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