Position monitoring system for subsea bellow compensators It is described an arrangement for providing a variable vol- ume size, comprising: an outer bellow (125) having an outer end wall (123) closing the outer bellow at one end; an inner bellow (131) having an inner end wall (133) closing the inner bellow at one end, the inner bellow being arranged at least partly inside the outer bellow; a base (127) at which another end of the outer bellow and another end of the inner bellow are attached; and a position measuring apparatus (137) for measuring at least one position indicative of a distance (d) between the outer end wall (123) and the inner end wall (133). (Figure 1)