Molecular spectroscopy analysis of SF6 absorption bands for the design of highly sensitive NDIR sensors for industrial applications

Sulfur hexafluoride (SF₆) is a widely used gas in the industry in high- and medium-voltage equipment that, in the long-term, can suffer from gas leaks. SF₆ leak detection with optical means has been reported previously using the primary absorption band $v_3$ for gas concentration below 0.1 %. This work aims to use molecular spectroscopy to examine the SF₆ absorption bands as an alternative approach to develop non-dispersive infrared (NDIR) sensors capable not only of monitoring real-time gas changes in gas-insulated substations (GIS), but also of improving sensitivity and detection percentages above 0.1 %. An experimental setup is proposed using a Fourier Transform Infrared Spectrophotometer (FTIR) to analyze the SF₆ concentration changes. Results show that the $v_3$ primary absorption band exhibits a pronounced sensitivity to concentrations below 0.2 %, whereas at elevated levels, it exhibits a diminished sensitivity, and at concentrations above 10 %, it demonstrates low sensitivity. In contrast, the combination bands do not demonstrate sensitivity for concentrations below 0.2 % but exhibit an increase in sensitivity above that concentration. A basic SF₆ NDIR sensor was implemented to validate the results obtained. The implemented sensor has a detection limit of 25 ppm, and it was demonstrated that it is possible to enhance the sensitivity of NDIR sensors that solely consider the $v_3$ band by employing the combination bands for detection in gas concentrations above 0.2 %. This work serves as a basis for enhancing the design of NDIR sensors by modifying their configuration to optimize accuracy and robustness.