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