FIELD DEPLOYABLE SIGNAL PROCESSING SYSTEM FOR MEASUREMENT OF PATH INTEGRATED GAS CONCENTRATION IN REAL-TIME

Abstract

Detection of multiple re signatures in the incipient stage of combustion can im- prove the re detection response time and minimize the false alarm. Fire mainly consists of combustion gases like carbon dioxide (CO2), carbon monoxide (CO), methane (CH4) whose analysis brings early re detection. The simultaneous mea- surement of concentration of target gas species is based on near infra-red (1.2 to 2.0 m) absorption spectroscopy and sensitive detection techniques coupled with advanced diode lasers and optical bres. It is based on the principle that gas molecules absorb energy in narrow bands (about few cm􀀀1) surrounding speci c wavelengths in the electromagnetic spectrum. In proposed method, the laser wave- length sinusoidally modulated at a frequency in the range of kHz and transmitted this modulated beam thorough the gas sample in protected space. Due to presence of the gases, this modulated beam get distorted and this signal is detected at In- GaAs detector as a combination of harmonics of the original modulating sinusoidal frequency. From this detected signal, the fundamental and second harmonics are extracted and their amplitudes are measured. The ratio of amplitudes of second harmonics to fundamental is proportional to the concentration of the gas present. The proposed scheme to extract required harmonic amplitudes is Lock-in-ampli er followed by moving average lter. The lock-in ampli er locks the reference fre- quency of signal with input signal and produces the DC output proportional to the amplitude of the particular frequency present in the input signal.