I require an inflatable pipe plug for temporary sealing a 1m diameter pipe pressurised with 1 bar Helium at about 350 deg C or lower. Can someone point me in the right direction for a possible supplier or recommend some materials to use. Permeability tables of Helium at these temperatures for NRHB, Silicone, PVF etc will help. What should the inflatable length be to ensure a proper seal?