High-resolution measurements of the coexistence curve very near the 3He liquid-gas critical point

The shape of the liquid gas coexistence curve of He-3 very near the critical point temperature T-c was measured in the range -5 x 10(-3) < T/T-c - 1< -1.5 x 10(-6) using the quasistatic thermogram method. This study was performed in the Earth's gravitational field using two cells of very different heights (0.5 and 48 mm). The measured coexistence curve near the critical point was strongly affected by the gravitational field. Away from the critical point, we compare the coexistence curve obtained using the thermogram method with earlier work by Pittman et al. The recently developed crossover parametric model of the equation-of-state is used to take gravity effects into account. The shape of the measured coexistence curve very near the critical point is remarkably symmetric about the critical density. Our results close to the critical point are consistent with the slope of the rectilinear diameter obtained by Pitman et al. from measurement farther away from T-c. The deviation from a law, of rectilinear diameter predicted by revised scaling and the Yang-Yang anomaly were not observed in He-3 within the 0.1% accuracy in our measurements.