Method and apparatus for supplying mixed gas

An apparatus for supplying a mixed gas comprising a mixing container which is charged with a mixed gas in a high-pressure state prepared by mixing a low-vapor-pressure first gas having a boiling point equal to or higher than 0° C. and equal to or lower than 100° C. and a high-vapor-pressure second gas having a boiling point equal to or lower than −30° C., and from which the mixed gas is supplied to a use point at a pressure equal to or higher than 0.5 MPa and equal to or lower than 15 MPa in terms of absolute pressure, and means for replenishing the mixing container with the mixed gas in the high-pressure state when the pressure in the mixing container is reduced to a setpoint as a result of supply of the mixed gas to the use point,

• wherein a replenishment container having an internal capacity equal to or larger than 1/100 of the internal capacity of the mixing container and equal to or smaller than ⅕ of the internal capacity of the mixing container is formed and the equivalent circle diameter of a cross-section of the internal space of the replenishment container defined by equation (1):
• S=πD2/4 (1)
• where D is the equivalent circle diameter (mm), S is the sectional area of the internal space of the replenishment container (mm2), and π is the ratio of the circumference of a circle to its diameter, is equal to or larger than 1/100 of the height of the internal space of the replenishment container and equal to or smaller than ½ of the height of the internal space of the replenishment container,
• wherein the replenishment container and the mixing container are connected to each other by a replenishment line having a replenishment valve;
• a first container containing the first gas is connected to the replenishment container by a first gas line having a first gas valve;
• a second container containing the second gas is connected to the replenishment container by a second gas line having a second gas valve;
• and a vacuum pump is connected to the replenishment container by a discharge line having a discharge valve,
• wherein the replenishment container in an evacuated state is charged with a predetermined amount of the first gas by opening the first gas valve, and
• wherein detection means for detecting the pressure in the mixing container is formed and, when the pressure detected by the detection means is reduced to a setpoint as a result of supply of the mixed gas in the high-pressure state to the use point, the second gas valve is opened to charge the second gas into the replenishment container charged with the first gas and the replenishment valve is simultaneously opened to cause the first gas in the replenishment container to be forced out by the second gas, whereby the mixing container is replenished with the mixed gas in the high-pressure state.