ANALYZING CO2 CONCENTRATION WITH NON-DISPERSIVE INFRARED SENSOR (NDIR) USING FAST FOURIER TRANSFORM (FFT)

Abstract

In this paper, non-dispersive infrared gas sensor (NDIR) for analyzing CO₂ concentration was designed and fabricated. The gas sensor structure includes an infrared heat lamp (IR-lamp) and infrared thermopile device with two detectors integrated optical band-pass filters at 4.279mm and 3.91mm. Square-pulse voltage source with two levels of 5V and 0V at 0.1Hz was applied for operating the IR-lamp. Output voltages of the IR thermopile detectors were amplified by 10000 times to obtained the sensing voltage (U₁) and the reference voltage (U₂). The obtained voltage values (U₁ and U₂) were analyzed by fast Fourier transform (FFT) to eliminate the noise and get the signal in correspondence with the frequency of IR-lamp. The voltage values (U₁ and U₂) were investigated for the CO₂ concentration range of 0-80%vol. The linear dependence of gas-sensing characteristic ln(U₁/U₂) on the CO₂ concentration was found.