A medical apparatus for the preparation of medical fluid is disclosed, comprising a support structure, a movable member, biasing means, and locking means. The movable member is mounted to the support structure and the preferably magnetic biasing means are configured to operate in at least one of a repulsion mode and an attraction mode In the repulsion mode, the biasing means exert on the movable member a biasing force directing the movable member away from its retracted configuration, and in the attraction mode, the biasing means exert on the movable member a biasing force directing the movable member towards its retracted configuration. A blood treatment apparatus comprising the medical apparatus for the preparation of fluid, a method of setting up the medical apparatus, and a method for use of the apparatus are also disclosed.
