= Pressure calibration
{title2=$p=g(R,T,\text{history})$}
<Pressure calibration> assigns <pressure> values to a sensor's output by comparison with a reference under specified conditions. It is not the same as making the output more finely resolved. A smooth <electrical resistance> curve can be repeatable yet have an incorrect <pressure> scale.
Bridgman used pressure-dependent <electrical resistance> of <manganin> and established fixed points, including a <bismuth> transition. He reported poorer accuracy above the better calibrated range.<General Survey of Certain Results in the Field of High-Pressure Physics by Percy Williams Bridgman>[p. 56, pressure-measurement paragraph: manganin gauge, bismuth fixed point, and different accuracy estimates below and above 30000 kg/cm²]{ref}
A fixed-point specimen and a property-measurement sample have distinct roles: one helps assign the scale, while the other is studied on that scale. The reference transition must itself have an assigned <pressure> at the relevant <temperature>. Agreement at a single point does not determine a nonlinear sensor response elsewhere. Comparing increasing and decreasing <pressure> checks <hysteresis>; returning to a reference checks drift. These are checks one can design from the measurement model, not additional historical controls asserted for the lecture's experiments.
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