Design, build and initial tests of a portable methane measurement platform

The quantification of methane concentrations in air is essential for the
quantification of methane emissions, which in turn is necessary to
determine absolute emission and the efficacy of emission mitigation
strategies. These are essential if countries are to meet climate goals.
Large scale deployment of methane analyzers across the millions of
emission sites is prohibitively expensive and lower-cost instrumentation
has been recently developed as an alternative.  Currently, it is
unclear how cheaper instrumentation will affect measurement resolution or
accuracy.  To test this, the Wireless Autonomous Transportable
Methane Emission Reporting System (WATCH4ERS) has been developed and
comprises four commercially available sensing technologies: Metal Oxide
(MOx,); Non-dispersion Infrared (NDIR); Integrated Infrared (INIR); and
Tunable Diode Laser Absorption Spectrometer (TDLAS).  WATCHERS is
the accumulated knowledge of several long-term methane measurement
projects at Colorado State University’s Methane Emission Technology
Evaluation Center (METEC) and this study describes the integration of
these sensors to a single unit and reports initial instrument response to
calibration procedures and controlled release experiments. Specifically,
this paper aims to describe the development of the WATCH4ERS unit, report
initial sensor response, and describe future research goals, while future
work will use data gathered by multiple WATCH4ERS units to 1. better
understand the cost-benefit balance of methane sensors, and 2. identify
how decreasing instrumentation costs could increase deployment coverage
and therefore inform large‐scale methane monitoring strategies. 
 Both calibration and response experiments indicate the INIR has
little practical use for measuring methane concentrations less than 500
ppm.  The MOx sensor is shown to have a logarithmic response to
methane concentration change between background and 600 ppm but it is
strongly suggested that passively sampling MOx sensor cannot respond fast
enough to report concentrations that change on a sub-minute time
frame.  The NDIR sensor reported linear change to methane
concentration between background and 600 ppm, although there was a
noticeable lag in reporting changing concentration especially at higher
values, and individual peaks could be observed throughout the experiment
even when the plumes were released 5 s apart.  The TDLAS sensor
reported all changes in concentration but remains prohibitively
expensive.  Our findings suggest that each sensor technology
could be optimized by either operational design or deployment location to
quantify methane emissions and the WATCH4ERS units will be deployed in
real-world environments to investigate the utility of each in the future.