Abstract: Apparatus for maintaining a predefined pressure in one or more reactors to which pressures from 1 to 500 bar can be applied, with simultaneous measurement of the gas mass flow, comprising at least one buffer container (A) which is equipped with a pressure gauge (B) and can be filled with gas via at least one valve (C), and the buffer container (A) being connected to at least one reactor (F) which is likewise equipped with a pressure gauge (B′), characterized in that the connection between buffer container and reactor contains a restrictor (D) and a valve (E) which is controlled via a control unit (H) which is connected to the pressure gauges of the reactor (F) and of the buffer container (A) via control lines.