Ruby calibration against sodium chloride

The 1975 experiment mixed powdered sodium chloride and 0.5%-chromium ruby in a 4:1 volume ratio. An Inconel gasket, initially $0.15\mathrm{mm}$ thick with a $0.3\mathrm{mm}$ hole, held the powder and 4:1 methanol–ethanol. At $25{}^{\circ }\mathrm{C}$, X-ray diffraction rings on film $12\mathrm{cm}$ away gave salt compression; a reference equation of state assigned pressure. After decreasing load, they waited at least five hours and bracketed diffraction exposures with $R_1$ readings to check drift.^[ref]

The recommended linear slope was $2.740\mathrm{GPa}\mathrm{n}{\mathrm{m}}^{-1}$, with a $95\%$ fit confidence interval of $\pm 0.016\mathrm{GPa}\mathrm{n}{\mathrm{m}}^{-1}$, to $19.5\mathrm{GPa}$. Reference-scale systematic measurement uncertainty remained separate. Above about $10.4\mathrm{GPa}$ the medium was only quasihydrostatic; separate range fits checked consistency.^[ref]

The logical chain is compression to reference pressure, paired with optical shift, then a fitted response. X-ray diffraction supplies the reference comparison during calibration; subsequent ruby readings transfer that scale to other samples. The reference equation of state is therefore part of the meaning of the reported pressure. Changing it can move all calibrated readings together, even if the optical measurements repeat perfectly. A regression residual tests agreement with the chosen reference within the sampled conditions; it cannot independently certify that reference.