Pressure controller for use in operating parallel reactors, said pressure controller being adapted to control the pressure in a fluid flow, said pressure controller comprising: - a flow channel for the fluid flow of which the pressure is to be controlled, said flow channel having a cross sectional area, - a movable valve member, said moveable valve member being adapted to control the size of the cross sectional area of the flow channel in order to control the pressure of the fluid flow in the flow channel, - a valve actuator, said valve actuator being adapted to control the position of the valve member, said valve actuator comprising a control chamber having a fluid under a reference pressure therein, said fluid engaging a pressure surface of the valve member for exerting a control force thereon, - a reference pressure controller, said reference pressure controller being adapted to control the reference pressure in said control chamber, said reference pressure controller comprising: - a first restrictor channel, said first restrictor channel having an inlet and an outlet, - a second restrictor channel, said second restrictor channel having an inlet and an outlet, - a fluid passage, which fluid passage extends between the outlet of the first restrictor channel and the inlet of the second restrictor channel, said fluid passage allowing fluid communication between the first restrictor channel and the second restrictor channel, - a pressure control fluid source, said pressure control fluid source being adapted to provide a flow of pressure control fluid through the first restrictor channel, the fluid passage and the second restrictor channel, said flow of pressure control fluid having an entrance pressure at the inlet of the first restrictor channel and an exit pressure at the outlet of the second restrictor channel, said entrance pressure being higher than said exit pressure, said flow of pressure control fluid experiencing a first pressure drop Δp1 over the first restrictor channel and a second pressure drop Δp2 over the second restrictor channel, - a connector connecting the fluid passage to the control chamber of the valve actuator, said connector being in fluid communication with said control chamber, the pressure control fluid at the connector having an intermediate pressure which is lower than the entrance pressure but higher than the exit pressure, said intermediate pressure being determined by the ratio between the first pressure drop Δp1 and the second pressure drop Δp2, - a controllable thermal device, said thermal device being adapted to heat and/or cool the first restrictor channel and/or the second restrictor channel, therewith influencing the ratio between the first pressure drop Δp1 and the second pressure drop Δp2, said thermal device comprising a thermal controller for controlling the thermal output of the thermal device. - a flow channel for the fluid flow of which the pressure is to be controlled, said flow channel having a cross sectional area, - a movable valve member, said moveable valve member being adapted to control the size of the cross sectional area of the flow channel in order to control the pressure of the fluid flow in the flow channel, - a valve actuator, said valve actuator being adapted to control the position of the valve member, said valve actuator comprising a control chamber having a fluid under a reference pressure therein, said fluid engaging a pressure surface of the valve member for exerting a control force thereon, - a reference pressure controller, said reference pressure controller being adapted to control the reference pressure in said control chamber, - a first restrictor channel, said first restrictor channel having an inlet and an outlet, - a second restrictor channel, said second restrictor channel having an inlet and an outlet, - a fluid passage, which fluid passage extends between the outlet of the first restrictor channel and the inlet of the second restrictor channel, said fluid passage allowing fluid communication between the first restrictor channel and the second restrictor channel, - a pressure control fluid source, said pressure control fluid source being adapted to provide a flow of pressure control fluid through the first restrictor channel, the fluid passage and the second restrictor channel, said flow of pressure control fluid having an entrance pressure at the inlet of the first restrictor channel and an exit pressure at the outlet of the second restrictor channel, said entrance pressure being higher than said exit pressure, said flow of pressure control fluid experiencing a first pressure drop Δp1 over the first restrictor channel and a second pressure drop Δp2 over the second restrictor channel, - a connector connecting the fluid passage to the control chamber of the valve actuator, said connector being in fluid communication with said control chamber, the pressure control fluid at the connector having an intermediate pressure which is lower than the entrance pressure but higher than the exit pressure, said intermediate pressure being determined by the ratio between the first pressure drop Δp1 and the second pressure drop Δp2, - a controllable thermal device, said thermal device being adapted to heat and/or cool the first restrictor channel and/or the second restrictor channel, therewith influencing the ratio between the first pressure drop Δp1 and the second pressure drop Δp2, said thermal device comprising a thermal controller for controlling the thermal output of the thermal device.