Characterization and Optimization of Optical Gas Sensors



Dipl.-Ing. Florian Dietachmayr


The negative impact of bad air quality in buildings on human performance and well-being is well known. The most common effects are tiredness, a lack of concentration, and headache.

An efficient way of improving the air quality in buildings are fully automated air conditioning systems. A number of parameters have to be measured in order to guarantee a good performance of those systems. Especially the concentrations of carbon dioxide (CO2) and water vapor (H2O) are important indicators for air quality.

Sensors used in fully automated air conditioning systems have to be long-term stable and should require little maintenance. Chemical gas sensors are not suitable for this task as they usually exhibit a high temporal drift. Therefore, optical gas sensors based on the non-dispersive infrared (NDIR) principle are typically used in automated air conditioning systems.

Figure 1: Set of different gas absorption spectra in the mid-infrared region from [Graf; Software-tailored Non-dispersive Infrared Sensors]. Figure 2: Schematic set-up of an NDIR gas sensor.

NDIR gas sensors utilize the absorption of infrared light by gas molecules. Many gases have absorption spectra in the infrared region (Fig. 1) and can therefore be measured by optical sensors.

Figure 2 shows the schematic set-up of an NDIR gas sensor. A light source emits infrared light which passes through a gas chamber. In the gas chamber the light gets partially absorbed by the molecules of the gas components. An optical bandpass filter is used to block the wavelength-bands of the radiation that are unaffected by the measurand. The filtered and attenuated radiation is measured by a detector.

All components of the optical gas sensor system will be analyzed and a simulation model of each component will be devised. These models will be used to evaluate the accuracy of different set-ups. Based on this research an optimized version of the sensor system will be built. Additionally, the feasibility and performance of different approaches of multi-gas measurement will be analyzed.