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 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.
