Abstract: The disclosure is directed to automatic flow blending of gas mixtures useful for calibrating gas analyzing instruments of the kind used in measuring atmospheric pollutants and the like. According to the disclosure the component gases reduced in pressure from the storage pressure, are continually mixed in a mixing zone at low pressure and pumped to high pressure for charging high pressure storage vessels. The mixture in the high pressure charging line is continuously monitored with a gas analyzer instrument. Variations from the desired blend are converted into an analog signal which is used to control various valve mechanisms for regulating the flow of gases to the low pressure mixing zone so that a match of the desired blend of gases is obtained.