Abstract: A method and an apparatus for treatment of a first liquid which is a plasma of human blood comprising cyclically feeding the first liquid and a second liquid to the treatment device, and measuring a component of the first and second liquids so that a ratio between the first and second liquids are determined by a measuring arrangement. The first and second liquids are transferred to predetermined collection points by initiating and terminating a collection of the first liquid during one of the feeding cycles based on the ratio between the first and second liquids. The volume of at least the first liquid conveyed to the treatment device is measured during one of the cycles by a volume measuring arrangement. A theoretical time is determined between the initiating and terminating of the collection of the first liquid.