US Patent 7137400 Bypass loop gas flow calibration

Described herein are apparatuses, methods and systems to monitor the performance of one or more mass flow controllers that supply gases to deposition, etching, and other manufacturing processes. A bypass loop is provided in fluid connection from either the process line or the vent line. In the bypass loop is a flow detector, such as a digitized mass flow controller. The flow detector takes one or more measurements of flow of gas from a mass flow controller, and data from such one or more measurements is used to provide information about the accuracy and/or precision of the mass flow controller. Also disclosed are ways to correct for back pressure or back vacuum in the process line.