The design and performance of an accurate flow system utilizing critical velocity orifices is described. Results indicate that an absolute accuracy of three parts in a thousand is realized over a flow range of 0.002 to 2 cubic feet per minute. Calibration of the flow system was accomplished by means of a specially modified and carefully standardized wet test gas meter. Descriptions of the components of the flow system and of subsidiary calibrating equipment are presented. Critical flow orifice discharge coefficients are found to be independent of the type of gas; however, because of gas impurities, prediction of relative rates of flow of various gases through an orifice by use of theoretical relationships is uncertain, and experimental data are given to illustrate this.