to be adequately lubricated by a fluid film. Vacuum applications In vacuum applications, the permeability of the material renders the point contacts unusable. Instead, higher mounting forces are used and the ring fills the whole groove. Also, round back-up rings are used to save the ring from excessive deformation. Because the ring feels the ambient pressure and the partial pressure of gases only at the seal, their gradients will be steep near the seal and shallow in the bulk (opposite to the gradient of the contact stress (See Vacuum flange#KF.2FQF.) High-vacuum systems below 10−9 Torr use copper or nickel O-rings. Also, vacuum systems that have to be immersed in liquid nitrogen use indium O-rings, because rubber becomes hard and brittle at low temperatures. High temperature applications In some high-temperature applications, O-rings may need to be mounted in a tangentially compressed state, to compensate for the Gow-Joule effect. Sizes O-rings come in a variety of sizes. Society of Automotive Engineers (SAE) Aerospace Standard 568 (AS568) specifies the inside diameters, cross-sections, tolerances, and size identification codes (dash numbers) for O-rings used in sealing applications and for straight thread tube fitting boss gaskets. British Standard (BS) which are imperial sizes or metric sizes. Typical dimensions of an O-ring are internal dimension (id), outer dimension (od) and thickness / cross section (cs) Metric O-rings are usually defined by the internal dimension x the cross section. Typical part number for a metric O-ring - ID x CS [material & shore hardness] 2x1N70 = defines this O-ring as 2mm id with 1mm cross section made from Nitrile rubber which is 70Sh. BS O-rings are defined by a standard reference. The World's Largest O-ring was produced in a successful Guinness World Record attempt by Trelleborg Sealing Solutions Tewkesbury partnered with a group of 20 students from Tewkesbury School. The O-ring once finished and placed around the Medieval Tewkesbury Abbey had a circumference, an approximately inner diameter, and a cross section of . Material O-ring selection is based on chemical compatibility, application temperature, sealing pressure, lubrication requirements, durometer, size and cost. Synthetic rubbers - Thermosets: Butadiene rubber (BR) Butyl rubber (IIR) Chlorosulfonated polyethylene (CSM) Epichlorohydrin rubber (ECH, ECO) Ethylene propylene diene monomer (EPDM): good resistance to hot water and steam, detergents, caustic potash solutions, sodium hydroxide solutions, silicone oils and greases, many polar solvents and many diluted acids and chemicals. Special formulations are excellent for use with glycol-based