THERMOMOLECULAR PRESSURE CORRECTIONS IN HELIUM VAPOUR PRESSURE THERMOMETRY - EFFECT OF TUBE SURFACE

The equation of Weber and Schmidt has been used for some time to describe thermomolecular pressure difference corrections necessary in precise vapour pressure thermometry. Roberts' and Sydoriak's tables of corrections are based on this equation. Several investigations have shown significant deviations from the constants given by Weber and Schmidt when metallic tubes are used in the measurement. It is shown that constants to fit the data can be found by modifying the equation to explicitly contain the momentum accommodation coefficient, which is a measure of the interaction of the gas with the tube wall. Data are given to show that polishing the tube surface decreases the thermomolecular pressure difference. A procedure is given to obtain single tube thermomolecular pressure corrections for vapour pressure corrections from a single differential measurement. © 1969.