 properti impact pressur probe free molecul flow properti impact pressur probe free molecul flow express deriv mass flow circular tube free molecul flow tube ga rel motion ga enter tube assum maxwellian distribut function molecular reflect process wall assum diffus theori determin pressur read impact probe free molecul flow gener express deriv appli ga veloc tube size detail calcul pressur probe difficult case low speed long tube experiment check theori carri impact probe whirl arm apparatu utia low densiti wind tunnel agreement theori experi satisfactori 