Investigation of gas-sensitivity of sensor structures to carbon monoxide in a wide range of temperature, concentration and humidity of gas medium

Response to CO of seven sensor structures of SnO2 with various catalytic additives, made by thick-film technology, was investigated in a dry gas medium at a concentration of 200 ppm CO in a temperature range of 100-600(circle)C. Concentration dependence of four sensors (pure SnO2 and doped with three catalytic additives: 3% La2O3, 1% Sb2O5 + 3% La2O3 and 1% Pt + 3% Pd) was investigated in a range of 0.5-98 ppm CO of different relative humidity (RH) in a range of 0-100% RH. The response and resistance of the sensors weakly depended on the humidity of gas medium at RH >= 10%. These parameters were averaged for each CO concentration in the range of 10-100% RH. Arithmetic mean values of response and resistance were approximated by linear functions of lg(S-ma - 1) = B + n 1g C and R-ma = D + k 1g C, respectively. It is shown that the relative error of the arithmetic mean values of resistance sd[(R-ma)/R-ma] in long-duration continuous measurements of the concentration dependence can be a criterion of reproducibility of sensor indications. The resistance drift of the sensors in pure air did not depend on the humidity of gas medium and did not exceed 0.4-0.5% h(-1) for the best sensors. The response threshold C-min in a dry gas medium and the arithmetic mean values in a range of 10-100% RH were determined. Based on the comparative analysis of experimental data (response to CO, reproducibility of indications, influence of humidity on the resistance and response, response threshold and a resistance drift in pure air), the best for CO detection was the sensor SnO2 + 1 % Sb2O5 + 3% La2O3, working at temperature 450(circle)C and consuming 210 mW. Formulas for response and resistance of this sensor were presented, which allowed estimation of CO concentration in air based on the measured parameters. The error of calculation of 20ppm CO in a range of 10-100% RH did not exceed 10-18%. Reproducibility of indications in the range of 0.5-98 ppm CO was within 6-10%. The response threshold was 0.80 +/- 0.03 ppm CO in the mentioned range of humidity. The present sensor can be recommended as the primary transducer of CO concentration in air for the gas control devices. (c) 2006 Elsevier B.V All rights reserved.