A thermal mass flow meter measures the movement of a fluid in a pipe or duct. So in this meter’s simplest configuration, gas flows past a heated velocity sensor and a temperature sensor. Then the device works to maintain a constant difference of 50 degrees Celsius between the gas and the heated sensor.

As the gas flows past the heated sensor, its molecules steal some of the heat, creating a cooling effect. So the watts needed to maintain the temperature difference between the two sensors is directly proportional to the mass flow rate. And the amount of heat lost depends on the thermal properties and flow rate of the gas.

 

To know more about the various types of flow meters, you can read the Visaya Artcile on the flow meter types

Thermal mass flow meter working principles

The market offers two principles for thermal mass flow meters. Some vendors will carry both and some just one. However, we need to know how they work and how they differ to choose the right one for an application.

We shall find devices based on thermal dispersion and others that use thermal profiling. After having a context about each, we can review some examples on the market.

Endress+Hauser
Proline t-mass B 150 Reliable and cost-effective flowmeter for utility gases
in the shop
6BABL1/2/3/4

Thermal dispersion

Here the meter will have two sensors, one heated and the other unheated, in contact with the product. The heated sensor does the detection and the unheated stands as a reference, with both of them connected through a Wheatstone bridge arrangement.

Wheatstone bridge demostration
Courtesy of Wikipedia

Now, we have two types of device for this principle. In one, the heated sensor has a constant current. If we have variation, then the resistance will change. We’ll see a correlation between the temperature (resistance) and the product flowing in the pipe.

The other keeps the resistance constant instead of the current. Here, we’ll have variance in the power when the flow shifts, with the power variance proportional to the flow.

We can easily install this meter if we pick the insertion version rather than flanged. The meter will measure the mass flow directly, and we can measure different gases and mixtures as well. Usually, we’ll have a database with the gases that can be measured, but in some cases, we may need to calculate mixtures.

Thermal profiling

On the other side, this principle applies the heat to the flow instead of a sensor. We still have two sensors, but we’ll measure the change of temperature in two areas. Usually, we can install the sensors at the inlet and the outlet after the heating system.

When we have zero flow, we’ll see the same temperature in both sensors. But with flow in the pipe, it creates a temperature difference proportional to the mass flow, which the sensors will read and the meter will translate.

Thermal mass flow meter designs

Now that we know these principles and how each measures mass flow, we should discuss thermal mass flow meter design. That is because each principle must have a particular design to work, and we need to know how those work too!

Insertion

If we have large pipes and don’t want to cut them to install a meter, then maybe we can use an insertion thermal mass flow meter. This one’s easy to install and comes in different sizes.

To find the total mass flow, this meter factors in the flow rate, compensation, and cross section. When we scale out a new insertion meter, we need to make sure we check the insertion length in your pipe. There might be measurement problems where the customer had the wrong length.

Bypass

Here, we have two names for the device. We can call it a bypass flow meter or a capillary tube flow meter. This type has a laminar flow element, with the capillary tube installed at the inlet and outlet of the flow element.

 

capillary tube thermal flow meters
Courtesy of sierrainstruments.com

The capillary tube houses the entire system, from the sensors to the heating element. However, we may find variations on this theme, with some vendors offering two heating systems and up to three sensors in the capillary tube. These flow meters combine with flow controllers and a final element to control the sensor. You get it all as one device.

In-line

This device combines the body, sensor, and transmitter. It has different  process connections, materials, and integration options. It uses thermal dispersion.

Hot-wire anemometer

As the most straightforward type of flow meter, we’ll find this device commonly used in research applications. We can get it in a variety of designs from simple to complex. And the fine wire sensor comes in materials such as nickel, platinum, and tungsten.

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Flowphant T DTT31 Flow switch for monitoring flow rates and temperature
in the shop from 270 €
DTT31

Thermal mass flow meter advantages and disadvantages

Here, we’ve listed a few of the pros and cons of thermal mass flow meters.

Advantages

  • No moving parts
  • High sensitivity
  • Wide range of pipe sizes

Disadvantages

  • Sensitive to gas composition
  • Delicate calibration
  • Sensitive to contaminated fluids

Fast facts

  • A thermal mass flow meter uses molecular heat transfer to measure fluid mass flow rate.
  • It contains a heated velocity sensor and a temperature sensor.
  • Fluid passing the heated sensor cools the sensor.
  • This cooling effect is proportional to the fluid’s velocity.
  • But the fluid’s thermal properties (amount and rate at which it can transport heat) influence this effect.
  • You can find the mass flow rate by measuring the temperature difference between the heated sensor and the temperature sensor.
  • The watts needed to maintain the temperature difference is directly proportional to the fluid’s mass flow rate.
  • These devices measure without needing pressure and temperature compensation or flow computers.
  • Thermal mass flow meters are the only direct gas mass flow meters other than Coriolis meters.
diagram of how a thermal mass flow meter sensor works
Courtesy of Sierra


To know about electromagnetic flow meters, you can read the Visaya Article on how does an electromagnetic flow meter work

Fast factors

Many other factors will affect how well this meter works:

  • Thermal characteristics of sensor materials
  • Construction of sensor
  • Heat loss other than that being carried away by the fluid
  • Uniformity of flow in the pipe

To know more about thermal mass flow meters, you can get in touch with our engineers and we will be happy to help.

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