26 also directed toward line 23 conventional acoustic wave directive components not shown may be used on both transmitter 52 and receiver 53 to focus the transmission and reception of the sonic wave path 54 improving the overall signal to noise ratio of the system under 34 no flow 34 conditions transmitter 52 radiates a sine wave produced by signal generator 55 preferably operating at an audio frequency the acoustical wave readily passes through the line 23 before it encounters the relatively rigid wall of recess 33 at reflection point 56 bouncing off point 56 the wave is redirected toward the receiver 53 passing again through the line 23 the wave is detected by receiver 53 and the output signal is fed to a microprocessor 57 the output of generator 55 is also delivered to microprocessor 57 which notes the phase relationship between the two signals this step initially establishes a 34 no flow 34 reference signal 58 depicting the phase relationship between the transmitted and received signals see fig6 under vapor flow conditions the acoustical wave is physically displaced downstream by the passing vapor both before and after bouncing off reflection point 56 this displaced wave 61 arrives at receiver 53 later than the no signal wave 54 the resultant electrical signal 59 is shown in fig6 showing the phase shift or offset existing between the 34 no flow 34 and flow conditions the microprocessor 57 integrates the difference between the phase shift under 34 no flow 34 conditions and the instantaneous phase shift over the driving test cycle to determine a value 34 a 34 and as with the thermal loss system described above the value a is proportional to the total flow 34 f 34 so as to provide.