A New Electro-Optical-Thermal Modelling for Non-Dispersive IR Sensing Technique of Gas Concentration

Abstract

In this research, a new electro-optical-thermal modeling is proposed and built by simulation program with integrated circuit emphasis (SPICE). In particular, it is constructed for use in the non-dispersive infrared (NDIR) sensing technique of gas concentration. This model, based on the theory of circuitry and the Beer-Lambert law, includes various equivalent elements for the optics, sensor, and circuits. To build and investigate the validity of the proposed model, an NDIR for measurement of CO2 is built with the hybrid combination of a thermopile sensor with a specific wavelength filter, an infrared micro electro mechanical systems (MEMS) heater, an optical tube, amplification circuits with a chopper amplifier, advanced RISC machine (ARM)-based micro processing unit and discrete electronic devices. The thermal properties of the light source with periodic modulation have been studied from the output signal of a thermopile within the limit of modulation frequency. Based on the thorough measurements of output signals and transient responses, the thermal and optical parameters of the sensor and optical components for this model are extracted. The comparison of the simulation and experimental data of the NDIR measurement for different CO2 concentrations shows a great agreement with a maximum error of 0.27% at 3500 ppm. This approach allows for the development of a high-level sensor and circuit integrated simulation based on the most fundamental principles and multiple variables.