Thermal Mass Airflow Measurement White Paper | FTM06D Insertion vs. FDM06-I Venturi Principles, Design and Applications
A comparison of sensing position, flow-path design and signal processing to explain the differences between two thermal flow measurement architectures and how to select them.
In compressed-air systems, production equipment needs stable supply pressure, while operators need to know where air goes, when it is used and how much is consumed over time. As monitoring needs expand from the shop floor to production-line comparison, preventive maintenance, energy saving and cost management, sensing principles, installation requirements and data use must all be considered.
eyc-tech FTM06D uses an insertion probe to sense airflow directly inside a pipe. It suits applications that primarily need air-velocity monitoring and a measurement point installed through a pipe opening. FDM06-I combines a Venturi measuring section with hot-wire sensing and provides instantaneous and totalized flow for downstream management. Both can be integrated into larger systems, but they differ in how measurements are established and how the necessary data are obtained and used. [1][2]
1. From Stable Air Supply to Transparent Consumption
Pressure indicates supply conditions but cannot, on its own, reveal air consumption. When compressors continue supplying air, pressure may remain similar even as equipment demand rises or a pipeline leak develops. Reading only the pressure gauge can conceal changes in the air-supply load.
Two machines may have similar instantaneous flow at startup but very different daily consumption because of operating hours, standby practices and production cycles. Monitoring must move beyond “is air available?” to “how much is being used now?” and “how much was used over the period?”
Flow monitoring can provide clues to abnormal operation. For example, persistent flow after production stops may prompt inspection of standby consumption, continuous blowing or leaks. The actual cause must still be verified against equipment status and on-site findings.
2. Air Velocity, Flow Rate and Totalized Flow
Air velocity describes how fast gas moves. Volumetric flow rate is the gas volume passing a cross-section per unit time, while totalized flow is the accumulated quantity over a period. These quantities are related, but they describe different information.
Under the same reference conditions, volumetric flow rate equals cross-sectional mean velocity multiplied by the internal cross-sectional area. Mean velocity is required: a single reading at an arbitrary location cannot simply be treated as the whole cross-section average. Insertion point, velocity profile and calibration relationship all matter when converting velocity to pipe flow.
Gas is compressible, and its volume changes with pressure and temperature. Actual volumetric flow in a pipe and standard volumetric flow converted to specified reference conditions must therefore not be mixed. Before comparing machines or time periods, confirm that instruments and downstream systems use the same reference pressure, temperature and units.
3. FTM06D Sensing Architecture: Insertion Air-Velocity Measurement
The insertion probe extends into the pipe through an installation port. Measurement representativeness depends on probe position, insertion depth, orientation and the on-site velocity profile. To convert velocity into total pipe flow, the correct internal pipe diameter must also be set, and the effects of flow distribution addressed according to installation requirements.
Airflow DirectionInsertion ProbeInstallation concept only. Refer to the product manual and on-site requirements.
Measurement Principle: Directly Sensing Airflow-Induced Heat Transfer
FTM06D uses a hot-wire sensing principle. Its probe includes a reference element for medium temperature and a heated sensing element. Air passing the probe removes heat; changes in velocity change the heat transfer between the element and the gas.
The product uses constant-temperature anemometry (CTA). Changes in the heating power needed to maintain the specified thermal state are related to velocity, then converted into measured output through calibration and signal processing. This allows the probe to obtain air-velocity information directly from the pipe.
The basic operating principle of a hot-wire anemometer is to measure gas velocity through heat transfer between a heated element and the fluid. When current passes through a heater, such as a platinum-resistance sensing element, heat is generated. Passing air removes some heat and changes the element’s temperature or resistance. Measuring that change, or the power needed to maintain the set temperature, and applying calibration and gas-property information provides an estimate of velocity.
The heating element need not be a traditional metallic wire. Platinum resistors or other suitable resistance heaters can also be used, depending on sensor construction and control strategy.
This is a conceptual illustration of the sensing principle. Refer to the product’s technical documentation for the actual sensing configuration and circuitry.
Figure 1. Thermal air-velocity measurement and constant-temperature control concept.
The principle diagrams explain measurement mechanisms, not the actual internal construction of the product.
4. FDM06-I Sensing Architecture: Venturi-Based Flow Measurement
FDM06-I is installed inline by connecting its complete measuring section into the pipeline. The Venturi flow path and sensing elements are integrated into that section, so insertion depth does not need to be adjusted on site. Flow direction, connection size and required straight pipe runs must still be checked to ensure suitable inlet conditions.
