This gasket is identical to the basic type, however an inner ring has been inserted to enhance gasket performance and an external guide ring accurately centers gasket on flange face. The inner ring is added to prevent the possibility of the gasket imploding into the pipe during installation, to protect the sealing element from extreme temperatures and mediums, fill the void between flanges to prevent erosion of the flange, and to reduce the possibility of failure. The inner ring is normally made of the same alloy as the winding. The DIN 2699 standard (German) specifies inner rings in all spiral wound gaskets. Inner rings are required for gasket with PTFE filler according to ASME B16.20 standards, and considered important for graphite fillers. This profile is normally used in raised and flat faced flanges. The external ring a provides additional radial strength to prevent gasket blowout and acts as a compression stop. The outer ring is often made of carbon steel (painted or zinc plated to prevent corrosion) but can be made of alloys for higher temperature and more severe medium applications.