= 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.<Calibration of the pressure dependence of the R1 ruby fluorescence line to 195 kbar by Gasper J. Piermarini and others>[pp. 2774–2775, Experimental Procedures]{ref}
The recommended linear slope was $2.740\,\mathrm{GPa\,nm^{-1}}$, with a $95\%$ fit <confidence interval> of $\pm0.016\,\mathrm{GPa\,nm^{-1}}$, 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.<Calibration of the pressure dependence of the R1 ruby fluorescence line to 195 kbar by Gasper J. Piermarini and others>[pp. 2775–2776, Table I and recommended calibration; p. 2778, pressure-distribution discussion]{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.
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