Digital ph meter

Claim 1

IN A PH METER OF THE CLASS DESCRIBED, THE COMBINATION COMPRISING: AN INDICATOR ELECTRODE AND A REFERENCE ELECTRODE FOR GENERATING A FIRST ELECTRIC POTENTIAL PROPORTIONAL TO THE HYDROGEN ION ACTIVITY OF A SAMPLE SOLUTION INTO WHICH BOTH SAID ELECTRODES ARE IMMERSED, A VARIABLE SOURCE OF SECOND ELECTRIC POTENTIAL, SAID SOURCE COMPRISING A POTENTIOMETER CONNECTED TO A STABLE REFERENCE VOLTAGE SOURCE HAVING A TEMPERATURE COMPENSATION RESISTOR IN SERIES THEREWITH AND A CALIBRATION RESISTOR IN PARALLEL WITH SAID POTENTIOMETER FOR CALIBRATING THE RESPONSE OF THE APPARATUS BY THE USE OF STANDARD SOLUTIONS, COMPARATOR MEANS FOR OBTAINING AN ERROR SIGNAL REPRESENTATIVE OF THE DIFFERENCE BETWEEN SAID FIRST ELECTRIC POTENTIAL AND SAID SECOND ELECTRIC POTENTIAL, SAID COMPARATOR MEANS INCLUDING FIRST AND SECOND HIGH-INPUT-IMPEDANCE BUFFER MEANS RESPECTIVELY CONNECTED TO SAID FIRST AND SAID SECOND ELECTRIC POTENTIALS AND CHOPPER MEANS FOR ALTERNATELY SAMPLING THE RESPECTIVE OUTPUTS OF SAID FIRST AND SAID SECOND BUFFER MEANS AT A PREDETERMINED FREQUENCY RATE, MEANS FOR AMPLIFYING SAID ERROR SIGNAL, CONTROLLED MEANS HAVING A ROTABLY POSITIONABLE OUTPUT RESPONDING TO THE MAGNITUDE AND POLARITY OF SAID AMPLIFIED ERROR SIGNAL, ROTATION-COUNTER MEANS DRIVEN BY SAID OUTPUT OF SAID CONTROLLED MEANS, TRANSLATING MEANS CONNECTED TO SAID OUTPUT OF SAID CONTROLLED MEANS FOR VARYING THE SETTING OF SAID POTENTIOMETER IN A CLOSED-LOOP SIGNAL-FEEDBACK MANNER UNTIL A NULL IN SAID ERROR SIGNAL IS REACHED CORRESPONDING TO A CONDITION OF SUBSTANTIAL EQUALITY BETWEEN SAID FIRST AND SAID SECOND ELECTRIC POTENTIALS, WHEREBY THE READOUT OF SAID ROTATIONCOUNTER MEANS IS THEN INDICATIVE OF THE HYDROGEN ION ACTIVITY OF SAID SAMPLE SOLUTION.