Electronic pH probes/meters

Hi, everyone.

This request was originally posted on the discus-king list, but on of the
respondees thought a lot of "pH knowledgeable" people were on this list, so
... here it is.

I want to set up my next large Discus tank with an automatic CO2 injection
system. What I hope to gain from this is an optimal pH for my Discus and a
healthy plant growth (I don't care much for plastic plants!) Being a big fan
of DIY projects (and also an electronics engineer), I have decided to try to
build my own electronic pH measurement/controller system.

I recently got a pH meter which supposedly was accurate to 0.01 pH (cost
around USD 320), but it required a lot of calibration knob fiddling to be
that accurate. The unit was made by a German company called LC Electronic
and was intended as a general-purpose pH meter, not especially for aquarium
use. After having tried it a few days, I came to doubt that it could be used
for *continous* pH monitoring (value changed half a degree over the course
of a few hours, value also started to fluctuate alot), which is of course
necessary if it is to be used as a controller. Also, the actual probe tip
was supposedly filled with an electrolyte containing some silver compound,
and I worried that this stuff would (eventually) leak into the tank and kill
fish. There was no English version available for the accompanying (German)
manual, so it was hard for me to figure all the details. Today I returned
the meter to the store.
[end background]

Has anyone any experience building an electronic pH meter/controller based
on commerically available (non-toxic!) probes intended for continous monitoring?

Does anyone know how/where to get such probes (at a reasonable cost)?

Any other comments on the subject?

I guess I will adhere to the rather interesting signature practice of this
mailing list: 
  -- Erik in foggy/rainy (but still some snow left) Norway