Precision Control: The Science Behind Thermal Mass Flow Controllers

What is a Mass flow controller?

• In order to precisely control the flow rate of gases, a specialized device called a mass flow controller (MFC) is employed in industrial and scientific applications. 
• It is made up of sensors that measure gas flow, control electronics, and a control valve or other device that controls flow. 
• The MFC accurately regulates the gas flow to maintain the flow rate that the user has specified. 
• MFCs are essential in fields like semiconductor production, ensuring accurate and reliable gas delivery in procedures like gas chromatography and chemical vapor deposition. 
• They are crucial in maintaining consistency and quality across a range of gas-dependent applications.

How does a mass flow controller operate?

• A mass flow controller is a compact instrument that accurately controls the flow rate to supply one or more chemical compounds—gases or liquids—to a process. 
• To achieve this, a source of voltage, current, or digital (fieldbus) information controls the flow rate of the compound(s) to be delivered electronically. 
• An internal sensor measures the amount of gas or liquid that passes through the device. 
• The setpoint value will be compared with this measurement value. 
• A control valve may change the size of the flow path (by opening or closing) to ensure that both values are equal. 
• A mass flow controller, in its simplest form, consists of a mass flow meter (i.e., the sensor), a control valve, and feedback electronics between the sensor and valve. 
• This control function (also known as PID, proportional-integral-derivative) is frequently a standard component of the electronics of the device, although the control features can be changed for quick or smooth control using user software.

Why would you control mass flow rather than volume flow?

• For the simple reason that in a lot of different research and production processes, mass is the crucial variable to focus on rather than volume. Mass flow can be directly controlled via mass flow controllers. 
• From an economic standpoint, the mass of a gas or liquid is an independent value, regardless of the process’s operating temperature or pressure. 
• As a result, mass flow devices are unaffected by changes in the incoming flow’s temperature and pressure.
• Conditions under which mass flow is turned into volume flow are agreed upon to satisfy users’ preferences for expressing compressible gas flow as volume flow anyway. 
• These “normal” reference conditions, denoted by the subscript “n” in the unit of volume used, are a temperature of 0 °C and an absolute pressure of 1 atm.

Thermal mass flow controllers – construction

A fundamental mass flow controller is made up of a thermal mass flow sensor, a laminar flow element that serves as a bypass, a proportional control valve that is operated by a solenoid, and a digital electronic PC-board that is used for PID control and communication. 

Operating principle of Thermal Mass Flow Controller

Thermal Gas Flow Sensor Principle

• The gas flow sensor is based on the concept of heat transfer, and it measures the difference in temperature along a capillary tube that has been heated in one section. 
• The use of a laminar flow element in the mainstream that generates a pressure difference allows for a portion of the total flow to be channeled via the capillary.
• The design of the laminar flow device is such that the flow conditions in both the capillary flow device and the laminar flow device are comparable. 
• As a consequence, the flow rates that are measured by the meter are proportional. 
• The quantity of heat that is absorbed by the gas flow determines the delta-T that is measured by the temperature sensors that are located both upstream and downstream on the capillary.
• The equation that can be used to represent the transfer function between gas mass flow and signal is as follows:

V_signal = K.Cp. Φm

V_signal = Output signal

 Cp = Specific heat

 K = Constant factor

 Φm = Mass flow

Thermal sensor in a bridge configuration

• A bridge circuit integrates the temperature sensors as a component. 
• The imbalance is linearized, and then the signal is amplified to the level that is required. 

• The sensor, which is made up of a capillary tube made of stainless steel and resistance thermometer elements, is the most important component of a thermal mass flow meter or controller. 
• This bypass sensor allows some of the gas to travel around it, where it is warmed by elements of the heating system. 
• As a direct consequence of this, the temperatures that were measured, T1 and T2, begin to diverge. 
• The difference in temperature is exactly proportional to the amount of mass flow that is passing through the sensor.
• The laminar flow element, which consists of a stack of stainless steel discs with precision-etched flow channels, is located in the primary channel. 
• Because of the perfect flow-split, the output of the sensor is directly proportionate to the total mass flow rate.

Bypass Principle

• A temperature sensor and a laminar flow element (LFE) constitute the mass flow controller’s measurement portion. 
• Discs are stacked together to form a laminar flow element, and each disc has accurately etched flow channels. Each channel’s flow is proportional to the flow through the sensor. 
• In this manner, the total flow rate of an instrument can be modified while maintaining the same sensor flow rate by adding more or fewer laminar flow discs.
• In general, equipment with these sensors can be positioned vertically or horizontally at low working pressures.
• The mass flow controller employs a conventional, direct-operated control valve. 
• It is a solenoid valve that is generally closed. The magnetic field of the coil’s force lifts the plunger. 
• For the purpose of the process application, the orifice’s diameter below the plunger is optimized.
•  Positive shut-off is not a feature of the control valve. If necessary, it is advised to install a separate shut-off valve in the line. Additionally, pressure spikes that could happen during system pressurization need to be prevented.

Solenoid Control Valve Principle

• The mass flow controller employs a conventional, direct-operated control valve. It is a solenoid valve that is generally closed. 
• The magnetic field of the coil’s force lifts the plunger.
• For the purpose of the process application, the orifice’s diameter below the plunger is optimized. 
• Positive shut-off is not a feature of the control valve. If necessary, it is advised to install a separate shut-off valve in the line. 
• Additionally, pressure spikes that could happen during system pressurization need to be prevented.

PID controller role in Mass flow controller

• A PID (Proportional-Integral-Derivative) controller plays a crucial role in the operation of a Mass Flow Controller (MFC). 
• It is responsible for maintaining the desired flow rate of gas or liquid by continuously adjusting the control valve based on feedback from the flow sensor. 

• The overall function of the PID controller in an MFC is to bring the measured flow rate as close as possible to the setpoint and maintain this balance over time. 
• By continuously adjusting the control valve based on the proportional, integral, and derivative terms, the PID controller can achieve precise and stable control of the flow rate, even in the presence of disturbances or changes in operating conditions.
• The specific tuning parameters (proportional gain, integral time, derivative time) of the PID controller are often adjusted to optimize the controller’s performance for a particular MFC and the characteristics of the process it’s controlling. Proper tuning is essential to ensure fast response, minimal overshoot, and minimal steady-state error, as well as to prevent instability or oscillations in the control system.