Dembous,
I took a look at your suggestion, it is very similar to the solution I came up with except I used a standard PLC so it was easy to connect into the control system. The measuring cells put out a significant mV that can be measured directly by the PLC without any additional electronics. It took a bit of work to get right but the end result allowed the plant to run 24 hours rather than have to have lab staff on hand. The chemicals are quite nasty, requiring a separate enclosure for the electronics.
If anyones interested further.
rmatson@noram-eng.com
Regards
Roy