Ring gaskets are a flat sealing element, consisting of an inner and outer diameter sized to fit a flange's bore and bolt circle. Ring gaskets sit inside the flange's bolt circle. ASME B16.21 and ASME B16.47 govern the dimensions of ring gaskets for standard and large diameter pipe flanges. EPDM, Ethylene Propylene Diene Monomer, is an "M" class synthetic rubber elastomer. Relatively immune to sunlight, water and low temperatures, EPDM is often a smart choice for gaskets that will be outside. Good compression set resistance means it recovers well in applications where a joint may open up from time to time, and high electrical resistance means EPDM gaskets may be appropriate for some electrical installations.