Abstract: The paper introduces how PLC(programmable logic controller)is used in a real-time monitoring process of piping flowrate and valve-back pressure of motor-driven control valves to dynamically keep them within the set value, based on 2-way or multi-way flowrate ratios calculated from multi-way gas's thermal values, composition, density and specific gravity, and artificial parameters(thermal value and so on) provided by means of chromatograph.Through flowrate and pressure control, real-time and/or dynamic gas mixing can be achieved.