Oxygen (O₂) Measurement by the Paramagnetic Method

How does paramagnetic oxygen measurement work?

Oxygen is one of the few common gases with strong paramagnetic properties, meaning it is attracted toward a magnetic field. Nitrogen, CO₂, CO and most combustion gases show this effect very weakly or are slightly repelled by a magnetic field. Paramagnetic analyzers use this difference as their measurement principle.

In the most common design (dumbbell type), nitrogen-filled glass spheres are suspended on a fine thread within a magnetic field. When oxygen molecules are drawn into the field, they exert a force on the spheres and the assembly tries to rotate. This rotation is balanced by optical or electrical feedback; the feedback signal is proportional to the oxygen concentration.

What the paramagnetic method measures and where it is used

The method measures oxygen at percent level in gas mixtures. Typical applications:

• Combustion control and efficiency optimization in boilers, furnaces and burners
• Flue gas emission monitoring; correction of measured values to a reference O₂ value
• Oxygen monitoring in inert and process gases
• O₂ measurement in portable flue gas analyzers

Where the paramagnetic method works well and where it struggles

The paramagnetic method is selective; because the measurement does not consume a sensor, it provides long-term stability and avoids the aging seen in chemical sensors. The method gives accurate, stable results for oxygen at percent level.

Limitations: paramagnetic gases such as nitric oxide and nitrogen dioxide can cause cross-effects, even if small. The measurement cell is sensitive to dust and condensation, so the sample should be clean, dry and supplied at a constant flow. Cell temperature and pressure changes need to be compensated. For trace-level (ppm) oxygen measurement, other sensor technologies are generally preferred.

Related standards

Paramagnetic analysis is included as the standard reference method for measuring oxygen in flue gas in EN 14789.

Choosing an instrument for the paramagnetic method

The measurement range, sample conditioning requirement (dry, clean sample), output type and ambient conditions are the deciding factors. In combustion control and emission monitoring, the accuracy of the O₂ value also affects the reference correction of all other measurements, so regular calibration matters.

Tetra and paramagnetic oxygen analyzers

Tetra supplies, installs, calibrates and services paramagnetic oxygen analyzers in Türkiye. The product range includes M&C PMA10, PMA20 and PMA30 oxygen analyzers, the paramagnetic O₂ option of the Thermo Scientific 60i NDIR multi-gas analyzer and the Mega System Thyris portable flue gas analyzer. These instruments can be integrated into Tetra's CEMS and sample conditioning systems.