Zirconia Oxygen Sensor mV Calculator
A zirconia oxygen cell produces a voltage set by the oxygen ratio between the reference air and the sample, by the Nernst equation. Convert between cell millivolts and oxygen percent at the cell temperature.
- E = (2.303RT ÷ 4F) × log₁₀(O₂ref ÷ O₂) = 48.27 × log₁₀(20.95 ÷ 2) = 49.25 mV
Nernst Equation for Zirconia
R = 8.314 J/mol·K, F = 96,485 C/mol, T the cell temperature in kelvin and the oxygen values partial pressures (or percent at the same total pressure). Four electrons transfer per O₂ molecule. Real cells add a small cell constant (offset) found by calibrating with air and a test gas.
Worked example
At 700 °C a cell with air reference and 2 % O₂ sample gives about 49.2 mV; the slope is 48.3 mV per decade of oxygen.
Frequently Asked Questions
Why must a zirconia cell be hot?
Zirconia only conducts oxygen ions at high temperature, typically above about 600 °C, so probes have a heater or sit in hot flue gas.
What is the reference gas?
Usually clean ambient or instrument air at 20.95 % O₂ on the inside of the cell.
What do combustibles do to the reading?
Unburned gases react with oxygen on the hot electrode, so the probe reads net (excess) oxygen, lower than the true free O₂.
Learn more on the blog
Instrumentation and control standards
A practical, field focused guide on instrumentationblog.in that explains the theory behind this calculator and how engineers apply it on real plants.
Read Full Article