Chamber walls, subjected to "in situ" baking, sometimes show a pumping function for a high vacuum, while a high vacuum pump sometimes shows outgassing in an ultrahigh vacuum. Such functions of the system elements can be represented by the internal pressures P-X. All the system elements, such as chamber walls, pumps, and pin holes through a pipe wall, can be replaced by a pressure generator with the internal pressure P-X and the internal flow impedance R-X in the equivalent vacuum circuit. The internal pressure P-X of the chamber wall varies depending on the wall history under high vacuum. The equivalent vacuum circuit composed of many characteristic values (P-X, R-X) and flow impedances R can represent the gas flows in the original high-vacuum system.