Comprehensive Analysis of Duct Air Velocity Transmitter Working Principles and Calculation Methods

In industrial ventilation, building HVAC systems, kitchen exhaust applications, and cleanroom environments, air velocity and air volume are directly related to ventilation efficiency, energy consumption control, and operational safety. Compared with outdoor meteorological wind speed monitoring, air velocity measurement inside ducts involves more complex environments and harsher operating conditions. Therefore, specially designed duct air velocity transmitters are required to provide long-term, stable, and quantifiable data output.

Definition and Function of Duct Air Velocity Transmitters

A duct air velocity transmitter is an industrial measurement device installed inside or on the sidewall of an air duct. It measures the gas flow velocity within the duct and converts the measurement result into a standard electrical signal output. Some models can directly calculate and output air volume based on air velocity, providing reliable data support for ventilation system regulation, energy efficiency evaluation, and intelligent control.

Its core functions include:

• Real-time monitoring of airflow conditions inside ducts

• Determining whether the ventilation system operates under designed conditions

• Providing a basis for variable frequency control of fans and energy-saving operation

• Enabling remote centralized monitoring of air velocity and air volume

Basic Physical Principles of Duct Air Velocity Measurement

Relationship Between Dynamic Pressure, Static Pressure, and Total Pressure

Under the condition of stable airflow in a duct, gas pressure can be divided into three types:

• Static pressure: the pressure exerted by the gas itself on the duct wall

• Dynamic pressure: the pressure generated by gas flow velocity

• Total pressure: the algebraic sum of dynamic pressure and static pressure

The relationship among them is:

Total pressure = Static pressure + Dynamic pressure

In engineering measurements, air velocity cannot be measured directly and is instead calculated indirectly through dynamic pressure.

Basic Formula for Duct Air Velocity Calculation

According to Bernoulli’s equation, the air velocity at a measurement point can be calculated from dynamic pressure as follows:

v = √(2 × pd / ρ)

Where:

• v — air velocity at the measurement point, unit: m/s

• pd — dynamic pressure, unit: Pa

• ρ — air density, approximately 1.2 kg/m³ under normal conditions

In practical engineering applications, values from multiple measurement points are required to obtain the average air velocity across the duct cross-section.

Typical Measurement Methods of Duct Air Velocity Transmitters

Indirect Measurement (Differential Pressure Method)

This is currently the most widely used and highest-accuracy method in industrial ventilation systems.

The process includes:

• Using a Pitot tube to collect dynamic pressure at multiple points across the duct cross-section

• Calculating air velocity at each measurement point

• Applying the root mean square method to calculate the average air velocity of the cross-section

• Combining the duct area to calculate air volume

The average air velocity calculation follows engineering standards and is suitable for system commissioning and precision testing.

Direct Reading Measurement (Thermal Method)

Direct reading duct air velocity transmitters typically adopt thermal sensing principles. They directly output air velocity signals based on the cooling effect of airflow on a heated sensing element.

Main features include:

• No complex calculations required

• Real-time continuous output

• Suitable for online monitoring and automatic control

However, this method requires relatively stable and clean airflow conditions.

Structural Composition of Duct Air Velocity Transmitters

A typical duct air velocity transmitter generally consists of the following components:

• Probe assembly: detects changes in airflow velocity

• Sensing element: converts physical quantities into electrical signals

• Signal processing module: performs filtering, compensation, and linearization

• Output interface: provides standard industrial signal outputs

• Protective housing: adapts to high-temperature, corrosive, or dusty environments

Nexisense duct air velocity transmitters feature reinforced structural designs tailored for industrial duct environments, balancing reliability and long-term stability.

Technical Advantages of Nexisense Duct Air Velocity Transmitters

Represented by the Nexisense RS-FS-*-9TH duct air velocity transmitter, the core advantages include:

• High-temperature-resistant and corrosion-resistant materials suitable for complex operating conditions

• High measurement accuracy, wide range ratio, and low long-term drift

• Simultaneous measurement of air velocity and air volume

• Device address configuration supported by both software and DIP switch settings

• Stable output signals for easy system integration

Even in continuous operation environments, the device maintains excellent repeatability and consistency.

Air Volume Calculation Method and Engineering Significance

Air volume refers to the volume of air passing through a duct cross-section per unit time and is a key indicator of ventilation capacity.

The calculation formula is:

L = F × V × 3600

Where:

• L — air volume, unit: m³/h

• F — effective area of the duct or air outlet, unit: m²

• V — average air velocity, unit: m/s

By obtaining stable air velocity data from the air velocity transmitter, automatic calculation and monitoring of air volume can be achieved.

Typical Application Scenarios

Duct air velocity transmitters are widely used in the following scenarios:

• Industrial kitchen exhaust and waste gas discharge ducts

• Building ventilation and exhaust systems

• HVAC supply and return air ducts

• Fresh air and return air monitoring in data centers

• Ventilation control in laboratories and cleanrooms

In these applications, air velocity and air volume data directly participate in system control logic.

Installation, Maintenance, and Servicing Considerations

To ensure measurement accuracy, the following points should be considered in engineering applications:

• Install the device in straight duct sections with relatively stable airflow

• Avoid installation near bends, valves, or fan outlets

• Regularly inspect probes for dust accumulation or oil contamination

• Increase maintenance frequency in high-dust environments

Proper maintenance can significantly extend the service life of the device.

Frequently Asked Questions (FAQ)

Can a duct air velocity transmitter directly output air volume?
Models with air volume calculation functionality can directly output air volume signals, provided that duct area parameters are correctly configured.

Which is more suitable for long-term online monitoring: thermal or differential pressure measurement?
Thermal methods are more suitable for continuous online monitoring, while differential pressure methods are more suitable for system commissioning and precision testing.

Does the device support integration with building management systems?
Models supporting standard industrial communication interfaces can be directly integrated into BMS or environmental monitoring systems.

Conclusion

Duct air velocity transmitters are fundamental devices for achieving visualized, quantifiable, and controllable operation of ventilation systems. Through scientific measurement principles, standardized installation practices, and stable signal outputs, they provide reliable data support for energy saving, operational optimization, and safety management.

With stability, adaptability, and engineering practicality as core values, Nexisense duct air velocity transmitters offer a long-term, reliable solution for air velocity and air volume monitoring in complex industrial and building environments.