I hope i get this right:
I dont think pH sensors are usually made for permanently exposing them to the substance u want to measure, as the tip usually has to get put a solution of KCl, also measurement isnt like just reading some stuff with analog in , you also need measurement bridges (hope thats the english term) and to amplify, as they have usually just changes in milivolts, and as it has been said continious calibration is an issue. And thats only pH...