Development of a prototype for dissolved CO_2 rapid measurement and preliminary tests

摘要

The measurements of dissolved CO_2 in seawater is of great significance for the study of global carbon cycle. At present, the commercial sensors used for dissolved CO_2 measurements are mostly equipped with permeable membranes for the purpose of gas-liquid separation, with the advantages of easy operation, low cost, etc.. However, most of these devices measure CO_2 after reaching gas equilibrium, so it takes a few minutes to respond, which limited its applications in rapid measurements. In this paper, a set of prototype was developed for the rapid measurements of dissolved CO_2. The system was built basing the direct absorption TDLAS. To detect the CO_2 absorption line located at 4991.26 cm~(-1), a fiber-coupled DFB laser operating at 2004 nm was selected as the light source. A Herriott type multi-pass cavity with an effective optical path length of 10 m and an inner volume of 90 mL was used for absorption measurements. A detection limit of 26 μatm can be obtained with this compact cavity. To realize the rapid measurements of dissolved CO_2, a degasser with high degassing rate was necessary. A hollow fiber membrane with a large permeable area used in this paper can achieve degassing rate up to 2.88 kPa/min. Benefitted from the high degassing rate of the degasser and high sensitivity of the compact TDLAS system, a rapid measurement of dissolved CO_2 in water can be achieved within 1s time, and the response time of the prototype when the dissolved CO_2 concentration changed abruptly in actual measurement was 15 s. To evaluate the performance of the prototype, comparison measurements were carried out with a commercial mass spectrometer. The dissolved CO_2 in both seawater and tap-water was measured, and the experimental results showed good consistent trends with R~2 of 0.973 and 0.931. The experimental results proved the feasibility of dissolved CO_2 rapid measurement. In the near future, more system evaluation experiments will be carried out and the system will be further optimized focusing on the underwater in-situ detection system.