System for pressurising a volume needing to have a relative air pressure, in comparison with an ambient pressure, within a specified interval, the system comprising - a source of pressurised air, - a conduit system for delivering pressurized air from the source into the volume, - a first sensor which is arranged to measure the pressure inside the volume, - a second sensor which is arranged to measure the ambient pressure, - a first regulator arranged on a first part of the conduit system, which regulator is arranged to regulate a first airflow, from the source of pressurised air to the volume through the first part of the conduit system, in order to obtain a first relative pressure difference value between the pressure inside the volume and the ambient pressure, in response to measurement results from the first and second sensors (33,35), - a first ejector arranged on the first part of the conduit system, downstream of the first regulator, characterised in that the system comprises - a second regulator which is arranged in parallel to the first regulator and the first ejector on a second part of the conduit system and which is arranged to regulate a second flow, from the source of pressurised air, to the volume through the second part of the conduit system, in order to obtain a second relative pressure difference value between the pressure inside the volume and the ambient pressure, in response to measurement results from the first and second sensors (33,35), - a second ejector arranged on the second part of the conduit system, downstream of the second regulator, - an upper check valve arranged on the second part of the conduit system downstream of the second ejector, blocking airflow from the volume into the second ejector. - a source of pressurised air, - a conduit system for delivering pressurized air from the source into the volume, - a first sensor which is arranged to measure the pressure inside the volume, - a second sensor which is arranged to measure the ambient pressure, - a first regulator arranged on a first part of the conduit system, which regulator is arranged to regulate a first airflow, from the source of pressurised air to the volume through the first part of the conduit system, in order to obtain a first relative pressure difference value between the pressure inside the volume and the ambient pressure, in response to measurement results from the first and second sensors (33,35), - a first ejector arranged on the first part of the conduit system, downstream of the first regulator, - a second regulator which is arranged in parallel to the first regulator and the first ejector on a second part of the conduit system and which is arranged to regulate a second flow, from the source of pressurised air, to the volume through the second part of the conduit system, in order to obtain a second relative pressure difference value between the pressure inside the volume and the ambient pressure, in response to measurement results from the first and second sensors (33,35), - a second ejector arranged on the second part of the conduit system, downstream of the second regulator, - an upper check valve arranged on the second part of the conduit system downstream of the second ejector, blocking airflow from the volume into the second ejector.