A mid-infrared carbon dioxide(CO_2) sensor is presented for the application in greenhouse environment. An integrated multi-pass gas chamber and a dual-channel differential detection method are adopted to decrease response time and suppress environmental influence, respectively. An optical module is developed using a cost-effective wideband mid-infrared light source, a dual-channel pyre electrical detector and a spherical mirror, and the moisture-proof function is specially designed for enabling the application of this sensor in greenhouse with high humidity. Experiments are carried out to evaluate the sensing performance on CO_2 concentration. According to the experimental results, the limit of detection(Lo D) is about 3×10^(-5) with an absorption length of 30 cm. The relative detection error is less than 5% within the measurement range of 3×10^(-5)—5×10^(-3). Based on 10 h long-term stability measurement on 5×10^(-4) and 2×10^(-3) standard CO_2 samples, the maximum fluctuations are 1.08% and 3.6%, respectively. By using a 2.4 GHz wireless network communication system for remote monitoring and data recording, a field measurement of this sensor in a greenhouse is conducted, and good performance is proven in such circumstance.
By adopting a distributed feedback laser(DFBL) centered at 1.654 μm, a near-infrared(NIR) methane(CH4) detection system based on tunable diode laser absorption spectroscopy(TDLAS) is experimentally demonstrated. A laser temperature control as well as wavelength modulation module is developed to control the laser's operation temperature. The laser's temperature fluctuation can be limited within the range of-0.02—0.02 °C, and the laser's emitting wavelength varies linearly with the temperature and injection current. An open reflective gas sensing probe is realized to double the absorption optical path length from 0.2 m to 0.4 m. Within the detection range of 0—0.01, gas detection experiments were conducted to derive the relation between harmonic amplitude and gas concentration. Based on the Allan deviation at an integral time of 1 s, the limit of detection(Lo D) is decided to be 2.952×10^(-5) with a path length of 0.4 m, indicating a minimum detectable column density of ~1.2×10^(-5) m. Compared with our previously reported NIR CH_4 detection system, this system exhibits some improvement in both optical and electrical structures, including the analogue temperature controller with less software consumption, simple and reliable open reflective sensing probe.
A differential carbon monoxide(CO) concentration sensing device using a self-fabricated spherical mirror(e.g.light-collector) and a multi-pass gas-chamber is presented in this paper.Single-source dual-channel detection method is adopted to suppress the interferences from light source,optical path and environmental changes.Detection principle of the device is described,and both the optical part and the electrical part are developed.Experiments are carried out to evaluate the sensing performance on CO concentration.The results indicate that at 1.013×10~5 Pa and 298 K,the limit of detection(LoD) is about 11.5 mg/m^3 with an absorption length of 40 cm.As the gas concentration gets larger than115 mg/m^3(1.013×10~5 Pa,298 K),the relative detection error falls into the range of-1.7%—+1.9%.Based on 12 h long-term measurement on the 115 mg/m^3 and 1 150 mg/m^3 CO samples,the maximum detection errors are about0.9%and 5.5%,respectively.Due to the low cost and competitive characteristics,the proposed device shows potential applications in CO detection in the circumstances of coal-mine production and environmental protection.
