Septa, 17.5mm, Headspace, White Silicone Rubber / Blue PTFE. For use in 18mm Aluminum, Screw Caps, sold separately. MicroSolv Brand. Additional Info: Silicone Rubber / PTFE Septa, Durometer of 55 Shore A (hardness) and 1.5mm thick. Temperature resistant from -60°C up to 200°C. Use in ND18, Precision Thread Screw Caps for Headspace.