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Small-diameter tubes and hypodermic needles are used to meter the flow rate in a continuous-flow gas mixing device by means of changing the upstream stagnation pressure. Critical Fanno flow of a perfect gas is assumed under laminar flow conditions. The theoretical and experimental aspects of the problem are considered, and a computer program is introduced to find the flow rates, Reynolds numbers, stagnation and maximum downstream pressures, as functions of the Mach number at the inlet section for different gases and different tube geometries at room temperature. (Author)