Martin Knudsen published three papers in 1909 in which he explained theoretically and demonstrated experimentally for the first time our understanding of the flow of gases at low pressure in terms of molecular flow. These, together with contributions in the immediately following years, provided the stimulation for responses from Smoluchowski, Dunoyer, Gaede, Langmuir and Dushman. Between them they set the scene for vacuum science and technology as well as laying the base for what are now well developed disciplines and quite separate subjects. 

W Steckelmacher, Knudsen flow 75 years on: the current state of the art for flow of rarefied gases in tubes and systems, Rep. Prog. Phys. 49 (1986) p.1083

The major feature of Knudsen, molecular, or rarefied gas flow is that molecule-wall collisions dominate over molecule-molecule collisions.  This inturn led to the development of the Knudsen cell, which is a heated small enclosure which contains a condensed phase in equilibrium with a gas.  A small orifice provides a means of measuring the vapor pressure.
The early measurements of the quantity of material effusing from the cell orifice used target collections.  Later weight losses of the cell and vapor vaporizing material were employed. Ionov(1948) first used a mass spectrometer to analyze the high temperature vapor from an inorganic substance, The KEMS technique was born.
Martin Knudsen, 1871-1949
at the international conference
on physics, London, 1934
  (Photo owned by Gerhard F. Hund)