Cascaded high-density multipoint gas detection with branched gas chambers

Abstract

Distributed or multipoint gas detection is crucial for several industrial and civil applications. Due to various losses, the traditional series–parallel method has a restricted number of measurement points. In this paper, an innovative cascaded connection method has been introduced for measuring a large number of locations. It is basically a branching route that contains a gas cell and a reflecting mirror. A small amount of light is coupled to the branch channel, leading to limited effects on transmission power. Using a light source that corresponds to the gas absorption line, an optical time domain reflectometer (OTDR) device interrogates each cell by branching. After establishing a length of delay fiber at one of the branch locations, the signals coming from each air cell are recognized individually at the receiving end. In this experiment, 99:1 couplers are used, and only 1% of the total light is transferred to the branch route gas cell. The signal-to-noise ratio increases dramatically compared to previous studies based on Rayleigh scattering. This is because the amplitude of the reflected light is one-half order more than that of the scattered light. A 1530 nm laser for acetylene and a 1653 nm laser for methane with a short pulse width are used for evaluating the novel multipoint gas detection technique. There is an analysis and discussion of the number of measurement points. The proposed method is capable of optimally laying 220 cells at a maximum monitoring distance of 8.8 km within a dynamic range of ∼13.56 dB. Using the present wavelength power calibration method, the minimum detection limits for methane and acetylene along the 1% branching path are 0.093 low explosion level (LEL) and 0.321 LEL, respectively.