Characterising the methane gas and environmental response of the Figaro Taguchi Gas Sensor (TGS) 2611-E00
Authors: Shah Adil , Laurent Olivier , Lienhardt Luc , Broquet Grégoire , Rivera Martinez Rodrigo , Allegrini Elisa , Ciais Philippe 
Published: July 2023

Abstract. In efforts to improve methane source characterisation, networks of cheap high-frequency in situ sensors are required, with parts-per-million-level methane mole fraction ([CH4]) precision. Low-cost semiconductor-based metal oxide sensors, such as the Figaro Taguchi Gas Sensor (TGS) 2611-E00, may satisfy this requirement. The resistance of these
sensors decreases in response to the exposure of reducing gases, such as
methane. In this study, we set out to characterise the Figaro TGS 2611-E00
in an effort to eventually yield [CH4] when deployed in the field. We
found that different gas sources containing the same ambient 2 ppm
[CH4] level yielded different resistance responses. For example,
synthetically generated air containing 2 ppm [CH4] produced a lower
sensor resistance than 2 ppm [CH4] found in natural ambient air due to possible interference from supplementary reducing gas species in ambient
air, though the specific cause of this phenomenon is not clear. TGS 2611-E00
carbon monoxide response is small and incapable of causing this effect. For
this reason, ambient laboratory air was selected as a testing gas standard
to naturally incorporate such background effects into a reference
resistance. Figaro TGS 2611-E00 resistance is sensitive to temperature and
water vapour mole fraction ([H2O]). Therefore, a reference resistance
using this ambient air gas standard was characterised for five sensors (each inside its own field logging enclosure) using a large environmental chamber, where logger enclosure temperature ranged between 8 and
38 ∘C and [H2O] ranged between 0.4 % and 1.9 %.
[H2O] dominated resistance variability in the standard gas. A linear
[H2O] and temperature model fit was derived, resulting in a
root mean squared error (RMSE) between measured and modelled resistance in
standard gas of between ±0.4 and ±1.0 kΩ
for the five sensors, corresponding to a fractional resistance uncertainty
of less than ±3 % at 25 ∘C and 1 % [H2O]. The TGS 2611-E00 loggers were deployed at a landfill site for 242 d before and
96 d after sensor testing. Yet the standard (i.e. ambient air) reference
resistance model fit based on temperature and [H2O] could not replicate
resistance measurements made in the field, where [CH4] was mostly
expected to be close to the ambient background, with minor enhancements.
This field disparity may have been due to variability in sensor cooling
dynamics, a difference in ambient air composition during environmental
chamber testing compared to the field or variability in natural sensor
response, either spontaneously or environmentally driven. Despite
difficulties in replicating a standard reference resistance in the field, we devised an excellent methane characterisation model up to 1000 ppm [CH4] by using the ratio between measured resistance with [CH4] enhancement and its corresponding reference resistance in standard gas. A bespoke power-type fit between resistance ratio and [CH4] resulted in an RMSE between the modelled and measured resistance ratio of no more than ±1 % Ω Ω−1 for the five sensors. This fit and its corresponding fit parameters were then inverted and the original resistance ratio values were used to derive [CH4], yielding an inverted model [CH4] RMSE of less than ±1 ppm, where [CH4] was limited to 28 ppm. Our methane
response model allows other reducing gases to be included if necessary by
characterising additional model coefficients. Our model shows that a 1 ppm
[CH4] enhancement above the ambient background results in a resistance
drop of between 1.4 % and 2.0 % for the five tested sensors. With
future improvements in deriving a standard reference resistance, the TGS 2611-E00 offers great potential in measuring [CH4] with
parts-per-million-level precision.