Optical Path Optimal Length for Ultraviolet Ammonia Detection With a Compact and Flexible Gas-Absorption Cell

A flexible and detachable gas-absorption cell with variable optical path length was designed to obtain the optical path optimal length for gas detection based on ultraviolet differential optical absorption spectroscopy. A model of the gas-absorption cell structure was established based on Lambert-Beer law. This model demonstrates the effect of different optical path lengths on ammonia detection, and the optical path optimal length of the proposed system is 1.6 m. A gas detection system was developed with the proposed gas-absorption cell, and the feasibility and stability of the system were evaluated at different ammonia concentrations. Owing to the influence of light loss induced by spherical concave mirror reflection and ammonia absorption, the detection results for ammonia at a concentration of 17 ppm are more accurate when the optical path length is 1.6 m versus 0.8 and 2.4 m. The error is less than 0.6% that is consistent with the simulation results. When the optical path length is 1.6 m, the linearity between the detection and the standard values reaches 0.9993. The standard deviation of the detection results is less than 0.3 ppm, indicating good stability of the system. The minimum detection limit of 0.43 ppm per meter is achieved. Water has a slight influence on the results of ammonia concentration detection. When the relative humidity is 7.5%, the response time of the system is 120 s that becomes longer with the increase of relative humidity. This method can be used to detect other gases with high precision.