The Pressure Distribution in a Liquid-Nitrogen Cold Trap

Abstract

Considerable controversy exists in the literature with respect to the pressure drop across a cold trap. In 1915, W. Gaede [¹] reported the results of his investigation designed to check the statement of Muller-Pouillet that a balance of pressure exists across a cold trap and the connecting tube used in determining the ultimate vacuum of mechanical mercury-sealed pumps. Gaede presented equations for the counter-diffusion of air and mercury vapor in the tube connecting the pump and cold trap over the whole range of pressures from molecular to laminar-viscous flow. In contradiction to Muller-Pouillet’s statement, Gaede showed that at low pressures a considerable pressure drop exists in the connecting tube because the conductance of the cold trap is small compared to its pumping rate. Although Gaede’s equations do not apply across the cold trap itself, his report is the earliest known to the present investigator indicating that a pressure drop is expected when a cold trap is used.

Work supported in part by National Science Foundation Grant Number GK-1606.