How Oxygen Probes Work: Zirconia, Reference Air, and the Nernst Equation

• Zirconia conducts oxygen ions at high temperatures to form an oxygen concentration difference battery
• Reference air (20.9% O₂) is passed through the tube, and the outside of the tube is exposed to furnace gas.
• Nernst's equation relates mV, temperature, and oxygen partial pressure
• Carbon potential = f (mV, temperature, CO%)
• With in-situ measurement and fast response, it is the mainstream sensor for carbon potential control in the furnace.

Structure

A typical oxygen probe consists of the following parts:

• Zirconia sensing element: Located at the front end of the probe, exposed to high-temperature furnace gas;
• internal electrode: Usually platinum, contacting the reference air in the tube;
• external electrode: Heat-resistant metal outer tube or electrode, in contact with furnace gas;
• Reference air channel: Clean air is continuously supplied by a small air pump;
• Thermocouple: Measure probe tip temperature (some probes are built-in);
• Charcoal air connection: Used to regularly remove carbon on the surface of the probe.

Common zirconia structures on the market include integral zirconia tubes, zirconium balls, zirconia and ceramic welding, and zirconia tips. No matter which type, good sealing between the zirconia and the ceramic inner tube is a prerequisite for accuracy.

Working principle: Oxygen concentration difference battery

Zirconia (stabilized by doping) allows the passage of oxygen ions at high temperatures. When the oxygen partial pressure inside and outside the tube is different, oxygen ions migrate from the side with high oxygen partial pressure to the side with low oxygen partial pressure, forming a potential difference between the two poles.

The oxygen partial pressure of the carburizing atmosphere in the furnace is extremely low (usually in the order of 10⁻¹⁵ bar or even lower), whereas the reference air contains 20.9% oxygen, thus producing a signal of the order of approximately 1 V.

Nernst equation

E = (RT / 4F) · ln(pO₂,ref / pO₂,gas)

Substituting constants and calculating with reference air pO₂,ref = 0.209 bar, it is often written as:

E (mV) = 0.0496 × T × lg(0.209 / pO₂)

Where T is the probe tip temperature (K), pO₂ is the oxygen partial pressure of the furnace gas (bar).

From mV to carbon potential

In an equilibrium atmosphere with a known CO content, the oxygen partial pressure is related to the CO₂/CO ratio (2CO + O₂ ⇌ 2CO₂), which in turn is related to the carbon activity. The controller therefore uses three inputs to calculate the carbon potential:

1. Probe mV;
2. temperature;
3. CO% (set value or actual measured value).

At the same temperature and CO%,The higher the mV, the higher the carbon potential. Signals in carburizing atmospheres are typically in the 1100–1200 mV range, depending on temperature and carbon potential settings.

Why oxygen probes are suitable for closed-loop control

• Directly inserted into the furnace, no sampling required, short response time;
• The signal is continuous, suitable for PID closed-loop adjustment of enriched gas or diluted air;
• The cost is relatively low and it is one of the most effective and economical sensors for carbon potential control.