Abstract: A method for calibrating a gas flow metrology system for a substrate processing system includes a) measuring temperature using a first temperature sensor and a reference temperature sensor over a predetermined temperature range and determining a first transfer function; b) measuring pressure using a first pressure sensor and a reference pressure sensor over a predetermined pressure range using a first calibration gas and determining a second transfer function; c) performing a first plurality of flow rate measurements in a predetermined flow rate range with a first metrology system and a reference metrology system, wherein the first metrology system and the reference metrology system use a first orifice size and the first calibration gas; and d) scaling temperature and pressure using the first transfer function and the second transfer function, respectively, and determining a corresponding transfer function for the first calibration gas based on the first plurality of flow rate measurements.