Ultrasonic Velocity in Carbon Dioxide and Ethylene in the Critical Region

Abstract

Measurements of the velocity of ultrasonic waves in carbon dioxide and ethylene are presented; the velocity was obtained while the gas was allowed to follow a p—v isothermal. For carbon dioxide the pressure range is 5 to 98 atmospheres at temperatures of 28.0°, 30.0°, 31.0°, 32.0°, 33.0°, 35.0° and 38.0°C. For ethylene the pressure range is 35 to 75 atmospheres at temperatures of 9.7°, 18.7° and 23.0°C. For both gases, as pressure increases, the velocity decreases, falls to a sharp minimum, and then rises steeply on the high pressure side of this minimum. The minima have a discontinuity of slope for isothermals below the critical isothermal, which persists for a limited temperature range above the critical temperature. Above this range the curves become continuous in slope through the minimum. The curve log p vs. 1/T for the minima is linear through the critical point and, below the critical point, coincides within experimental error with standard vapor pressure data. Some calculations of ratio of specific heats near the critical point are included.