A mass flow controller (MFC) is a device used to measure and control the flow of gases. A mass flow controller is designed and calibrated to control a specific type of gas at a particular range of flow rates. The MFC can be given a setpoint from 0 to 100% of its full scale range but is typically operated in the 10 to 90% of full scale where the best accuracy is achieved. The device will then control the rate of flow to the given setpoint. MFCs can be either analog or digital, a digital flow controller is usually able to control more than one type of gas whereas an analog controller is limited to the gas for which it was calibrated. All mass flow controllers have an inlet port, an outlet port, a mass flow sensor and a proportional control valve. The MFC is fitted with a closed loop control system which is given an input signal by the operator (or an external circuit/computer) that it compares to the value from the mass flow sensor and adjusts the proportional valve accordingly to achieve the required flow. The flow rate is specified as a percentage of its calibrated full scale flow and is supplied to the MFC as a voltage signal. Mass flow controllers require the supply gas to be within a specific pressure range. Low pressure will starve the MFC of gas and it may fail to achieve its setpoint. High pressure may cause erratic flow rates.
Theory of operation
The heart of a mass flow controller is a thermal sensor. It consists of a small bore tube with two resistance-thermometer elements wound around the outside of the tube. The sensor tube is heated by applying an electric current to the elements. A constant proportion of the gas flows through the sensor tube, and the cooling effect creates a temperature differential between the two elements. The change in the resistance due to the temperature differential is measured as an electric signal.
