A typical system for measuring pressures in a rarefied gas consists of a vacuum gage connected to the gas by a length of tubing, usually with an orifice of diameter smaller than that of the tubing at the end. The interior surface of this system will outgas at a fairly constant rate for a long period of time so that an equilibrium state will be reached with a definite non‐constant pressure distribution along the tubing. The response time constant for the system to come to equilibrium and the difference in pressure between gas and gage are functions of the outgassing rate and the geometry of the system. An analysis is made assuming molecular flow, and formulas are obtained for the response time and the pressure difference. It is shown that there is an optimum value of the tubing diameter for a given orifice diameter, gage volume, and length of tubing. This optimum is rather critical; if the diameter is too small, response time constants of the order of hours may result. Experimental work has been done and the results compared with theory. This work was done under ONR contract.