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Auto-zero for low-cost NO2 sensor

Nowadays, gas sensors have been widely employed to monitor ambient gas concentrations, due to its cheap price, small size and convenient deployment. However, the drawbacks of theses gas sensors, like gas cross impact, temperature and humidity impact, environmental sensitivity, also cannot be ignored during the research and application. According to our previous study, sensors can’t stand sudden temperature/humidity change in an environment or the change from one environment to another one. When it happens, it may take few hours for the sensor to warm up to the normal working mode and the calibration equation would also shift with an offset. To overcome this problem, a portable apparatus based on an electrochemical sensor was built with an auto-zero function to measure indoor and outdoor NO2. A chemical media, which can remove NO2 by chemisorption, was used to generate a gas free of NO2. Then the gas was flushed through the NO2 sensor to do the zero calibration and the sensor drift was obtained. When the media was exposed to the air, the relative percentage of water of the media can change based the moisture content of the air and it will equilibrate with the ambient moisture and the media will either gain or lose moisture. Due to this special characteristic, a Nafion tube was also utilized to balance the humidity change brought by the chemical media to avoid unexpected sensor response to it. Then, the apparatus was set in three environments including indoor and outdoor with a reference NO2 analyzer. The results showed that after the zero calibration, the measured gas concentration of our designed NO2 apparatus had good agreement with that of the reference analyzer.

Fig. 1 Schematic diagram of the auto-zero apparatus

Fig. 2Preliminary test of the auto-zero apparatus after it was moved from our lab to an apartment

Project contact: Li SUN