Thermal Mass Flow Controllers / Mass Flow Meters Basics: Definition, Design and Advantages
For over 20 years Bürkert has been a reliable expert in the field of thermal mass flow controllers (MFC)/meters (MFM) for gases. But what is an MFC/MFM? And what advantages does MFC/MFM offer for gas flow measurement/control in general and Bürkert MFC/MFM in particular? Find out everything you need to know about this topic in this article.
What is a mass flow controller (MFC) / mass flow meter MFM?
A mass flow controller / meter is a compact, closed loop control system for controlling (MFC) or measuring (MFM) gas flow rates.
MFC / MFM: Structure & Function
The difference between mass flow controllers and mass flow meters
Mass flow meters consist of fluidics, sensors and electronics to measure the mass flow of gases.
Mass flow controllers are of identical design. They also have an actuator to not only measure the mass flow rate but also to actively control it to a specified flow setpoint. The setpoint value is transferred to the device electrically via a standard signal, a fieldbus or Industrial Ethernet.
The actual value recorded by the sensor is compared with the setpoint value in the controller. The controller sends a pulse-width modulated voltage signal to the actuator as the manipulated variable and varies it according to the detected control deviation.
Thermal flow sensors
Most manufacturers of MFC/MFM rely on thermal flow sensor technology because it offers high measurement accuracy and repeatability , fast response times and large measurement ranges. As there are no moving parts in the medium, thermal sensors are nearly wear-free.
The measuring principle of thermal mass flow measurement
Heat always flows in the direction of lower temperature. So if a body has a higher temperature than its surroundings, it gives off its heat energy to a mass flowing past. The thermal/calorimetric measuring method makes use of this principle of heat conduction and heat transport in gases.
Thermal flow sensors for determining the mass flow of gases consist of a heating element and temperature sensors. The heating element heats the gas flowing through, whereupon the temperature sensors detect the amount of heat dissipated. The amount of heat dissipated in each case is a measure of the existing mass flow of the gas.
What is mass flow?
The mass flow defines the mass, i.e. the weight of the flowing medium (gas) that flows through the device in a certain unit of time.
What is the difference to volume flow?
In contrast to mass flow, volume flow defines the volume of gas flowing through the device in a given unit of time. If you measure the gas volume, you also need the information on temperature and pressure for reasons of comparability, since gases are compressible. This means that their density and thus their volume changes depending on pressure and temperature.
Due to the strong pressure and temperature dependence of gas volumes, it is therefore recommended to measure mass flow - and not volume flow - for the precise and reliable flow measurement of gases, as it is independent of pressure and temperature.
Different types of thermal flow sensors and their advantages and disadvantages
Thermal sensors are distinguished as follows:
- Direct sensors: Sensor element sits directly in the gas flow. These include, for example, the "Inline" and "CMOS / MEMS" sensors.
- Indirect sensors: Sensor element is separated from the medium. These include capillary sensors, for example.
Advantages and Disadvantages of Inline & CMOS / MEMS Sensors (direct sensors)
Inline Sensor Measurement principle CMOS Sensor Measurement principle
Advantages
✓ Fast response time
✓ High measuring accuracy
✓ Long-term stability of the flow calibration
✓ Low pressure drop
✓ Insensitive to contamination
Disadvantages
- Not suitable for aggressive gases
- Gas conversion using conversion factors only possible to a limited extent
Advantages and Disadvantages of Capillary Sensor (indirect sensor)
Capillary Sensor Measurement principle
Advantages
✓ Suitable for aggressive gases
✓ Gas conversion using conversion factors possible for many gases
Disadvantages
- Slower response times due to the thermal barrier between sensor and gas
- Loss of accuracy in case of gas conversion
- Flow calibration less long-term stable
- Higher pressure drop
- Thin capillary tubes are prone to contamination
Applications of mass flow controllers
Mass flow controllers/ meters have long since become an integral part of modern process automation. The automated and precise control or measurement of gas flow rates plays a decisive role in metal and glass production or processing, for example, in the form of burner control systems, process gas supply or atmosphere control.
In fermentation processes, gases are fed to the bioreactor by means of MFC, and mass flow controllers also provide the necessary amount of protective gas in filling or packaging machines.
Find the right mass flow controller & mass flow meter for your application.
The difference between mass flow controllers and mass flow meters
Mass flow meters consist of fluidics, sensors and electronics to measure the mass flow of gases.