A vacuum-tight stainless steel piston pump containing mercury is described which can be attached to a glass high-vacuum line. The movement of the piston is controlled to raise or lower the level of mercury in a glass burette so as to keep the pressure in the system constant. The movement of the piston is recorded upon a chart driven by a synchronous motor to give a record of the volume change with time in the gas phase at constant pressure; three chart speeds are provided. Pressure control is by matched Pirani gauges in a Wheatstone bridge network at low pressures, or by a mercury manometer with sealedin metal contacts at higher pressures; associated circuits are described.