Installation concept only. Refer to the product manual and on-site requirements.
Measurement Principle: Venturi Differential Pressure and Heat Transfer
As gas passes the Venturi throat, its velocity rises and static pressure falls, generating a pressure difference between the inlet and throat. This differential pressure drives a small portion of the gas through an internal sensing passage, where the sensing element obtains a signal related to the main-pipe flow.
This Venturi diagram illustrates the concept. Refer to the product’s technical documentation for actual sensor construction and flow conversion.
Measurement Principle: Converting Heat Transfer to Flow by Hot-Wire Sensing
The sensing passage uses hot-wire measurement technology. As gas passes the heated element, it changes heat transfer and distribution. Sensing elements detect those changes, which are combined with the Venturi flow-path design and product calibration to determine main-pipe flow.
This heat-transfer illustration is conceptual. Heat-field shapes and colors do not represent measured temperatures or CFD simulation results.
The principle diagrams explain measurement mechanisms, not the actual internal construction of the product.
In short, FTM06D inserts a probe into the main pipe to sense local airflow-related heat transfer directly. FDM06-I first creates a pressure difference using the Venturi geometry and then obtains a flow-related signal from heat transfer in its internal passage.
5. Installation Conditions and Data Reliability
Measurement quality depends on both instrument performance and field conditions. Selection should consider normal consumption, peak demand, the minimum flow of interest and the pipe layout—not just the connection size. A straight pipe run allows the flow profile to recover from disturbances caused by elbows, valves and other components.
Gas conditions also matter. Water droplets, oil contamination or deposits can change heat transfer; changes in gas composition may also invalidate an existing calibration relationship. This article focuses on dry compressed-air measurement. The same configuration must not automatically be applied to wet, corrosive or mixed gases without confirmation.
6. Temperature Compensation and Three-Cycle Verification
Thermal flow sensing uses heat exchange between the gas and sensing element. Therefore, medium temperature affects readings as well as velocity. FTM06D includes temperature compensation to reduce the influence of temperature changes on air-velocity measurement.
Temperature Compensation: Velocity Response at Different Temperatures
Key observation: Compare reference air velocity, reference temperature and transmitter readings across different temperature conditions to assess whether the measurement output follows changes in airflow velocity. The chart below illustrates the verification trend.
Reference velocity, reference temperature and instrument velocity are compared on the same time axis. Air velocity is increased and decreased while temperature is changed progressively, to observe whether the transmitter output continues to follow the reference velocity.
The reference and instrument velocity curves show similar trends, which can help examine temperature compensation. The focus is performance when temperature changes—not just agreement at one fixed temperature.
Figure 4 | Temperature Compensation Verification
Time (s) | Shared schematic scale for velocity (m/s) and temperature (°C)
Note: HTML trend illustration reconstructed from the appearance of the Word chart. It is not the original test dataset and must not be used to calculate accuracy or error.
Three-Cycle Linearity: Comparing Repeated Velocity Steps
Key observation: At 25°C, repeat three stepwise increases and decreases in air velocity to compare readings at each step, repeatability between cycles, and differences between upward and downward sequences. The chart below illustrates the verification trend.
The three-cycle linearity chart compares reference and instrument velocity during three repeated stepwise increases and decreases at 25°C.
Repeating the same sequence reveals how readings match at different velocity levels, how consistently the measurement behaves across cycles and whether rising and falling steps differ materially. The two curves remain close over the three cycles and provide a basis for assessing repeatability.
Figure 5 | Three-Cycle Linearity Verification
Time (s) | Air velocity (m/s) | Test at 25°C
Note: HTML trend illustration reconstructed from the appearance of the Word chart. It is not the original test dataset and must not be used to calculate accuracy or error.
Temperature compensation and three-cycle testing evaluate FTM06D from two perspectives: changing medium temperature and repeated velocity sequences. In practice, catalogue specifications and installation requirements must also be observed when applying these validation capabilities to field monitoring.
7. FDM06-I Data Integration: From Flow Signal to Consumption Records
FDM06-I displays instantaneous and totalized flow and can transmit both values through Modbus to PLC, HMI or an energy-management system. Instantaneous flow helps track current demand. Totalized flow supports consumption summaries by shift, day or machine, reducing the need for downstream accumulation logic.
When integrating systems, confirm flow units, reading intervals and how the totalizer handles resetting or power loss so that downstream records remain consistent with the local display.
8. Monitoring Layout: From Equipment to Zone Consumption
Measurement locations should reflect the intended management boundary. Points may be established at machine inlets, distribution branches or treated-air headers, provided gas and installation conditions are suitable.
