIC input & Opamp output. I used a 0.1UF & 10uF capacitor in
> parallel. I am also averaging 256 samples before displaying it. My
> display has stabilised now and I have also removed all junk
> characters on my display. 
> 
> When I remove my probe from the BNC connector, I am able to see that
> the displayed value is dropping to zero immediately. I shall check
> the oscilloscope output and get back to you. I am currently searching
> for a possible solution for controlling the erratic behavior when the
> probe is removed out of water but still connected to the PIC. 

What may work, depending on the impedances involved, is to inject a low
current at the input. The current should be so small that it doesn't
affect the measurement significantly when the probe is in the solution
(low impedance) but does bring the opamp output to one of the rails (the
lower is probably best) when the probe is out of the solution (high
impedance). 

Something similar is used for break detection with thermocouples.

If it doesn't work directly (that is, if the impedance difference is not
so big that the necessary current wouldn't affect the measurement, but
not so small that it couldn't be measured), you can switch the current
on and off periodically and use it to measure the impedance, and discard
the measurements while the impedance is indicating that the probe is not
in the solution.

Gerhard
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