 Pressure calibration

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.^[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.