At the equipment level, monitoring can compare production, standby and shutdown to see airflow changes under different operating conditions. Consider FTM06D where insertion velocity measurement is the priority; consider FDM06-I when an integrated flow section and totalized consumption are required, after checking the actual operating range.
Distribution branches can reveal differences in consumption across lines or plant areas. The FDM06-I totalizer supports shift and daily records. However, a rise in branch consumption must be interpreted alongside output, operating hours and new equipment; it does not automatically mean lower efficiency.
When comparing multiple points, the difference between header and branches is not necessarily all leakage. Receiver charging and discharging, unmonitored branches, drain or dryer consumption, unsynchronized time windows and measurement error can all contribute. Confirm the system boundary and identical time windows before narrowing the cause with branch isolation or field inspection.
CDA Supply Header
Monitor the main supply at a pipe section with suitable gas and installation conditions.
Production Zone Consumption
Compare air use across production zones, considering operating hours and output.
Equipment Airflow Monitoring
Compare velocity or flow trends during production, standby and shutdown.
Shift Consumption Reporting
Use the totalizer to generate consistent shift and daily consumption records.
Energy Management Integration
Combine consumption, power and machine operation data to track savings and costs.
9. Energy Management: Consumption, Savings and Cost Control
FTM06D is suitable for establishing velocity monitoring points through pipe installation openings. FDM06-I integrates instantaneous and totalized flow for period-based consumption. Their different installation and data characteristics support different levels of energy management.
Energy Saving: Find Waste and Track Improvements
Where an existing pipe can accommodate an insertion point, FTM06D can show velocity changes during production, standby and shutdown to identify clues of continued air consumption. To compare total use before and after an improvement, FDM06-I totalized flow provides a direct period total. Together with actual electricity records, energy savings can be assessed, and applicable electricity emission factors can be used to estimate the resulting change in emissions. [4]
Preventive Maintenance: Use Flow Trends to Plan Inspections
FTM06D can continuously monitor velocity at an equipment inlet or air-supply pipe, helping identify changing airflow under comparable operating conditions. FDM06-I can monitor instantaneous flow and period consumption together, identifying unexpected increases in usage. Combined with pressure and equipment status, these trends can guide inspections of pipes, valves and pneumatic equipment and support comparisons before and after maintenance.
Energy Profit Centers: Account for Consumption and Cost
For existing pipes suitable for insertion installation, FTM06D can provide flow information once internal diameter and measurement conditions are correctly configured, allowing downstream accumulation based on the selected output. If a meter totalizer should be read directly, FDM06-I reduces the need to build integration logic downstream. With agreed point ownership, electricity rates and cost-allocation rules, downstream systems can summarize air-related cost by line or department for energy profit-center management.
ESG and Carbon Accounting: Build Traceable Records
FTM06D and FDM06-I can provide velocity, flow or totalized-flow data, according to configuration, for analysing compressed-air use and tracking improvements. Use in energy management or carbon inventories also requires electricity consumption, relevant emission factors and the company’s management and accounting procedures.
10. Selection and Implementation Based on Measurement Goals
Selection can be narrowed by four questions: what must be answered, which installation methods are acceptable, what is the actual operating range and what downstream data are needed?
FTM06D uses a direct insertion probe to measure airflow, providing a straightforward way to establish velocity monitoring points. Derived flow and downstream functions require correct configuration and selected outputs. FDM06-I combines a Venturi measuring section with instantaneous and totalized flow, facilitating consumption records and management integration. [1][2][3]
During commissioning, record measurement location, piping conditions, configuration, reference conditions and communication mapping. Verify local and downstream readings. Traceable measurement sources, data definitions and use procedures are essential for meaningful consumption analysis, preventive maintenance, cost allocation and energy-saving follow-up.
Technical References
Technical References
[1] eyc-tech FTM06D product page and catalogue, accessed October 21, 2026.
https://www.eyc-tech.com/ftm06d-thermal-mass-flow-transmitter
[2] eyc-tech FDM06-I product page, accessed October 21, 2026.
https://www.eyc-tech.com/fdm06-i-venturi-thermal-mass-flow-meter
[3] eyc-tech FDM06-I Modbus Protocol, 20250304.
https://zh-tw.eyc-tech.com/pdf/620346/eyc-tech-FDM06-I-Modbus-Protocol-20250304.pdf
[4] GHG Protocol Scope 2 Guidance, accessed October 28, 2026.