Accurate measurement of gas mass flow rate is crucial in many modern industries such as aerospace, oil and gas, semiconductor manufacturing, and many other. Proper and routine calibration of gas flow measurement device is important in order to ensure that the error and uncertainty of the reading of the gas flow meter is within the tolerance of the instrument’s manufacturer and/or the user. Among several common gas flow measurement devices commonly used in industries, critical flow venturi (hereafter refered to as its alternate name, sonic nozzle) has an advantage of not having a moving part and being able to maintain a constant mass flow regardless of downstream disturbances and pressure changes. The accuracy and consistency of the reading of sonic nozzle makes it a suitable choice for high accuracy flow measurement, flow rate control device, and even as a standard for calibration for other (less accurate) flow measurement devices.