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As a leading manufacturer of high-performance thermal mass flow meters, we regularly address key questions about these precision instruments and their applications in gas flow measurement. This comprehensive guide explores the fundamental principles of thermal mass flow meters, their distinct advantages over conventional flow measurement technologies, and the diverse applications where they excel.
What Is a Thermal Mass Flow Meter?
A thermal mass flow meter is a precision instrument that directly measures gas mass flow using heat transfer principles. Unlike other flow measurement technologies, thermal mass flowmeters don’t require additional temperature or pressure compensation and deliver accurate readings regardless of most operating conditions.
Actual Flow versus Standard Flow
Gas is compressible, making accurate measurement of gas flow rates challenging. As temperature increases, gas molecules move further apart. Conversely, as pressure rises, gas molecules move closer together.
Most gas flow meters (differential pressure, turbine, positive displacement, vortex shedding) measure gas flow at actual operating conditions where the measurement is made, often called ACFM (Actual Cubic Feet per Minute). However, what’s more important is the flow rate adjusted for a particular temperature and pressure, frequently referred to as STP (Standard Pressure and Temperature) and measured in SCFM (Standard Cubic Feet per Minute).
Most flow meters require temperature and pressure correction to convert from actual (ACFM) to standard (SCFM) conditions. To determine mass flow rate in standard conditions, you need to measure the actual flow rate, operating pressure, and operating temperature. This is not necessary with thermal mass flow technology.
How Thermal Mass Flow Meters Work
Thermal mass flowmeter technology measures heat transfer as gas flows past a heated sensor. These meters represent an entirely different method for measuring flow rate compared to traditional technologies.
The Sage meter consists of two high-precision platinum sensors protected in stainless steel. The temperature sensor serves as the reference and measures gas temperature. The flow sensor is heated to maintain a slight temperature difference (overheat) above the temperature sensor.
As gas flows past the heated flow sensor, heat transfer occurs. The instrument measures the power required to maintain the desired overheat, which is proportional to the mass flow rate. The amount of energy applied to the sensor is very low, allowing this technology to be used safely with natural gas, hydrogen, or other flammable gases.
Thermal mass flow measurement is an inferred flow measurement. These meters measure heat transfer and then relate it to mass flow rate based on NIST traceable calibration. During calibration, known amounts of gas flow past the sensor while the signal is measured multiple times across the instrument’s operating range. A curve showing signal vs. flow rate is created for each meter (Figure 2).
Advantages of Thermal Mass Flow Meters
Thermal mass flow meters offer significant advantages over other flow measurement technologies:
- Direct Mass Flow Measurement: No need for pressure or temperature corrections
- Superior Low-End Sensitivity: Accurately measures even minimal flow rates
- Exceptional Turndown Ratio: Typically 100:1 with resolution as high as 1000:1; higher turndown is available in optimal conditions.
- Easy Installation: Simple insertion or inline installation options
- No Moving Parts: Ensures long-term reliability and minimal maintenance
- Low Pressure Drop: Conserves energy with negligible flow restriction
- Versatility: Effective across a wide range of pipe sizes and applications
Top Applications for Thermal Mass Flow Meters
Thermal mass flow meters excel in numerous industrial, commercial, and environmental applications. Here are seven key areas where these instruments deliver exceptional performance:
#1. Natural Gas to Combustion Sources
Combustion sources such as boilers and furnaces have different efficiencies. Users can determine the more efficient operation by measuring natural gas flow rates to combustion sources. Thermal mass flow meters are ideal for measuring natural gas flow rates to individual combustion sources.
#2. Natural Gas Submetering
In facilities with different cost centers, submetering natural gas is frequently performed for cost allocations. When utility costs are allocated to various departments, there’s an increased incentive to improve efficiency and reduce natural gas usage.
#3. Compressed Air
Industrial air compressors use more electricity than most other industrial equipment and can account for as much as a third of energy consumption in a plant. Thermal mass flow meters help determine the optimal number of compressor units needed. Additionally, air leaks waste up to 30% of an industrial compressor’s output. These meters help define the extent of leakage in a system and quantify the lost energy. For more information, read “ISO 50001 Energy Management: The Use of Thermal Mass Flow Meters in ISO 50001 Energy Management Systems.”
