Look into :Ion sensitive Field Effect Transistors.and books on Biomedical
transducers.
Wen Ko

To whom it may concern:
>    My name is Russell Mueller and am a Masters student in Animal
>Science at Oklahoma State University.  My research project entails
>trying to develop a radiotelemetry pH sensor that will measure hind-gut
>pH in Horses feed varying pH  affecting diets.  We are having
>difficulties finding a sensor that will fit in a capsule about 1 inch
>long and a diamter of 3/4 of an inch.  If you have any information,
>ideas, or products on the market, your help would be greatly
>appreciated.  Thank you.
>Russell Mueller
>205 Poultry Science
>Oklahoma State University
>Stillwater OK 74078
>(405) 744-7783
>
>