Pressure Calibration to 60 kbar Based on the Resistance Change of a Manganin Coil under Hydrostatic Pressure

The relative resistance change of a properly seasoned manganin coil was measured at the center of the region of indifference for the bismuth II-III, thallium II-III, and barium I-II phase transformations in a hydrostatic system. The equilibrium pressures for these transformations were obtained from the relative resistance change of manganin which was calibrated with pressure by means of the mercury L-I and bismuth I-II transition pressures. Equilibrium pressures corrected to 25°C of 26.861±0.042, 36.569±0.153, and 56.273±0.521 kbar were determined for the bismuth II-III, thallium II-III, and barium I-II phase transformations, respectively. Only the barium I-II point deviated from the presently accepted high pressure scale. On the basis of the manganin pressure gauge, either the presently accepted value of the barium I-II point is too high by about 2.5 kbar or the pressure coefficient of manganin resistance changes sharply between 37 and 59 kbar. A two-point quadratic calibration curve was satisfactory for pressures to 37 kbar.