NDIR against Metal Oxide Films in CO2 Measurement
NDIR sensors provide definitive CO2 accuracy through direct mid-IR light absorption, while low-cost metal oxide films infer CO2 via prone cross-reactive surface chemistry.
Spectral Absorption
Radiation Interaction
Electromagnetic energy reduction describes the process by which specific wavelengths of light are attenuated as they pass through a substance according to its molecular structure. This phenomenon, known as spectral absorption, is the basis for all optical gas sensing technologies. When a photon of infrared light matches the natural vibration frequency of a gas molecule, the molecule absorbs the energy and enters a higher energy state.
This causes a measurable drop in the intensity of the light that reaches the detector on the other side of the gas cell. Because every molecule has a unique set of vibration frequencies, the absorption pattern acts as a fingerprint for identifying different chemicals. It is a highly specific method that minimizes the risk of cross sensitivity between different gases.
Molecular Transition
The energy levels of a molecule are quantized, meaning that only specific amounts of energy can be absorbed to move from one state to another. During spectral absorption, the molecules undergo rotational and vibrational transitions that correspond to the bonds between the atoms. For example, the carbon oxygen bonds in a CO2 molecule absorb light at a wavelength of four point three micrometers.
This specificity allows a sensor to detect carbon dioxide in a complex mixture of other gases without any interference. The strength of the absorption depends on the number of molecules in the path and the physical properties of the gas. Sensors use this relationship to calculate the exact concentration of the gas in the air.
Beer-Lambert Law
Mathematical relationship between the light intensity and the gas concentration is described by a fundamental law of physics. As the concentration of the target gas increases, more light is absorbed and the signal at the detector decreases following an exponential curve. This means that a sensor using spectral absorption is most sensitive at low concentrations where the change in light intensity is the largest.
The length of the optical path also plays a role, with longer paths allowing for the detection of lower concentrations of gas. However, a longer path also makes the sensor larger and more difficult to integrate into portable devices. Engineers use reflective mirrors and folded optical designs to create long paths in a small volume.
Frequency Window
Optical sensors must be designed to work within a specific part of the spectrum where the target gas has its strongest absorption peaks. This frequency window is selected to avoid the absorption regions of common background gases like water vapor. If the spectral absorption of water overlaps with the target gas, the sensor will give false readings whenever the humidity changes.
Narrow bandpass filters are used to isolate the desired wavelengths and block out the rest of the infrared source. The stability of the light source and the detector is also important, as any change in their performance could be mistaken for a change in gas absorption. Regular calibration with a zero gas confirms that the optical system is aligned and the filters are working correctly.