The acoustic signal generated by the absorption of the third laser beam is proportional to {HEAD ~ the propane concentration} in {TAIL ~ the flowing gas sample} 
The acoustic signal generated by the absorption of the third laser beam is proportional to { HEAD ~ the propane concentration } in { TAIL ~ the flowing gas sample }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL