Air Velocity Sensor Principle | Hot-wire Air Velocity Sensor Technology

eyc-tech Hot-wire Air Velocity Sensing Technology Overview

Hot-wire Air Velocity Sensor (CTA) Measurement Principle

A hot-wire air velocity sensor measures airflow velocity based on the heat transfer between the sensing element and the surrounding fluid. Typical sensing elements include a heated wire or a thin metal film.

When fluid flows across the sensing element, part of the heat is carried away, causing the temperature of the sensing element to decrease and its electrical resistance to change.

By measuring the electrical power required to maintain the sensing element temperature, the airflow velocity can be calculated. This measurement method is commonly used in a Constant Temperature Anemometer (CTA).

CTA Anemometer System Architecture

Modern CTA anemometers typically consist of the following three main components:

• Analog-to-digital converter (ADC) and data acquisition system
• CTA control unit with user configuration options
• Air velocity calibration system and calibration facilities

The data acquisition system receives the airflow signal from the CTA sensor and performs velocity calculation and signal analysis through digital signal processing.

Air Velocity Measurement and Calibration System

CTA anemometers usually require calibration systems to ensure measurement accuracy.

Liquid flow calibration systems typically cover velocity ranges from several cm/s to approximately 10 m/s, while gas calibration systems may range from several cm/s up to nearly 1 Mach.

Signal Processing and Output

After the hot-film probe is connected to the sensor circuit, the airflow signal is converted into a voltage signal. The signal is processed through differential and non-inverting amplifier circuits and then transmitted to the 12-bit ADC channel of the MSP430 microcontroller.

The microcontroller calculates the airflow velocity based on the measured signal and can output a 4–20 mA analog signal according to user requirements.

Nonlinear Characteristics of CTA Anemometers

The output voltage (E) of a CTA anemometer and the fluid velocity (U) generally exhibit a nonlinear relationship. This relationship is commonly modeled using power-law equations such as King’s Law or polynomial calibration models.

In addition, factors such as fluid density and temperature can affect heat transfer. Therefore, CTA airflow sensors typically require velocity response calibration and temperature compensation before use.

FAQ

How does a hot-wire air velocity sensor work?

A hot-wire air velocity sensor measures airflow speed by detecting heat transfer between the sensing element and the airflow. When airflow velocity increases, heat loss from the sensor increases, resulting in changes in resistance and power consumption.

What is a CTA anemometer?

A CTA (Constant Temperature Anemometer) is a type of airflow sensor that maintains the sensing element at a constant temperature. The airflow velocity is calculated by measuring the electrical power required to maintain this constant temperature.

Why do air velocity sensors require calibration?

Factors such as fluid density, temperature, and environmental conditions can influence airflow measurement results. Therefore, air velocity sensors typically require calibration using wind tunnel systems to ensure measurement accuracy.

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