A New Portable Instrument for In Situ Measurement of Atmospheric Methane Mole Fraction by Applying an Improved Tin Dioxide–Based Gas Sensor

Abstract

A new portable instrument based on a tin dioxide natural gas leak detector was developed to monitor the atmospheric methane mixing ratio in areas lacking sufficient infrastructure to sustain a conventional measurement system, such as a large power source, carrier gas supply, and temperature-stabilized laboratory. The effect of water vapor, the dominant factor influencing the stable measurement of methane, was examined. Reducing water vapor to less than 10 ppm in the sample air enabled stable and sensitive detection of atmospheric methane. The selectivity of the sensor to other flammable gases was also examined and improved, without additional power consumption, by applying an additional catalytic layer made of platinum-black powder suspended in quartz wool. The developed system is small (W ? D ? H = 300 mm ? 250 mm ? 200 mm) and lightweight (4 kg). The normal power consumption of the detector unit, consisting of a detector, a heater for controlling temperature, and an electronic circuit for each device, is less than 10 W. The precision of the tin dioxide?based sensor system for methane measurement in ambient air was compared with that of a conventional system, a gas chromatograph equipped with a flame ionization detector. Diurnal variations were clearly detected, and an excellent linear relationship was found between the two systems, with a standard deviation of 4 ppb (1σ).