Unlike conventional combination electrodes, differential pH sensors have
two pH glass measuring electrodes and a common solution ground
One of the glass electrodes is used as the active electrode which generates a potential E1
proportional to the process pH. The second glass electrode is used as a
reference electrode. It consists of an internal measurement electrode
immersed in a stable buffer solution and a salt bridge chamber (double
junction) with ceramic plugs. The internal electrode makes
electrochemical contact with the measured media via the salt bridge
chamber and generates a standard reference potential, E2.
Both glass electrodes have a common potential E3 developed at the third electrode, the ground electrode. Common ground rod materials are titanium and stainless steel.
The two electrodes and the ground electrode are connected to high impedance inputs of a differential preamplifier, see the figure to the right. The output of the preamplifier provides an output expressed as:
EpH = (E1 - E3) - (E2 - E3) which is, after canceling the E3 term: E1 - E2
It is also appropriate to note that a temperature sensor is built into a differential pH sensor for the same reason as for a combination electrode.
• Reduces ground loop problems.
Since ground loop currents will pass through only the ground electrode (E3), and not through the reference electrode (E2), the overall pH signal output (EpH) is unaffected by the ground loop potential.
• Stronger signal from the pH sensor.
The preamplifier located in either the sensor or in a nearby junction box, boosts the sensor voltage and converts it from high impedance to low impedance before sending it to the pH transmitter. This results in the following benefits:
- Possible to have long cable runs, about 1000 meters (3000 feet). For a conventional sensor, without a preamplifier, the pH signal can be sent only about 5 meter (15 feet).
- Reduced interference from electrical sources, e.g. motors and pumps.
- Low noise and expensive coaxial sensor cables are not needed.
• Reduced reference junction problems.
The double junction salt bridge greatly reduces the possibility of contamination of the reference electrode solution compared to a conventional reference junction. Even contamination will be able to reach the stable buffer solution of the inner chamber it is less susceptible to contamination than the standard KCl reference solutions used in combination electrode systems. A buffer solution, by definition, resists pH changes and keeps the reference electrode potential stable (E2).
Longer electrode life in a wide range of measurement environments due to reduced reference junction problems. Some of the manufacturers of differential pH electrodes also offer the possibility to replace the reference solution and salt bridge when fouling occurs.
• Diagnostic functions
A solution ground in a pH sensor makes it possible to apply diagnostic functions to the pH system.