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CO2 concentration is considered as a very important index for the health of human respiratory system and industrial greenhouse gas emission so that CO2 sensing/monitoring becomes an interesting and challenging topic for both medical purpose as well as environmental engineering application. This paper proposes an innovative CO2 sensor by using the crystal resonators, including both Langasite and Quartz. The sensing principle is based on the frequency-mass effect, by which the adsorption of CO2 molecules on the crystal electrodes (typically Silver, Gold or Platinum) will cause the mass change and thereby determine the frequency shift according to the Sauerbrey equation. Lab experiments are carried out with both Langasite resonator and Quartz Crystal Microbalance. To evaluate their CO2 sensing performance, a mixed gas of CO2 and Nitrogen (reference gas) is applied to both Langasite CO2 sensor (with Platinum electrodes) and QCM CO2 sensor (with Gold electrodes), the concentration of CO2 is adjusted from 0% to 100% with a step of 25% by using a gas proportioner. Experiments results show that both Langasite resonator and QCM have a frequency shift with CO2 concentration change that is associate with the principles mentioned above. Moreover, Langasite resonator performs more accurately, stably and reliably, which is mainly due to the crystal and electrodes properties. The proposed CO2 sensor could be used as convenient breath monitoring for chronic respiratory disease, industrial greenhouse emission monitoring and chemical lab CO2 alarm.
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