The methane gas within cell weakly absorbs {HEAD ~ the infrared energy} of laser beam , and thus the resulting output signal produced by a transducer serves as {TAIL ~ a reference signal} for determining the methane concentration 
The methane gas within cell weakly absorbs { HEAD ~ the infrared energy } of laser beam , and thus the resulting output signal produced by a transducer serves as { TAIL ~ a reference signal } for determining the methane concentration
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