US Patent 8220538 Plug

A plug for installation in a well is described, which comprises a housing (1) that carries at least two discs (5, 6) made from a brittle material which can be crushed with mechanical stress. Between at least two of the discs (5, 6) is a gas-filled cavity (15) that is in connection with a drainage channel (14) and a closing device (16) which is set up to open to release the gas from the cavity (15). Arranged in the cavity is at least one break bar 7 or a break bushing (8-12) that is set up to crush at least one of the discs (5, 6). Also arranged is a shear pin (13) that holds the discs (5, 6) apart, but which is set up to be broken when the pressure difference across at least one of the discs (5, 6) exceeds a given value.