Abstract: A pressure control system allows gas to be evacuated out of a semiconductor process chamber at a substantially constant rate of mass flow. A gas line connects the process chamber to a vacuum pump. A controllable valve having a variable sized opening is positioned between the process chamber and the vacuum pump. A pressure sensor is in turn positioned between the valve and the vacuum pump, proximate the inlet to the vacuum pump. The size of the variable sized opening is regulated based upon the pressure in the gas line measured by the pressure sensor. The size of the valve opening is varied to maintain the pressure measured by the pressure sensor at a constant value. As a result, because the quantity of gas flowing through the gas line is proportional to the gas pressure, a substantially constant mass flow of gas out of the chamber and into the pump can be achieved.