Mass Flow Controllers
Mass Flow Controller (MFC) is a device used to measure and control the flow of fluids and gases. A mass flow controller is designed and calibrated to control a specific type of fluid or 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 Controllers are usually able to control more than one type of fluid or gas whereas Analog Controllers are limited to the fluid 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 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.
- The base houses the channels that make up the gas’ main flow path. All other parts of the mass flow controller are mounted on the base.
- The thermal sensor is made up of a small-bore tube with two resistance-thermometer elements wound around it. When an electric current is applied to the resistors, the tube is heated and then a temperature differential is created between the two resistors when gas passes through the tube.
- This is measured as an electrical signal.The bypass, also known as the flow splitter, ensures equitable flow rates between the sensor and the main flow path. This makes it possible to measure total flow rate by measuring only the gas that flows through the sensor.
- The control valve establishes the gas flow in response to a signal comparing the actual flow to the set point.
- Printed circuit board