or outlet for drilling fluid from or to the drilling fluid circulation system as said pressure chambers are able to be closed and separated by an intermediate valve arranged for circulation of drilling fluid into or out of the drill string during coupling up or disconnecting of a new length of drill string a method to maintain the mainly constant pressure of drilling fluid in a drill string is also described application_number US 66540908 A description the invention comprises as shown a common pressure container 60 in which several components are localised the components can be threaded flanged or machined such that they can be put together to create a common pressure container function 60 an entering cone 80 a can be found uppermost the function of the entering cone 80 a is to guide the drill string into the invention an upper seal 20 a can be found below the entering cone 80 a the seal has a composition which enables it to make a seal around a chosen drill string 100 including the variable diameter which the drill string represents the seal allows for movement by the drill string 100 both axially and rotationally at the same time as it seals against the working pressure which is defined in advance an upper locking anchor 30 a is arranged below the upper seal 20 a the locking anchor is arranged so that when it is not connected up deactivated it allows a drill string 100 to freely move through when the locking anchor is connected up activated the bottom of the drill string the pin end 120 is hindered.