The thermal mass flow meter is an accepted instrument that accurately and reliably measures gas mass flow, making it a cost-effective solution for many industries, including oil and gas, refining, chemical, mining, and wastewater.     However, as with any technology, there are certain limitations and challenges associated with the flow meter that must be addressed before implementation. Here are nine of those restraints and how to effectively overcome them. 
1. The presence of condensation may result in inaccurate measurements.
The gas temperature is measured at two points, and the difference between those temperatures is used to calculate airflow. As the gas passes through the heated sensor, its condensed moisture rapidly increases heat transfer, resulting in an inaccurate spike in flow measurement.
How to triumph over this challenge: Install a knockout drum (KO drum or knockout pot) to reduce gas velocity and change the flow direction. The gas will flow upwards, effectively removing moisture droplets from the gas due to gravity.
Angle the flow sensor in the pipe so that if any condensation develops, it will flow away from the sensor. This will help to mitigate any inaccuracies associated with condensed moisture.
2. Sensitivity diminishes as velocity increases.
In thermal flow measurement, mass velocity and heat transfer are nonlinear relationships. Thermal flow meters provide excellent sensitivity for low gas flow, but the sensitivity decreases at higher rates. Therefore, when selecting a mass flow meter for any given application, it is essential to size the meter based on mass velocity relative to the standard temperature and pressure (STP) conditions. Â Â Â Â In contrast, other flow meter types are sized based on the velocity at their respective operating conditions.
As gas pressure increases, the gas molecules become more tightly packed, resulting in higher gas density. This increased gas density directly affects the mass velocity measured by thermal mass flow meters, as it increases in proportion with the density of the gas. However, despite this mass velocity increase, the gas’s actual velocity at operating conditions remains the same.
How to triumph over this challenge:Â It is necessary to use a flow measurement method other than thermal mass flow meters when dealing with compressed gas systems with pressures greater than 150 PSI. Â Â Â Â Two potential alternatives include vortex-shedding and differential-pressure flow measurement. Both of these methods are more reliable than thermal mass flow meters because they are not affected by the loss of sensitivity at higher mass velocities. Additionally, these methods are better suited to handle higher pressures, allowing for more accurate measurements.
3. In the case of calibration with an actual gas type, that cannot be determined.
For accurate mass flow measurement of gas or gas mixtures, it is essential to use a thermal mass flow meter calibrated for the specific gas or gas mixture. This calibration process involves a controlled flow of the specified gas passing through the meter’s sensor on a flow bench and measuring the signal. This process is repeated over the meter’s entire operating range, establishing a relationship between the mass flow and the signal for the specific gas. This relationship is then used to calibrate the meter for the indicated gas or gas mixture, ensuring accurate measurement of mass flow.
How to triumph over this challenge: Overcoming safety or hazardous conditions that make it impossible to use the actual gas (or gas mixture) during calibration can be challenging. However, you can use a surrogate gas with similar heat transfer characteristics and combine it with analytical adjustments based on the relative heat transfer properties of the actual and calibration gases to achieve an accurate calibration. Â Â Â Â This will help ensure the accuracy and reliability of your measurement results.
4. The cost of annual calibrations is high, and the process is inconvenient.
Once a thermal mass flow meter has been calibrated, : when should it be re-calibrated? To ensure optimal performance and accuracy, some manufacturers require their meters to be returned to the factory or an authorized NIST-traceable facility for recalibration. Â Â Â Â While this may be expensive and inconvenient, recent technological advances have enabled flow meter manufacturers to develop alternative ways to verify that their meters remain calibrated. These calibration verification procedures can range from simple to complicated; some may even require the meter to be taken out of the pipe. To ensure accuracy and performance, it is crucial to understand the calibration verification methods employed by each manufacturer and adhere to their suggested recalibration intervals.
How to triumph over this challenge:Â In one manufacturer’s verification process, the user can ensure that the meter is calibrated by comparing one data point with the original calibration data. To do this, a retraction assembly with a compression seal and ball valve is used to withdraw the sensor from the pipe while keeping the gas line in service. This creates a no-flow condition, allowing the user to compare the zero-flow signals during the test and calibration. If the signals match, this verifies that the meter is still in calibration.
