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A diamond anvil cell compresses a sample between opposed diamond tips. The flat diamond anvil culets load a metal gasket whose hole holds the specimen and a pressure-transmitting medium. Transparent diamond admits excitation and returning fluorescence, enabling an internal ruby pressure sensor.^[ref]

The functions in the drawing answer different questions. The drive applies force, the gasket confines the sample, the medium transmits stress, and the ruby pressure scale assigns a local pressure from an optical signal. A large drive reading cannot establish either the sample's pressure or the absence of deviatoric stress. Optical access also permits repeated readings without unloading the sample: a change during a long measurement can be detected rather than silently folded into a material-property curve.

A diamond anvil culet is the small flat contact face at the tip of a diamond anvil. Its diameter $d_c$ determines its nominal surface area $A_c$. The gasket hole has a different surface area $A_h$; neither should be confused with the much larger drive mechanism. For an ideal static axial balance, $F$ is the compressive force on either anvil. The two equal opposing loads do not produce $2F$ through a single contact.

For an original chosen geometry, $F=1000\mathrm{N}$ and $d_c=0.300\mathrm{mm}$ giveHere $\bar{q}$ is the mean axial contact stress over the culet, not a measured chamber pressure. Using a $0.100\mathrm{mm}$ hole instead would give $F/A_h=127.324\mathrm{GPa}$, nine times larger. That substitution falsely assigns the entire applied force to the hole.

To see the missing load path, idealize the culet as a coplanar hole region plus a surrounding contact annulus. With a hydrostatic chamber pressure $p$ and axial resultant $F_g$ carried by the gasket, its force balance isIf the stipulated chamber pressure is $5.40\mathrm{GPa}$, the hole carries only $42.4\mathrm{N}$ and the gasket carries $957.6\mathrm{N}$. These are model loads, not measurements. Real gasket deformation and anvil elasticity alter the local contact stress; the scalar balance cannot predict $p$ without additional information. This is why internal pressure calibration matters even when applied force is accurately known.