Characterising the methane gas and environmental response of the Figaro Taguchi Gas Sensor (TGS) 2611-E00
<p>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 ([CH<span class="inline-formula"><sub>4</sub></span>]) precision. Low-cost semiconducto...
| Main Authors: | , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2023-07-01
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| Series: | Atmospheric Measurement Techniques |
| Online Access: | https://amt.copernicus.org/articles/16/3391/2023/amt-16-3391-2023.pdf |
| Summary: | <p>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 ([CH<span class="inline-formula"><sub>4</sub></span>]) 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 [CH<span class="inline-formula"><sub>4</sub></span>] when deployed in the field. We
found that different gas sources containing the same ambient 2 ppm
[CH<span class="inline-formula"><sub>4</sub></span>] level yielded different resistance responses. For example,
synthetically generated air containing 2 ppm [CH<span class="inline-formula"><sub>4</sub></span>] produced a lower
sensor resistance than 2 ppm [CH<span class="inline-formula"><sub>4</sub></span>] 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 ([H<span class="inline-formula"><sub>2</sub></span>O]). 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 <span class="inline-formula"><sup>∘</sup></span>C and [H<span class="inline-formula"><sub>2</sub></span>O] ranged between 0.4 % and 1.9 %.
[H<span class="inline-formula"><sub>2</sub></span>O] dominated resistance variability in the standard gas. A linear
[H<span class="inline-formula"><sub>2</sub></span>O] and temperature model fit was derived, resulting in a
root mean squared error (RMSE) between measured and modelled resistance in
standard gas of between <span class="inline-formula">±</span>0.4 and <span class="inline-formula">±</span>1.0 k<span class="inline-formula">Ω</span>
for the five sensors, corresponding to a fractional resistance uncertainty
of less than <span class="inline-formula">±</span>3 % at 25 <span class="inline-formula"><sup>∘</sup></span>C and 1 % [H<span class="inline-formula"><sub>2</sub></span>O]. 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 [H<span class="inline-formula"><sub>2</sub></span>O] could not replicate
resistance measurements made in the field, where [CH<span class="inline-formula"><sub>4</sub></span>] 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 [CH<span class="inline-formula"><sub>4</sub></span>] by using the ratio between measured resistance with [CH<span class="inline-formula"><sub>4</sub></span>] enhancement and its corresponding reference resistance in standard gas. A bespoke power-type fit between resistance ratio and [CH<span class="inline-formula"><sub>4</sub></span>] resulted in an RMSE between the modelled and measured resistance ratio of no more than <span class="inline-formula">±</span>1 % <span class="inline-formula">Ω</span> <span class="inline-formula">Ω<sup>−1</sup></span> for the five sensors. This fit and its corresponding fit parameters were then inverted and the original resistance ratio values were used to derive [CH<span class="inline-formula"><sub>4</sub></span>], yielding an inverted model [CH<span class="inline-formula"><sub>4</sub></span>] RMSE of less than <span class="inline-formula">±</span>1 ppm, where [CH<span class="inline-formula"><sub>4</sub></span>] 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
[CH<span class="inline-formula"><sub>4</sub></span>] enhancement above the ambient background results in a resistance
drop of<span id="page3392"/> 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 [CH<span class="inline-formula"><sub>4</sub></span>] with
parts-per-million-level precision.</p> |
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| ISSN: | 1867-1381 1867-8548 |