5. Composition variation of gas flows.
A thermal mass flow meter must be calibrated for the gas being measured; otherwise, any changes in the gas composition will result in inaccurate data. The degree of inaccuracy will depend on the size of the gas variation. Therefore, to ensure the most accurate readings, it’s essential to regularly check and adjust the meter’s calibration to account for any changes in the gas composition. The challenge of varying biogas compositions can be overcome by understanding the gas composition and applying a correction factor. In most cases, the heat transfer difference caused by the variation in design is minimal and impacts less than 5 percent on the accuracy. However, the composition variation is more significant in specific applications like flare gas at a refinery or chemical plant. Therefore, it could have an enormous impact on the meter’s accuracy. Knowing the gas composition, manufacturers can create tailored correction factors to ensure accurate readings.
How to triumph over this challenge: The challenge of varying biogas compositions can be overcome by understanding the gas composition and applying a correction factor. In most cases, the heat transfer difference caused by the variation in design is minimal and impacts less than 5 percent on the accuracy. However, the composition variation is more significant in specific applications like flare gas at a refinery or chemical plant. Therefore, it could have an enormous impact on the meter’s accuracy. Knowing the gas composition, manufacturers can create tailored correction factors to ensure accurate readings.
6. Profile of undeveloped flow.
Improving the accuracy of a thermal mass flow meter involves duplicating the fully developed turbulent flow profile that the test station provides at the sensor. The recommended upstream, and downstream straight run distances must be followed to ensure the most accurate results. However, achieving the ideal linear run distances in the field is often impossible. As a guideline, a minimum of 15-25 pipe diameters downstream of a single elbow is typically sufficient to create a suitable flow profile.
How to triumph over this challenge: Sometimes, clients opt to use a flow meter with an undeveloped profile, sacrificing some accuracy in exchange for highly repeatable data. Alternatively, a flow conditioner assembly may be employed when a short straight run is an issue. Flow conditioners provide a uniform flow profile at the sensor’s location rather than the fully developed flow profile occurring with an excellent straight run. Therefore, it is essential to calibrate the flow meter with the flow conditioner to create a uniform flow profile.
7. Flow sensor buildup damages accuracy.
The accuracy of your thermal flow meter can be significantly reduced due to buildup on its flow sensor. This buildup reduces the heat transfer between the sensor and the gas flow, resulting in lower readings than expected. Furthermore, the material that accumulates on the sensor increases its thermal mass, decreasing its response time to changes in gas flow rate.
How to triumph over this challenge: If you’re looking for an easy way to improve flow measurement accuracy in large ducts, the thermal mass flow meter with a retractable element is a perfect choice. The insertion probe of the meter contains a compression seal and ball valve, which allow for easy removal from the pipe and cleaning without disrupting service. And by increasing the number of measurement points, you’ll get even more accurate readings.
8. Accurately measuring flow in large ducts.
The thermal mass flow meter probe precisely measures gas flow at the sensor location when the recommended straight-run distance is available. However, obtaining the optimal straight-run distance for large duct runs can take time and effort, making it difficult to measure accurately. To overcome this challenge, our flow meter probe can be easily adjusted to fit any pipe diameter, allowing for accurate flow measurement in any size pipe.
How to triumph over this challenge:Â By increasing the number of measurement points, you can improve the overall accuracy of your process. Using multiple flow meters at different locations across the duct and averaging the flow measurements can further strengthen the readings’ accuracy. This is especially true when compared to using a single flow meter, as the single point of size may be impacted by irregularities or turbulence in the environment. Utilizing multiple flow meters at different locations can help to create a more accurate and reliable measurement.
9. Calibrating at low speeds.
Thermal mass flow meters are an excellent choice for measuring low-velocity flows. However, to ensure accurate readings, proper calibration is essential. Unfortunately, not all thermal mass flow meter manufacturers can calibrate their instruments at low flow rates. This means selecting a manufacturer with the necessary capabilities is vital to ensure accurate measurements.
How to triumph over this challenge:Â If you are considering a low-velocity application, it’s essential to contact the manufacturer to ensure they can accurately calibrate the meter over the specified velocity range. Â Â This is essential to ensure you get the most out of your application, so take notice of this critical step!
10. Conclusion
Every flow measurement technology has its limits, and challenges may arise when pushing the boundaries of a thermal mass flow meter. To ensure the best solution for any given application, it is vital to provide the manufacturer with all the necessary details about the operating environment and any specific requirements. Â Â This will help the solution provider to ensure that the technology they provide can meet the desired results. As a result, the manufacturer can ensure the best solution for any flow measurement challenges with