What is a thermal mass flow meter?

November 9, 2021 Wout van ‘t Wel

Thermal mass flow meters are often used to measure low gas flows, but what are they exactly? And what distinguishes these instruments from other gas flow meters? You may also find it interesting to read about the difference in flow sensors and their advantages and disadvantages. In this blog I will share my insights with you on these topics.

Mass Flow versus Volume Flow Measurement

In many research and production processes the important variable is mass and not volume. Measurements of volumetric flow are not as reliable as mass flow measurement due to the effects that changes in temperature or pressure have on the density of a fixed volume of gas.

Thermal Mass Flow Meter/Controller for Gas (by-pass design).  

Volume flow versus Mass flow
Volume flow versus Mass flow

Unlike volumetric flow measurement devices such as purge meters (variable area meters) or turbine meters, thermal mass flow meters are immune to fluctuations in temperature and pressure of the incoming flow. The mass flow meter can provide direct measurement of mass flow. Most other methods measure volumetric flow and require separate measurements for temperature and pressure to calculate density and, ultimately, the mass flow.

Mass flow meters measure the flow on a molecular level and so can provide you with an extremely accurate, repeatable, and reliable delivery of gas into the process.
The difference between volume and mass flow is further explained in our blog Do you know why Mass Flow reference conditions matter?

The advantages of the thermal mass flow measurement principle

Thermal mass flow meters are by far the most popular instruments for determining mass flow of low gas flow. Since the introduction of these instruments in the mid-1970s, they have undergone considerable development, from analogue to digital, from low flow to higher flow, from applications in laboratories and machines to industrial environments and even hazardous areas. There is a suitable thermal mass flow meter for almost every low-flow gas application. Other measurement principles that can be used for low-flow gas applications are Coriolis flow measurement and differential pressure measurement

Coriolis flow measurement

Coriolis-based mass flow meters offer advantages especially when measuring supercritical gases and when users want to measure the mass flow of varying or unknown gas mixtures.

Differential pressure measurement

In case of differential pressure measurement, the measured delta-p is converted to mass flow via a calculation that includes the gas temperature as well as the density and dynamic viscosity of the gas used. Compared to this method, thermal sensors are less vulnerable to pressure changes in your process and the conversion for other gases in thermal flow measurement is intrinsically more accurate.

Coriolis mass flow measuring principle 

Difference between thermal flow sensors

Thermal mass flow meters and thermal mass flow controllers make use of the heat conductivity of fluids. Within the range of thermal flow meters, we distinguish several types of sensors, namely:

  • inline sensors that measure directly in the main flow and
  • by-pass sensors, where a proportional part of the flow is passed through a flow restriction.

Thermal mass flow meter for gas with ‘inline’ principle;

Each technology has its advantages and disadvantages which are application specific: A clean, dry gas application where higher accuracy is as important as repeatability, may be a better application for a bypass instrument (e.g., our instruments of the EL-FLOW Select series and EL-FLOW Prestige series).

An application with a slightly moist gas, or where lower accuracy but high repeatability and robustness is required, may be a better application for flow meters with a robust inline sensor in combination with a straight flow channel (e.g., our instruments of the MASS-STREAM series)

There are also thermal mass flow meters that use chip sensors (MEMS, Micro Electro Mechanical System, or CMOS, Complementary Metal Oxide Semiconductor), at which the gas flow comes into contact with these sensors.

The advantage of a faster measuring signal is offset by the fact that these sensors are less robust and therefore often not suitable for use with aggressive or explosive gases. Moreover, these flow meters need to be calibrated on the actual gas (e.g., our instruments of the IQ+FLOW series).

Proportional Iintegral Derivative Bronkhorst flow meter solution
PID = Proportional Iintegral Derivative

Thermal Mass Flow Controllers

A thermal mass flow controller is in fact nothing more than a flow meter combined with an integrated or close-couple control valve. These compact instruments are used in applications where gases must be accurately controlled.
A mass flow controller continuously compares the output signal with a setpoint signal from a voltage, current or digital (fieldbus) source. Any deviations between setpoint signal and measured signal are translated into a solenoid control valve adjustment until the two signals are identical.

This control function (PID, proportional-integral-derivative) is often a standard part of the flow meter electronics, whilst the control characteristics can be adjusted for fast or smooth control by means of user software. Depending on your application, you can select a proportional, electromagnetic control valve for high or low (differential) pressure and for low to high flow.

More intelligence in thermal Mass Flow Meters & Controllers

There is still a continuous improvement of thermal mass flow meter and thermal mass flow controller technology, made possible by the integration of extra intelligence. Features like Multi-Gas/Multi-Range functionality or diagnostic functions are no longer an exception.

Thermal mass flow controller of the EL-FLOW Prestige series
Thermal mass flow controller of the EL-FLOW Prestige series

Bronkhorst’s EL-FLOW Prestige series features an incorporated gas database with physical properties of approx. 100 gases, enabling the instrument to automatically compensate for inlet pressure variations.

If you are interested in learning more about this thermal mass flow meter, download our white paper containing in-depth information about influential factors and the impact on accuracy, stability, linearity, and pressure correction.

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