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.
If you would like to know more about various types of flow meters, take a look at this article.
Thermal mass flow meter working principles
The market offers two principles for thermal meters. Some vendors will carry both and some just one. However, you need to know how they work and how they differ to choose the right one for your application.
You’ll find devices based on thermal dispersion and others that use thermal profiling. Let me give you more context about each, then we can review some examples on the market.
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.
Now, we have two types of device for this principle. In one, the heated sensor has a constant current. If you have variation, then the resistance will change. You’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, you’ll have variance in the power when the flow shifts, with the power variance proportional to the flow. (Beautiful!)
You can easily install this meter if you pick the insertion version rather than flanged. The meter will measure the mass flow directly, and you can measure different gases and mixtures as well.
Usually, you’ll have a database with the gases you can measure, but in some cases, you may need to calculate mixtures.
On the other side, this principle applies the heat to the flow instead of a sensor. You still have two sensors, but they’ll measure the change of temperature in two areas. Usually, you’ll install the sensors at the inlet and the outlet after the heating system.
When you have zero flow, you’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 for you.
Thermal mass flow meter designs
Now that you know these principles and how each measures mass flow, we should discuss design. Because each principle must have a particular design to work, and you need to know how those work too!
If you have large pipes and don’t want to cut them to install a meter, then maybe you 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 you scale out a new insertion meter, make sure you check the insertion length in your pipe. I’ve seen measurement problems where the customer had the wrong length.
Here, we have two names for the device. You 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.
The capillary tube houses the entire system, from the sensors to the heating element. However, you 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.
This device combines the body, sensor, and transmitter. It has different process connections, materials, and integration options. It uses thermal dispersion, and you remember how that works, right? Good! (If not, go ahead and take a peek at the earlier sections. Then come back.)
As the most straightforward type of flow meter, you’ll find this device commonly used in research applications. You 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.
Thermal mass flow meter advantages and disadvantages
Here I’ve listed a few of the pros and cons of thermal mass flow meters. Maybe you have more to mention? Feel free to add your input!
- No moving parts
- High sensitivity
- Wide range of pipe sizes
- Sensitive to gas composition
- Delicate calibration
- Sensitive to contaminated fluids
- 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.
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
Part of this article was written by Scott Rouse, Product Line Director @ Sierra Instruments.