pH Probe Placement

Engineers typically arrange the pH sensors for either feedforward or feedback process control. Three pH probes would be needed for a two tank-feedforward control setup. One probe is installed on the influent pipe, the second probe is installed in the first tank effluent pipe, and the third probe is installed in the final discharge pipe after the second tank. Only two probes are needed on the two tank-feedback control setup, one probe on the discharge pipe of the first tank and a second probe on the discharge pipe of the second tank.
The major difference between feedback and feed forward control is the location of the measured control variable. Feedback control measures the control variable at the system discharge point, while feedforward control measures the control variable at the inlet point. Feedforward control can predict changes or disturbances in the process (pH control) whereas feedback control responds only after the disturbance has occurred.
Both systems have advantages and disadvantages and both systems can be used to control pH neutralization. Feedback control is simpler but does not provide perfect control. Feedforward control is more complex and provides better control if needed.

Figure 3-2 Typical pH Neutralization Probe

The pH Probe

The most common way to measure pH involves a pH probe and a pH analyzer (a device that translates the electrical signal from the probe into the 0-14 pH unit scale).
A pH probe or sensor has 4 basic parts: the measurement electrode, the reference electrode, a reference liquid junction, and a gel coated glass membrane. (See Figure 3-2) Many probes are also equipped with a temperature compensation device.
pH is the measurement of hydrogen ion (H+) concentration. The pH probe measures the concentration of the hydrogen ions in the wastewater by comparing the electrical potentials of the measurement electrode with that of the reference electrode. The measurement electrical potentials result from the transfer of the hydrogen ions from the wastewater to the electrode. The reference electrode has its own electrical potential resulting from the transfer of ions from the reference solution to the reference electrode.
The pH analyzer needs an electrical signal, but where does the electrical signal come from? The pH probe is a lot like a battery. A voltage is created from the movement of electrons from one electrode (positive) to another electrode (negative). In the case of a pH probe, a voltage is created from the flow of ions between the reference and the measurement electrodes through the reference liquid junction, or “salt bridge.” The salt bridge completes the circuit. The greater the difference between electrical potentials, the greater the voltage created. The signal produced is from 0 to 300 millivolts (mV).