NEW DETERMINATION OF THE BISMUTH I-II EQUILIBRIUM PRESSURE - A PROPOSED MODIFICATION TO THE PRACTICAL PRESSURE SCALE

A new high-pressure instrument has been developed with the pressure un certainty characteristic of free-piston gauges in the 3000 MPa range. It consists of an end-loaded piston-cylinder apparatus modified to use argon as a pressure medium. Pressure is determined from the piston fo rce and the piston seal area with minor corrections for vessel bore di lation and seal friction. Bore dilation is approximately 0.36 % and do uble-valued friction for the piston seal ranges from about 1% to 2%. A force-balance analysis of the piston seal indicates that the friction is highly symmetric. The equilibrium pressure of the Bi I-II transiti on has been determined in this apparatus to serve as a reference point on the practical pressure scale. Transition pressure measurements wer e made on 99.9999 % pure polycrystalline bismuth. Small piston force e xcursions were made with both phases present to determine the equilibr ium pressure which is insensitive to reaction rate. The Bi I-II equili brium pressure is (2520+/-5) MPa at 25 degrees C. This value is 30 MPa (1.2 %) lower than Heydemann's [1] value of (2550+/-6) MPa. It is 8 M Pa (0.3 %) higher than the value of (2512+/-3) MPa from Sekoyan et al. [2].