Abstract
Environmental monitoring of carbon dioxide (CO2) emissions and their concentrations in the atmosphere is increasingly crucial to controlling the greenhouse effect and improving the operational safety of various industrial sectors, such as smart grid power, transportation, and the like. Optical-based nondispersive infrared (NDIR) CO2 sensors are highly regarded for environmental CO2 monitoring due to their unique robustness and stability against disturbances. Today, the most critical challenge to developing NDIR CO2 sensors with high compactness and performance is the lack of a sufficiently small optical sensing cell (OSC) possessing simultaneously high optical path length per unit volume (OPLV), relative ray intensity (RRI), and RRI consistency at maximum allowable placement error (RRIC).This thesis reports the research and development of a compact-type high-performance NDIR CO2 sensor featuring a novel OSC integrating with a signal conditioner and a controller for environmental CO2 monitoring based on ray-guided multi-reflector (RGMR) and dome collector (DC) techniques. The proposed OSC, called "RGMR–DC OSC," is tactfully created by specifically configuring an RGMR and a DC and integrating them with an IR source and a detector to simultaneously enable the ray-guiding feature, multi-pass feature and ray-focusing feature for synergically enhancing the OPLV, RRI, and RRIC at a small size. The application background and design, computational method and analysis, fabrication and evaluation, calibration and field demonstration are also presented and discussed for the RGMR–DC OSC and its resulting NDIR CO2 sensors.
A comprehensive computational model for designing, analyzing, and optimizing the proposed RGMR–DC OSC and its NDIR CO2 sensors is established using the COMSOL Multiphysics finite-element technique, TracePro ray-tracing technique, Multisim SPICE technique, and Python programming technique. Python serves as the primary computational platform for simulations and calculations, while COMSOL Multiphysics, TracePro, and Multisim perform specific/functional simulations supplementary to the Python platform. An optimal RGMR–DC OSC design is obtained with high OPLV, RRI, and RRIC levels of 4.4 cm/cm³, 2.86%, and 96% at a sufficiently small size of 5 cm × 5 cm × 1 cm (i.e., 25 cm³ volume). A signal conditioner is also designed to maximize the amplification gain while minimizing the noise and thermal drift. The sensor simulations show excellent detection performance of 55 µV/ppm sensitivity and 0.27 ppm resolution at 400 ppm CO2 concentration and 25°C temperature, with the maximum thermal drift of 3.96 ppm at a temperature variation of 25±3°C. The trade-off between power consumption and detection resolution is studied and balanced. Single-sided ventilation (SSV) and stack ventilation (SV) are developed to enhance the response time.
Three groups of RGMR–DC NDIR CO2 sensors, each with three samples and using different grades of signal-conditioning components, are fabricated, and their detection performances are evaluated to experimentally investigate the sensitivity, resolution, stability, power consumption, and response time. The best-performing group exhibits 61 µV/ppm sensitivity, 0.25 ppm resolution, 46.8 mW power consumption, and 15 s response time at 400 ppm CO2 concentration and 25°C temperature, together with an hourly fluctuation of 1.75 ppm and a weekly drift of 3.77 ppm at 25±3°C temperature. All-good agreements with the specifications and computations confirm the effectiveness of our fabrication technique and validate our design specifications, models, and solutions.
A proprietary calibration method and system are developed to streamline the process, improve the accuracy, and reduce the time and cost for calibration. The calibration results show a small indication error of 1.95 ppm at 400 ppm CO2 concentration using 4x3 calibration points, corresponding to 0.5% reading compared to the specification of <2% reading. The parallel architectural system design supports broadcasting-style calibration, allowing a simultaneous calibration of multiple sensors for mass production and promoting a greener production environment. A calibrated RGMR–DC NDIR CO2 sensor is deployed with a commercial sensor for field demonstration. The maximum deviation of only 3.6 ppm confirms the high suitability of our sensors for environmental CO2 monitoring.
| Date of Award | 2 Jun 2025 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Siu Wing Or (Chief supervisor) |