A portable circuit and method for performing a time study and analysis of bodily ionic characteristics as esophageal pH includes a transducer 11 for measuring pH and generating a proportional analog signal and an interface 12 for converting this signal into a representative digital signal including offset and gain controls 23,24 for permitting correction for transducer 11 calibration drift during the study period. A processor 14 receives and stores all digital signals for rapid transmission to a data analysis after the study period is ended. Esophageal pH values are measured and stored for a plurality of pre-study and post-study known conditions.