Temperature compensated differential acoustic sensor for CO2 sensing

Abstract

Temperature must be accounted for in order to provide accurate measurements in acoustic wave resonator based gas sensors. We present a flexible printed flexural plate wave (FPW) sensor for CO2 monitoring employing temperature compensation. The FPW sensor is based on a differential interdigitated transducers (IDTs) structure consisting of an input IDT and two identical output IDTs with same distance from the input IDT. Among the two outputs of the differential structure, one is sensitive to the target gas CO2 and the other one is insensitive to the target gas. The difference between the two output signals’ resonant frequencies is considered as the sensor’s differential response. Experiments at 25 ∘C and 45 ∘C show that the sensor’s differential response changes linearly with CO2 concentration over 20–3000 ppm range. The differential responses at both temperatures are very similar showing the ability of the sensor’s structure to compensate for the temperature change. Sensitivity and limit of detection for CO2 sensing are measured to be 543 Hz ppm−1 and 5.6 ppm, respectively. Further, the sensor shows good repeatability and low hysteresis. Sensor’s responses are mostly measured by measuring the scattering parameter, forward transmission (S21) between each output IDT and input IDT of the sensor in frequency domain. In the last section, a prototype time domain measurement system is demonstrated to show the feasibility of the time domain measurements with the sensors. The sensor with the potential to be printed with roll-to-roll printing can be a good candidate for CO2 monitoring where temperature varies.