A container having a fixed volume sealed from the atmosphere by an elastic septum is filled with liquid via a needle inserted through the septum, the needle providing a path for liquid flow into the fixed volume from an external source through the septum and a path for air flow from the fixed volume to atmosphere through the septum. The needle includes a stem having a hollow tube with a non-cylindrical section and a cover element cooperating with the non-cylindrical section of the tube to form an air passage and give the stem a cylindrical configuration even in the region of the non-cylindrical section of the tube. The configuration of the stem minimizes damage to the septum as the stem is inserted through or withdrawn from a septum, while the air passage permits air to exit from the container through the stem as liquid enters the container through the tube.