A few months ago, a pH controller I built in Feb 1992 broke.
It was stuck in the ON position, so I just throttled back the
CO2 via the needle valve until I could get some time to work on
This weekend I had that opportunity and I discovered that the
controller was fine, in fact it was still very well calibrated,
benefitting from the low-drift op amps I chose in the design.
What had happened was the the pH probe was no longer working.
It kept reporting a very high pH all the time.
I guess that's not too bad considering I had done zero maintenance
on it for the last few years.
Anyway I bought another one, this time the one had an exposed
glass bulb, whereas the original one had a plastic cover around the
tube. (The original was a Hannah 1910B, the new one a Hannah
1912B, perhaps the old one isn't mad anymore. except for the
protective plastic they look the same).
Unfortunately, after one day, the pH probe bumped into something
while I was doing water changes and the fragile exposed bulb
Moral - don't get that type of probe - or protect it with a porous
substance like a piece of sponge. The probe tip is very delicate,
you're not supposed to even touch it, so maybe pushing it into
a soft sponge isn't so good an idea. From what I recall when I was
researching the probes back in '92, the glass is a specially made
type and scratches will affetct the pH response.
For more info on this design check my web pages. The URL is in the
Jim Hurley hurleyj at arachnaut_org