Bridgman seal

A Bridgman seal, named after Percy Williams Bridgman, seals a high pressure volume by the use of a multi-stage mechanism.^[1] A cylindrical driving piston is mounted within a cylindrical channel that is closed at its far end. This piston presses against a hard steel ring, followed by a softer steel ring, and then a ring of more viscous or elastic material such as rubber, copper or soap stone, all within the channel. These three intermediate stages provide an inner cylinder, and the third stage bears on a specially shaped final steel piston that applies the force to pressurise material held at the end of the outer, enclosing channel. The final piston consists of a wider portion that fills the main channel, and a narrower cylindrical extension that leads back through the inner channel formed by the three ring-shaped intermediate stages, ending within the hard steel ring without making direct contact with the driving piston. This arrangement ensures that higher pressures create tighter seals that resist any leakage from the material at the end, since the pressure within the last and softest ring is greater than that in the material at the end.^[2]

This arrangement has much in common with the earlier de Bange breech obturator system used to prevent the escape of gasses from breech loading artillery, whether inspired by that or independently invented, with the important further feature that Bridgman's system does not merely resist the escape of material under pressure while stationary, it applies that pressure by movement within the pressurising equipment.

This seal allowed for pressure increases from 400 MPa to 40,000 MPa. These are typical pressures expected in the Earth's internal structure.^[2] For Bridgman a whole universe of possibility had opened. Everything he squeezed did something interesting and unexpected. Water froze into strange phases. Salts changed colour. Conductivities changed unpredictably. A new science world became explorable.^[3]

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