pH probes

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
the controller.

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
signature line.

 Jim Hurley  hurleyj at arachnaut_org