On 3 May 00, at 9:36, Stephan Kotze wrote:

> Jim,
>
> Interfacing a pH probe to PIC is not a simple exercise as I have found over
> the last three months.

 I'm agree with this conclusion...( I'm interfere here like fly's in the
milk...)
>
> The best types are the glass refillable type. Most manufacturers have them.
> The gel filled electrodes are not refillable and  have a finite lifespan of
> about one year. If the fish tank  is saline, you will need a special prode.

 You may use here metallic electrodes (with bad liniarity out of
4...11pH)

> Regardless you will need a special probe for very long life as the junction
> tends to clog up with protiens (common in the tank).
>
> Probes marked as ATC probes have a seperate temperature sensor in them
> usually Pt100 or Pt1000. To interface with a PIC you will either need a
> pre-amplified probe or you have to provide a FET input stage as the pH probe
> has an impedance of about 10^9 Ohms ( yes 1000 MOhms).
>
   If money are not a very important thing, you can use a uniFET-
electrodes with small output impedance direct conectable to a AD-
PIC (output impedance in 1..100kohm) from Norel.inc USA fax:609-
625-0526 or in Europe 02633/8198
   If no money, no fear...a FET operational amplifier not to expensive
like LF411 can solve your problem. Connect the electrodes between
in- and output and use a 100ohm + 47uF( not polarized )filter from
output to ground. The in+ can be connected to a offset reglage;
large value  resistors (10M) must be added to in+ and in- to
echilibrate bias current...
   If tank gradient temperature do not exceed few grades a manual
temperature compensation it's enough...
By the way what pH precision you need and what is exactly the
saltwater content ( % ) in your tank?
 Vasile

*********************************************
Surducan Vasile, engineer
mail: vasile@l30.itim-cj.ro
URL: http://www.geocities.com/vsurducan
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