The inverted attitude of 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 whose flow is being measured into the gas flow tube, thereby eliminating {HEAD ~ diffusion} of a lighter gas through {TAIL ~ the soap film} during measurement 
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The inverted attitude of 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 whose flow is being measured into the gas flow tube , thereby eliminating { HEAD ~ diffusion } of a lighter gas through { TAIL ~ the soap film } during measurement
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