Two ultrasonic measurement methods for pressure calibration to 4.4 GPa in a multi-anvil apparatus by measuring the travel times of longitudinal wave as a function of pressure are reported. The first method is to continuously calibrate pressure by combining the measured travel times of Z-cut quartz under hydrostatic pressure with the related equation of state and unit-cell parameters of quartz. The second method is fixed-point calibration, which is to calibrate the pressure by measuring the abrupt change of the longitudinal wave travel times since the samples, H2O, Hg and Bi used in this study will undergo pressure-induced phase transitions at room temperature. Experimental results of these two methods are in good agreement. The quartz pressure scale obtained from this study is expressed as P(GPa) = 28.7(1-t (p)/t (p0)). The two ultrasonic measurement methods might be complementary means for pressure calibration in situations where other probe method is not conveniently available.

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