Abstract
A key question to all high-pressure research arises from the reliability of pressure standards. There is some indication and discussion of an uncertainty of 10–20 % for higher pressures in all standards. Independent and simultaneous investigation of the dynamical (ultrasonic interferometry of elastic wave velocities) and static (XRD-measurement of the pressure-induced volume decline) compressibility on a sample reveal the possibility of a standard-free pressure calibration and, consequently an absolute pressure measurement. Ultrasonic interferometry is used to measure velocities of elastic compressional and shear waves in the multi-anvil high-pressure device MAX80 at HASYLAB Hamburg enabling simultaneous XRD and ultrasonic experiments. Two of the six anvils were equipped with lithium niobate transducers of 33.3 MHz natural frequency. NaCl was used as pressure calibrant, using the EoS of Decker (1971), and sample for ultrasonic interferometry. From the ultrasonic wave velocity data we calculated the compressibility of NaCl as a function of pressure independent from NaCl-pressure calibrant. The results were compared with data of static compression experiments up to 5 GPa (Bridgman, 1940) and up to 30 GPa (Birch, 1986) using experimental data from Boehler & Kennedy (1980) and Fritz et al. (1971). At 1.2 GPa and 5.3 GPa the results of static compression data agree with our velocity-derived compressibility data. In the range between 2 and 4GPa our dynamical data have 1.5–3% higher values. The pressure revealed according to Decker (1971) is in accordance to our standard-free pressure calibration. Consequently, up to 8 GPa the NaCl pressure standard has a reliability of at least 1%. However, there is some evidence that at higher pressures the inaccuracy of the NaCl standard becomes ≫ 1%.
