Re: Aquatic Plants Digest V2 #55

>From: Andrew Hamilton <andrewha at tafe_sa.edu.au>
>Date: Tue, 7 May 1996 10:53:48 +0930
>Subject: Ph Probe problem
>I recently replaced my lighting system and now have my fluro's run from two
>electronic ballasts which I have placed remotely in the aquarium cabinet.
>The tubes are suspended from the inside of the hood which rests on the
>glass. the wiring is therefore also contained under the hood.
>As I am not running a sump my Ph probe is also placed under the hood. I have
>it positioned as far away from the wiring as practicle.
>The problem I now have is that the lights seem to interfere with the Ph
>reading. Whilst the lid on the hood is closed I am getting a reading of
>close to 6.9, however as soon as I raise the lid the reading plumits to 6.5.
>If I turn the lights off the reading also drops to 6.5. Using a ph test kit
>my ph seems to be around 6.6 (the kit is not very easy to read). I have
>checked the wiring and nothing is coming in contact with the wiring to the
>probe. Obviously there is some electrical interfernce here that I must deal
>Would insulating the lighting section off to the Ph probe area work and what
>sort of insulating material should I use. Has anyone else had simular
>problems? Any advice would be greatly appreciated.
>In sunny Adelaide, South Australia.
Although I am a biologist, and don't know a whole lot about electricity, it
sounds as though there is some kind of electric field coming from your
lights that is inducing some current in your pH probe and altering the
reading.  The obvious remedy would be to shield the probe somehow.  Metalic
foil or window screening should do an adquate job of shielding, but the
foil or screening should be grounded.  You could try enclosing the probe in
some grounded screening, then raising and lowering the hood to see if the
pH reading changes as it did before.  If the screening works, you still
have a problem in that the screening will introduce toxic metal ions into
your water.  What is really needed is a shielded probe that has a non-toxic
outer covering to keep the shielding from contacting the water.  It seems
to me that the manufacturer of your probe should have done that.  I think
you have a legitimate complaint, and you should try to get satisfaction
from the manufacturer.

On the other hand, you might be able to compensate your system for the
altered reading.  I am assuming that the output from your pH probe controls
some kind of CO2 injection system.  If you assume your pH readings are 0.4
units too high during the day, and you want the pH to be kept at, for
example, 7.0, can you adjust the set point to inject CO2 if the pH is 7.4
or higher?

Paul Krombholz                  Tougaloo College, Tougaloo, MS  39174
  In already humid Mississippi