The inverted attitude of {TAIL ~ the gas flow tube} prevents backflow of the soap film solution down to the sensor assembly and prevents diffusion of a heavier gas than the gas {HEAD ~ whose flow} is being measured into the gas flow tube, thereby eliminating diffusion of a lighter gas through the soap film during measurement 
The inverted attitude of { TAIL ~ the gas flow tube } prevents backflow of the soap film solution down to the sensor assembly and prevents diffusion of a heavier gas than the gas { HEAD ~ whose flow } is being measured into the gas flow tube , thereby eliminating diffusion of a lighter gas through the soap film during measurement
OTH OTH OTH OTH TAIL 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 HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH