"Gennette, Bruce" wrote:
>
> Continuous pH is impossible.  Measurement works by comparing the effect of a
> known pH liquid on a specially coated electrode (reference) with the effect
> of an unknown pH liquid on the surfaces (note the s at the end) of a special
> type of glass bulb passed into an electrode identical to the reference.
>
> If the special composition glass bulb is left in the unknown pH liquid for
> an length of time it destroys the special oxides in the glass that pass the
> voltage differences.  To preserve the glass bulb you need to have the same
> pH liquid on both sides of it (usually KOH).
>
> So - you need a mechanical device that keeps a dual electrode (reference and
> test) pH probe in a container of KOH most of the time and moves it into the
> test liquid when required.  You still need to periodically check the level
> of KOH in the electrode which must never be allowed to drop below the level
> of the liquid you are measuring.
>
> Check out Vernier instruments for pH probes designed to connect through a
> CBL to a TI Graphing calculator.
>
> Bye.
>
>         -----Original Message-----
>         From:   Jim Ruxton [SMTP:cinetron@PASSPORT.CA]
>         Sent:   Tuesday, 2 May 2000 12:28
>         To:     PICLIST@MITVMA.MIT.EDU
>         Subject:        Looking for PH sensor
>
>         I was wondering if anyone can recommend a PH sensor that is fairly
>         simple to interface with a PIC either through A/D or serial etc. I'm
> not
>         looking for anything too fancy. I want to be able to monitor the PH
> of
>         the water in a fish tank I'll also be measuring temperature so if
> there
>         is something that does both that would be a bonus.
>         Thanks for any suggestions.
>         Jim

There are plenty of continuous PH monitors around for that are used in
fish tanks. Usually they are used in connection with a calcium reactor
they monitor the PH of the tank and adjust the amount of calcium reactor
effluent accordingly. They do need to be calibrated on occasion (I think
like once every month or two)


                        Damon Hopkins