Pressure experiments are experiments performed at pressures lower or higher than atmospheric pressure, called low-pressure experiments and high-pressure experiments, respectively. Pressure experiment are necessary because substances behave differently at different pressures. For example, water boils at a lower temperature at lower pressures. The equipment used for pressure experiments depends on whether the pressure is to be increased or decreased and by how much. A vacuum pump is used to remove the air out of a vacuum vessel for low-pressure experiments. High-pressures can be created with a piston-cylinder apparatus, up to 50.000 bar and ~2000degrees C. The piston is lowered with hydraulics, decreasing the volume inside the confining cylinder and increasing the pressure. For higher pressures, up to 250.000 bar, a Multi-Anvil cell is used and for even higher pressures the diamond anvil cell. The diamond anvil cell is used to create extremely high pressures, as much as a million atmospheres, though only over a small area. The current record is 5.6 Mbar, but the sample size is confined to the order of tens of microns.

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