Combined pH and dissolved carbon dioxide gas sensor

An apparatus for measuring pH and the concentration of dissolved CO.sub.2 in a fluid comprising:

• a measurement chamber for containing a sample of fluid to be measured;
• a pH sensor including
• a first wire having first and second ends, said first end exposed for electrical contact thereto,
• a first electrochemically active layer disposed covering said second end of said first wire for forming an electrochemical coupling to said first wire,
• a first dried electrolyte layer disposed covering said first electrochemically active layer, said first dried electrolyte layer when hydrated forming an aqueous solution buffered against changes in pH due to changes in dissolved CO.sub.2 concentration, and
• a first membrane having an outer surface exposed to the fluid in the said measurement chamber and an inner surface covering said first electrolyte layer, said first membrane being sensitive to the difference in pH between the inner and outer surfaces thereof to form an electrical potential thereacross and being water permeable;
• a CO.sub.2 sensor including
• a second wire having first and second ends, said first end exposed for electrical contact thereto,
• a second electrochemically active layer disposed covering said second end of said second wire for forming an electrochemical coupling to said second wire,
• a second dried electrolyte layer disposed covering said second electrochemically active layer, said second dried electrolyte layer when hydrated forming an aqueous solution including bicarbonate ions, and
• a second membrane having an outer surface exposed to the fluid in said measurement chamber and an inner surface covering said second electrolyte layer, said second membrane being sensitive to the difference in pH between the inner and outer surfaces thereof to form an electrical potential thereacross and being water and dissolved CO.sub.2 permeable;
• a ground electrode disposed in electrical contact with the fluid in said measurement chamber;
• a reference electrode disposed in electrical contact with the fluid in said measurement chamber for providing a stable reference potential independent of ionic activity of the fluid in said reference chamber;
• and
• an electrical circuit connected to said first ends of said first and second wires including
• a first differential signal means connected to said first wire and said ground electrode for generating a first difference signal proportional to the electrical potential difference between said first wire and said ground electrode,
• a second differential signal means connected to said second wire and said ground electrode for generating a second difference signal proportional to the electrical potential difference between said second wire and said ground electrode,
• a third differential signal means connected to said reference electrode and said ground electrode for generating a third difference signal proportional to the electrical potential difference between said reference electrode and said ground electrode,
• a fourth differential signal means connected to said first differential signal means and said third differential signal means for generating a fourth difference signal proportional to the electrical potential difference between said first difference signal and said third difference signal, said fourth difference signal being said pH signal,
• a fifth differential signal means connected to said second and third differential signal means for generating a fifth difference signal proportional to the electrical potential difference between said second difference signal and said third difference signal, and
• a sixth differential signal means connected to said fourth differential signal means and said fifth differential signal means for generating a sixth difference signal proportional to the electrical potential difference between said fourth difference signal and said fifth difference signal, said sixth difference signal being said CO.sub.2 signal.