Abstract: The invention relates to an internal solution ground for primary use with a pH sensor, and preferably with a multi-sensing instrument monitoring a pH sensor and one or more additional secondary sensors having non-isolated circuits. In one aspect, a pH sensor assembly includes a pH electrode, a reference electrode, and an internal solution ground wire. The internal solution ground wire shares a liquid junction with the reference electrode but is protected by the reference fill solution. In one embodiment, the internal solution ground can be electrically isolated through a capacitor or, alternately, a capacitor and resistor in parallel.

Abstract: The invention relates to an internal solution ground for primary use with a pH sensor, and preferably with a multi-sensing instrument monitoring a pH sensor and one or more additional secondary sensors having non-isolated circuits. In one aspect, a pH sensor assembly includes a pH electrode, a reference electrode, and an internal solution ground wire. The internal solution ground wire shares a liquid junction with the reference electrode but is protected by the reference fill solution. In one embodiment, the internal solution ground can be electrically isolated through a capacitor or, alternately, a capacitor and resistor in parallel.

Abstract: A method and apparatus are disclosed for measuring a resistance of a sensor. The apparatus injects a first test current into the sensor and then measures a substantially stable first voltage level across the sensor when the first test current is present. The apparatus then injects a second test current into the sensor, the second test current being substantially equal to but opposite in polarity to the first test current. A substantially stable second voltage level across the sensor is measured when the second test signal is present. Using the first measured voltage level and the second measured voltage level, the apparatus calculates the resistance of the sensor.

Abstract: An apparatus and method for measuring potentiometric electrode impedance for diagnostic purpose while continuously reading the results of the process value. A square wave is applied to an external circuit, which in turn is coupled to an electrode assembly output through a capacitor. The output of the electrode is converted into a pulse width modulated signal and sampled. These sampled values are used to calculate the impedance and process output value.

Abstract: A three wire amplifier circuit for use with a sensor in which the amplifier output drives the negative supply so that the negative supply becomes the circuit output and the system is capable of diagnosing the condition of the sensor.