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