pH probes

To: Aquatic-Plants at actwin_com
Subject: pH probes
From: Jim Hurley <hurleyj at arachnaut_org>
Date: Sun, 12 Jan 1997 19:45:52 -0800
References: <199701122039.PAA30171 at looney_actwin.com>

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
broke.

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
 <URL:http://www.webcom.com/hurleyj>