Thermal mass flow meters

What is thermal mass flow measurement?

Thermal mass flow measurement is all about heat transfer. When a gas or liquid flow passes a heated surface, it will take away the heat from the surface, absorbing this heat into the fluid. In a sense, such a flow has a cooling effect on the heated surface. The larger the flow, the more the heated surface will be cooled. By recording how much additional (electric) power is required to keep the cooled surface at the original temperature, you can calculate the amount of heat that is absorbed into the liquid flowing along the surface.

Heat capacity
Thermal mass flow measurements rely on thermal properties of the fluid, such as its specific heat capacity. The specific heat capacity of a liquid or gas is a characteristic material property of that fluid. It defines the rise in temperature of that fluid in degrees Celsius when a certain amount of heat energy is absorbed into the fluid. The mass of the fluid is relevant: 2 grams of fluid can absorb twice as much heat as 1 gram of the same fluid. This applies to static fluids as well as to moving fluids, so for low liquid or gas flows.
Thermal mass flow measurement makes quite straightforward use of this basic physical principle, which relates the flowing mass (in grams per second) to the applied power (in Joules of heat per second) with specific heat capacity (in Joules per gram per degree Celsius) and heat-induced temperature difference (in degrees Celsius) as proportionality factors.

What is mass flow rate?

This direct relationship between thermal properties of the fluid and its mass has the advantage that a mass flow rate is measured, which is not influenced by varying process conditions such as changes in temperature or pressure. Read more about the relationship between mass flow and volume flow. In many production and R&D processes such as chemical reactions, mass is the important variable, and not volume.

Why is calibration is needed?

The measurement principle of a thermal mass flow meter is based on heat transfer and depends on the density and specific heat capacity of the fluid. Therefore, thermal mass flow meters need to be calibrated for a specific fluid.
 

3 Types of sensor principles for thermal mass flow meters

So, a thermal mass flow meter uses the thermal properties of a fluid to measure (or ‘sense’) its mass flow rate. How does this work in practice? We distinguish three different sensor principles:
1. Thermal mass flow sensor for gases, using the bypass principle, where a proportional part of the flow is passed through a flow restriction.
2. Thermal mass flow sensor for gases, using the inline principle (CTA), that measures directly in the main flow channel.
3. Thermal mass flow sensor for liquids, using the inline principle (CTA/CPA).
 

More about MEMS-based thermal mass flow meters

Also, thermal mass flow meters for gases are available that use chip-based sensors. MEMS technology (Micro Electro-Mechanical System) is incorporated in Bronkhorst FLEXI-FLOW devices, where the gas flow encounters these sensors. In addition to mass flow rates, FLEXI-FLOW devices can also measure temperature and pressure of the gas. Highly compact MEMS-based IQ+FLOW devices are ideal for use in confined spaces.
 

Thermal mass flow meter or controller

A thermal mass flow meter is transformed into a mass flow controller – to accurately and repeatably control the mass flow rate of supplied gases or liquids – by adding an integrated or close-coupled control valve to the meter. Read more about mass flow controllers.