#4. Biogas Production
Biogas is produced in landfills as landfill gas and in anaerobic digesters as digester gas, among other places. High in methane, it’s often the focus of biogas-to-energy projects. Accurate gas flow measurement is needed throughout production, whether monitoring biogas destruction or cogeneration. Specific gas flow is also necessary for GHG emissions reporting to environmental agencies and carbon credit programs.
#5. Flare Gas
Flare gas measurement and monitoring ensure the system operates correctly. Varying gas compositions complicate this application. Thermal mass flow meters provide reliable measurement in these challenging conditions.
#6. Aeration Air
The activated sludge process used at wastewater treatment plants requires an air flow for microorganisms to break down organic waste. An optimum air flow throughout the system, sufficient to encourage waste consumption by microorganisms (yet not excessive), is continuously sought.
#7. Combustion Air
Optimizing air-to-fuel ratios through accurate and repeatable gas flow measurement achieves combustion efficiency and effective energy management. Direct measurement of combustion air flow and fuel flow provides the criteria for peak efficiency.
Frequently Asked Questions About Thermal Mass Flow Meters
How accurate are thermal mass flow meters?
The meters typically achieve accuracy ratings of ±1% of reading plus ±0.5% of full scale. This high accuracy, combined with excellent repeatability of ±0.2%, makes them ideal for critical gas flow measurement applications where precise readings are essential for optimizing system performance or meeting regulatory requirements. If needed, accuracies can be enhanced via flow conditioning or custom testing.
Do thermal mass flow meters work with all gases?
No. Thermal flow meters cannot measure condensing gases; dry or slightly humid gases are best. Also, each gas has different thermal properties that affect heat transfer rates, so for optimal performance, the meters should be calibrated using the actual gas composition for which the meter is intended. When ordering a thermal mass flowmeter, specify the gas to be measured to ensure proper calibration. For applications with variable gas compositions, such as biogas, variational adjustments to the gas mix can be made during the calibration process.
What’s the difference between insertion and inline thermal mass flow meters?
Insertion-style thermal mass meters feature a probe that inserts directly into the pipe through a compression fitting or ball valve assembly. They’re ideal for larger pipe sizes (typically 2″ and above) and allow for easy installation without cutting the pipe. Inline models have the flow body integrated into the pipe section and are generally used for smaller pipe diameters (typically under 2″).
Can thermal mass flow meters be used in hazardous locations?
Yes. Many thermal mass flow meters are designed and certified for use in hazardous locations. Models like the Sage Paramount are available with Class I Division 1 approval, with Division 2 approval being standard, making them suitable for environments where flammable gases or vapors may be present. Always verify the specific hazardous area certification needed for your application.
How often do mass flow meters require calibration?
Traditional thermal mass flow meters typically require annual factory recalibration. However, advanced models like those from Sage Metering feature in-situ calibration verification technology that allows users to verify calibration accuracy in the field without removing the meter. This innovative feature can eliminate the need for annual factory recalibrations while ensuring measurement integrity.
What communication options are available for thermal mass flow meters?
Modern thermal mass flow meters offer multiple communication options, including 4-20mA analog outputs, pulse outputs for totalized flow, and digital communications like Modbus RTU, BACnet, HART protocol, and Ethernet options. These communication capabilities allow for seamless integration with various control systems, energy monitoring software, and building management systems. Additionally, the Sage Paramount has optional Bluetooth capability that permits remote programming of hard-to-reach integral-style meters.
How do mass flow meters handle varying flow conditions?
Thermal mass flow meters excel in handling widely varying flow conditions thanks to their exceptional turndown ratio. With turndown ratios of 100:1 standard and higher in some models or applications, they can accurately measure both very low and high flow rates. This makes them ideal for applications with fluctuating demands or seasonal variations.
Conclusion
Since gas is compressible, its volume changes under pressure or when heated (or cooled). Thermal mass flow measurement is based on heat transfer and measures mass flow, not volumetric flow, thus eliminating the need for temperature or pressure correction.
Thermal mass flow meters deliver significant advantages for gas flow measurement applications. By directly measuring mass flow based on heat transfer principles, these instruments provide exceptional accuracy across various conditions while simplifying installation and reducing maintenance requirements.
For more information on thermal mass flowmeter technology and applications, explore our related resources:
Reference
Fundamentals of Thermal Mass Flow Measurement – by Wayne Shannon
