Abstract: A system that allows an operator to determine the true vapor pressure of a liquid composition from the bubble-point pressure for a given temperature at which the lightest component starts to flash includes a heated chamber with a retractable plunger. A sample of the liquid to be tested is trapped in a chamber at a pressure above the bubble-point pressure. A plunger inserted into the chamber gradually reduces the chamber internal pressure by retracting the plunger out of the chamber at a uniform (synchronous) speed from the start to the resting plunger rod position. The chamber pressure is recorded at each pressure increment reduction. The bubble point-pressure is determined from the point of inflection in the pressure decline curve.