A mono-atmospheric chamber module for placing on the top of a Christmas tree (1) in a well-head, to keep equipment and personnel at surface conditions during the execution of varied well operations. The chamber module comprises a mainly oblong cylindrical mantle (4) with associated base-ends. An opening (7) for tight engagement with the top of the Christmas tree (1) is arranged in the lower base-end (5). The upper base-end (6) is detachable. Inside the module, a pressure resistant personnel chamber (14) is arranged with direct connection to an access opening (19) placed in the cylinder mantle (4).