The screw caps are for use with Starna's screw top anaerobic cells: Type 1 (macro), Type 9 (semi-micro), Type 18 (micro), Type 3 (macro fluorometer), Type 9F (semi-micro fluorometer), Type 18F (micro fluorometer). The caps are approximately 20mm diameter at the widest point (the rim at the bottom of the cap) and approximately 18.25mm diameter for the ribbed area. The caps come with a silicon rubber insert installed that seals the cell when cap is tightened. PTFE faced inserts, GL14/ST-PTFE, may be purchased separately.