An arrangement for providing a variable volume size is described. The arrangement includes an outer bellow having an outer end wall closing the outer bellow at one end. The arrangement further includes an inner bellow having an inner end wall closing the inner bellow at one end, the inner bellow being arranged at least partly inside the outer bellow. The arrangement further includes a base at which another end of the outer bellow and another end of the inner bellow are attached. The arrangement further includes a position measuring apparatus for measuring at least one position indicative of a distance between the outer end wall and the inner end wall.