Spectral Fingerprint
Gas molecules have specific energy levels due to their unique individual combination of atoms and the associated binding energies. Infrared light can activate these energy levels, which reduces the transmitted light energy: The molecules absorb the light at specific wavelengths. Each type of molecule has its own typical absorption fingerprint. However, fingerprints of gases are often very similar. To avoid this cross-sensitivity, detectors can be designed to examine the absorption of one or more typical wavelength ranges of the target gas.
System Components
Nondispersive infrared sensors (NDIR sensors) are compact spectroscopic measuring systems which are often used for gas analysis. The key components are an infrared emitter, a sample chamber (gas cuvette), optical wavelength filters and infrared detectors. The selection of the wavelength is done by special infrared emitters and optical filters, rather than by a dispersive element such as a grating or prism. Since there are no moving components, NDIR detectors are very compact and robust.
m-u-t infrared multigas sensors are based on the measuring principles of NDIR spectroscopy (Figure 1). With this measuring principle, an optical filter is used to select a range of the infrared spectrum in which the measured gas strongly absorbs the radiation. This absorption strength depends on the gas concentration, allowing the gas concentration to be deduced from the detector signal. A further filter channel in a wavelength range in which the gas does not absorb the radiation, generates a reference signal with which various interferences are compensated. This 2-channel configuration makes NDIR measurement technology particularly stable.
The gas is led into the sample chamber (gas cuvette) and the infrared radiation (IR radiation) is directed through the sample chamber towards the detector. The detector has an optical filter that eliminates all light except the wavelength that the selected gas molecules can absorb.
The IR light intensity reaching the detector is inversely related to the concentration of the gas to be measured in the sample chamber. If there is no gas in the chamber, the detector receives the full light intensity. As the concentration increases, the intensity of the IR light reaching the detector decreases.
An infrared multigas sensor has at least one spectral channel for each gas component to be measured (up to eight channels are freely available), as well as a further joint reference channel. Often one gas component generates a signal at several filter channels. These cross sensitivities are determined in the m-u-t infrared multigas sensors during factory calibration and compensated directly in the sensor. Therefore, these dependencies are already considered in the measurement data output. In addition, the measurement values are linearized in the respective measuring ranges during factory calibration. This is done by measuring several reference samples of the respective gas which concentrations are spread over the entire measuring range.
