As part of the modernization project of C.-S. Chertal in view of the increase of its annual production to 3.5 Mt, the RH degassing unit has been revamped in order to produce 1 Mt of ultra-low carbon steel. Three goals had to be reached : avoid the raise of tapping temperature in the BOF - RH - CC4 route, reach 100% of heats with a carbon under 30 ppm in the tundish, achieve a high level of availability of the degasser. We present here the technical solutions and operating modifications chosen.