Zirconia oxygen probe in combustion monitoring A zirconia (zirconium dioxide, YSZ) probe installed in flue ducts is used to measure which flue-gas component in percent?

Introduction / Context:
Combustion control in boilers and furnaces relies on tight feedback from excess air levels. Zirconia oxygen probes are the industry standard for continuous measurement of oxygen concentration directly in hot flue gases, enabling optimisation for efficiency and emissions.

Given Data / Assumptions:

• Probe uses stabilized zirconia (yttria-stabilized ZrO2) as a solid electrolyte.
• A reference gas (usually ambient air) is compared to process flue gas.
• Operating temperature is high (typically 600–800 °C) to activate ionic conduction.

Concept / Approach:
The probe exploits the Nernst cell principle: oxygen ions conduct through hot YSZ. The cell generates a voltage proportional to the logarithm of the ratio of oxygen partial pressures between flue gas and reference. From this EMF, the instrument computes % O2 in the flue gas. While CO2 is a major flue constituent, zirconia cells do not directly sense CO2 or humidity; they measure oxygen potential. Proper calibration and temperature control are necessary for accurate oxygen readings.

Step-by-Step Solution:

Verification / Alternative check:
Cross-checks with paramagnetic O2 analysers at low temperatures confirm consistency when sampling and conditioning are comparable.

Why Other Options Are Wrong:

• % CO2: Requires infrared analyzers, not zirconia cells.
• Humidity: Measured by hygrometers; zirconia is insensitive directly to water vapour.
• Speed of a submarine: Completely unrelated.

Common Pitfalls:
Ignoring flue gas composition effects (combustibles) on sensor lifetime; zirconia reads oxygen, not CO or CO2.

Final Answer:
% O2 in flue gas