Tham khảo tài liệu 'monitoring control and effects of air pollution part 4', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Gas Sensors for Monitoring Air Pollution 51 Gas-sensing properties were measured in a conventional gas-flow apparatus by changing the mixing ratio between the parent gas 4 CO2 in an N2 balance and dry synthetic air. The operating temperature was controlled by monitoring the applied voltage and current using the power supply. The sensors were exposed to the flow 100 cm3 min of the required sample gases. The gas mixtures of CO2 air with the CO2 concentration varied from 1 000 to 10 000 ppm. Four types of sensors were fabricated from NASICON as a solid electrolyte. A series of Na2CO3-CaCO3 mixtures at the molar ratio range of 1 0-1 2 was attached to the sensing electrode. Figure 6 shows the EMF response to CO2 as a function of the CO2 concentration at various temperatures. The EMF variation for each sensor at 470oC agreed well with the theoretical value of mV decade based on a two-electron electrochemical reaction. As the temperature decreased however the slope tended to deviate from the ideal. Quite noticeably the deviation could be suppressed very effectively with Na2CO3-CaCO3 1 2 which allowed mV decade to be kept at temperatures as low as approximately 400oC. An increase in the amount of CaCO3 at the auxiliary phase is fairly effective for keeping the theoretical value at lower temperatures whereas an adverse effect occurred when the CaCO3 content was insufficient. The mechanism behind such improvements is not yet well understood though. It requires further research. HCHO gas sensor Formaldehyde HCHO is an achromatic toxic gas and has a stimulating scent. When exposed to HCHO gas even just for a short time a person may develop headache and vertigo and when exposed to it for a long time a person may develop asthma and other lung diseases. When exposed to high concentrations of HCHO a person may develop pneumonia or edema of the lungs 9 . Considering these the allowed concentrations of formaldehyde in Korea Denmark the Netherlands and Germany are only
