CO and CO₂ Measurement by NDIR and Gas Filter Correlation

How do NDIR and GFC work?

The non-dispersive infrared (NDIR) method is based on a gas absorbing infrared light at specific wavelengths. CO, CO₂, SO₂ and many hydrocarbons have characteristic absorption bands in the infrared. An infrared beam from the light source passes through the sample cell; a detector at the cell outlet measures the amount of light absorbed. The amount of absorption is proportional to the gas concentration.

Gas filter correlation (GFC) is a type of NDIR with increased selectivity. A rotating filter wheel sits in the light path; one part of the wheel is filled with the gas being measured (for example CO), and the other part is inert. The band absorbed by the gas to be measured is thus used as a reference, and the effect of other interfering gases is largely suppressed. Optical filter correlation instruments also have a similar wheel.

What NDIR and GFC measures and where it is used

The method is used to measure CO and CO₂, as well as SO₂ and some hydrocarbons. Typical applications:

• CO and CO₂ measurement at air quality and climate monitoring stations
• Flue gas emission monitoring and combustion control
• Process gas analysis, greenhouse and fermentation applications
• Reference measurement in calibration laboratories

Where NDIR and GFC works well and where it struggles

NDIR and GFC instruments measure continuously, are stable and require no chemical consumables. Standard, high-level and trace-level configurations can cover different ranges from ppb to percent.

Limitations: water vapor absorbs in the infrared, so the sample must be dried or the interference compensated. Contamination of optical elements can affect the measurement, so a filtered, conditioned sample should be supplied. Symmetrical molecules (such as O₂, N₂, H₂) do not absorb in the infrared and cannot be measured by this method; a paramagnetic or other sensor is added for oxygen.

Related standards

The NDIR method for measuring CO in ambient air is described in EN 14626. The NDIR method for measuring CO in flue gas relates to EN 15058. Instrument type approvals are assessed under the EN 15267 certification system.

Choosing an instrument for NDIR and GFC

When selecting, the gases to be measured, measurement range (trace-level or high-level) and options such as an internal zero/span valve and oxygen sensor are assessed together. Sample drying and filtration are the most important factors in long-term measurement stability.

Tetra and NDIR/GFC analyzers

Tetra supplies, installs, calibrates and services analyzers using this method in Türkiye. The product range includes Thermo Scientific 48i and 48iQ CO analyzers (GFC), 410i and 410iQ CO₂ analyzers (optical filter correlation), the 60i NDIR multi-gas analyzer and the Mega System Thyris portable flue gas analyzer. These instruments can be integrated into Tetra's air quality station cabinets and CEMS systems.