What Is A Thermal Mass Flow Meter And What Is Its Working Principle?

A thermal mass flow meter is a type of device that measures gas mass flow. And it works in a range of applications. But this does not gauge the volumetric flow, but compressed gas or other types that include:

  • Natural gas
  • Carbon dioxide
  • Air and biogas measurement 
  • Nitrogen and oxygen
  • Argon 

So, do these thermal mass flow meters apply only for gas flow applications? Yes, that’s right. They use heat to measure flow. Temperature sensors gauge how much heat dissipates and introduce heat into a flowing stream.  

It’s a process that only works with gas mass flow measurement. So, it would be pretty challenging to consider the heat absorption to get a strong signal if you use a thermal mass flow meter in a liquid. 

What Is The Difference Between These Thermal Flow Sensors?

Let’s distinguish the types of sensors within the scope of thermal flow meters. Both thermal mass flow meters and thermal mass flow controllers use the heat conductivity of fluids. 

Inline sensors: they directly measure the main flow

By-pass sensors: they measure where there is a proportional flow passing through a flow restriction. 

Maybe you could be wondering, which technology is better or more advantageous than the other? A by-pass sensor would be ideal for a clean, dry gas application since higher accuracy and repeatability are vital. But both of these may have their pros and cons depending on the specific application. 

Then an inline sensor would work best for application with moderately moist gas or lower accuracy. High repeatability and sturdiness make this instrument a better choice for this application, but one may also combine it with a straight flow channel. 

Working Principle Of A Thermal Mass Flow Meter

Since their invention in the 70s, a thermal mass flow meter has been one of the most popular devices for determining the mass flow of low gas flow. But they didn’t start that way. They have evolved from analog to digital, then they went from low flow to higher flow, from lab applications, and to industrial environments. Today you’ll find a suitable thermal mass flow meter for every low-flow gas application.

Then we have Coriolis flow measurement and differential pressure measurement that also use the measurement principle for low-flow gas applications. 

Methods For Measuring How Much Heat Dissipates

Let’s look at two other ways for measuring how much heat disperses.

Constant Temperature Differential 

One of these measures the temperature of the gas, while the other uses a heated sensor. And you calculate the mass flow rate based on the amount of electrical power expressed in milliwatts. It’s the energy required to sustain a steady difference in temperature between the two sensor-heated sensors and the passive temperature sensor.

Constant Current Differential Meter

This meter has a slow response; it’s a less popular flow meter. These meters have a heated sensor and another one that senses the temperature of the flow stream. And it maintains the heated power or energy delivered to the heated sensor constant. So, the mass flow measurement is the difference between the temperature of the heated sensor and the flow stream.

These two methods use the principle of higher velocity flows, resulting in a higher cooling effect while the heat redirects from the heated sensor.


Gas mass meters only calculate the mass flow instead of volumetric flow. Thermal mass flowmeters are the best when it comes to accuracy and repeatability over other flow rates. They are also excellent in measuring flow in large pipes. And since everyone wants to save on some bucks, these meters don’t require temperature or pressure correction. And this saves the person additional cost for buying and installing other equipment.

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