Using Thermal Dispersion Flow Meters for Mass Flow Measurement: Common Questions and Answers

During the past few months, Magnetrol® Product Manager Tom Kemme has fielded numerous questions about thermal dispersion technology on the thermal mass flow measurement portal for Flow Control magazine. This week, we are sharing some of the questions that Tom has received to help you understand this complex topic. For more information about using thermal dispersion technology for mass flow measurement, please review last week’s blog post.

Question: What is the typical accuracy of thermal mass flow meters?

Answer: Typically, thermal dispersion flow meters have an accuracy statement in terms of both percent reading and percent full scale. It is common to see a single point thermal dispersion flow meter have a percent reading accuracy of 1 percent plus 0.5% full scale. This statement will vary by manufacturer and design (i.e., high temperature, multi-point, etc.). The accuracy statement is a calibration accuracy under ideal conditions. The type of calibration, as well as the installation, will affect the true accuracy.

In terms of the calibration, the most accurate method is to have a thermal flow meter calibrated to the type of gas it is intended to measure. Depending on the application, the highest level of accuracy may not be needed, and repeatability may be the most important factor.

Many manufacturers calibrate with air instead of the particular gas, because it is less expensive and still provides good results. How the relationship between air and that gas is formed will depend on manufacturer, but it could be a theoretical comparison or actual data comparing air versus the specific gas (this is often referred to as an “equivalency” calibration). Sometimes it is simply not possible to calibrate with a specific gas (or gas mix), and a theoretical calibration may be the only option to provide the best results.

On the installation, single point thermal flow meters expect a fully developed flow profile. Most manufacturers will recommend the amount of straight run required before and after the flow meter, which will depend on what is upstream. A single 90-degree elbow may cause distortion, while a double elbow could induce swirl and require more straight run to develop the flow profile. Most manufacturers will advise against installing the flow meter downstream of control valves. The installation is pivotal in getting the best accuracy possible, but repeatable measurement can still be obtained in most cases.

There are many factors to take into account which makes the accuracy conversation an in-depth one.

Question: Can thermal mass flow meters be used with combustible gases?

Answer: Thermal dispersion flow meters can be used in combustible gases. Typically, very little heat is added to the system (the amount of heat depends on the manufacturer). Some customers ask this question because they are worried about exceeding the auto-ignition temperature of the particular gas. However, the maximum temperature rise of the MAGNETROL Thermatel® Model TA2 thermal mass flow transmitter recorded by FM during the approvals process was 4K above ambient. Therefore, this slight temperature increase to the process temperature does not cause ignition, but it should be evaluated as needed on an application basis with the manufacturer.

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THERMATEL TA2 thermal mass flow
meter with retractable probe assembly

Question: Thermal mass flow meters are good for air flow measurement, but for flammable gases (like natural gas), are thermal flow meters suitable from explosion point of view since the sensor requires current for flow detection?

Answer: Some of the most popular applications for thermal flow meters involve natural gas flow measurement or other gas mixes with high methane content (landfill/digester/biogas). How much heat the sensor is adding to the system will vary by manufacturer (please see the answer to the preceding question for a more detailed response).

Another common gas for thermal flow meters is hydrogen, which is used in many industries, including chemical, power and metals. You can find a summary of common applications for thermal mass flow meters on the thermal mass flow measurement portal.

 



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