Sensor for dynamically detecting the residual liquid volume Vres of a collapsible reservoir (1,3) characterized by the fact that it is adapted to detect a threshold pressure Pth which corresponds to a phase within said reservoir (1,3) when only said residual liquid volume Vres remains, said residual volume Vres corresponding to a safety volume sufficient to ensure a safety margin to alert the user before the reservoir (1,3) is empty.
