A novel wireless, passive CO2 sensor incorporating a surface acoustic wave reflective delay line

Abstract

A 440 MHz wireless and passive surface acoustic wave (SAW) chemical sensor was developed forCO2 detection. The developed SAW gas sensor is composed of single phaseunidirectional transducers (SPUDTs), three shorted grating reflectors, andCO2-sensitivepolymer film on 41° YX LiNbO3 substrate. Coupling of modes (COM) modeling was used to find optimal designparameters. Using the extracted design parameters, the SAW device wasfabricated. Teflon AF 2400 was used as the sensitive film because it provides highCO2 solubility, permeability and selectivity. In wireless device testing using anetwork analyzer, four sharp reflection peaks with high signal-to-noise(S/N) ratio, small signal attenuation, and few spurious peaks were observedin the time domain. The time positions of the reflection peaks werewell matched with the predicted values from the simulation. Infusion ofCO2 into thechamber induced large phase shifts of the reflection peaks. Good linearity and repeatability were observedfor a CO2 concentration of 0–450 ppm. The obtained sensitivity was1.98° ppm−1. Temperature and humidity effects were also investigated during the sensitivity evaluationprocess.