NDIR Gas Measurement with an Active Multicolor Detector and Pulse Response Evaluation for Low Power Applications

摘要

Monitoring the concentration of carbon dioxide is one of the future tasks to decrease the energy-consumption of a private home. Demand-oriented air condition stands for high quality room climate and saves heating energy. At present, procurable CO_2-sensors do not fulfill the requirements of home applications. Easy handling, maintenance-free, low power consumption and a low price are the main parameters a sensor must fulfill to be used in every room of a private home. To achieve long term (up to 5 years) high precision measurements the Non Dispersive Infra Red (NDIR) measurement is qualified for a postulated accuracy of 100 ppm over a total range from 0 ppm to 2000 ppm. To keep down the costs of production low cost pyroelectric sensors are used. They are irradiated by a micro incandescent lamp. Pyroelectric sensors are sensitive on the gradient of the change of the radiation. Therefore, the IR source is usually electrically or mechanically chopped with the frequency the detector is most sensitive for. The power consumption of the IR source is very high. This is especially true for battery powered applications. To decrease the wattage a new method is used to solve this problem. With only one shot of radiation the pulse response of the pyroelectric detector is evaluated by a smart microcontroller. The microcontroller also calculates the corresponding CO_2 concentration. To achieve constant good measurement values a new detector design was developed to provide maximum amplitudes in pulse mode and also in the chopping mode. The newly developed pulse mode does not achieve the same accuracy as the common chopping mode. Therefore, a smart measurement algorithm alternates between the two modes and recalibrates the pulse mode from time to time. If the application should essentially watch a special CO_2 concentration threshold the algorithm is able to change the measuring algorithm in defined areas, too. All results have been achieved for the Project "ECOS" which is sponsored by the "Deutsche Bundesstiftung Umwelt (DBU)".