Class VI Perfluoroelastomer with 3 tabs each spaced 120 degrees apart along the outer edge to aid in gasket alignment. The chemically resistant perfluoroelestomer has a soft pliable sealing surface that provides improved sealing under full vacuum, as opposed to encapsulated o-ring materials and envelope gaskets. Gasket is for use with 400mm Chemglass flat ground flange on Chemglass 50L and 100L vessels.