eyc-tech Air Velocity Sensing Principle
eyc-tech Air Velocity Sensing Principle
Overview
Hot-wire air velocity sensors operate based on heat transfer between the sensing element—such as a heated wire or metal film—and the surrounding fluid. When the fluid moves, it causes heat loss from the sensing element, resulting in a change in electrical resistance. The cooling effect is proportional to the fluid velocity. In the circuit, the sensor temperature is maintained at a constant level; therefore, the voltage drop caused by the fluid flow can be directly determined from the power consumed by the sensor. The figure illustrates the basic design of a Constant Temperature Anemometer (CTA).Modern CTA Anemometers Typically Consist of Three Key Elements:
- An analog-to-digital converter (ADC) and data acquisition software to sample the CTA velocity signal and perform digital signal processing.
- A CTA system that provides a range of user-configurable options.
- Calibration facilities covering the measurement velocity range. Liquid calibration systems typically operate from several cm/s up to 10 m/s, while gas calibration systems usually cover velocities ranging from several cm/s up to Mach 1.
The hot-film probe is connected to the sensor circuit, converting the gas flow signal into a voltage signal. This signal is processed through differential and non-inverting amplifier circuits, then fed into the 12-bit ADC channel of an MSP430 microcontroller. Based on the acquired signals, the microcontroller calculates the gas velocity and outputs a 4–20 mA current signal according to user requirements.
The CTA output voltage (E) and fluid velocity (U) exhibit a nonlinear relationship described by a power-law transfer function. A fourth-order polynomial is commonly used for modeling this relationship. In addition, flow-related variables such as fluid density and temperature influence heat transfer. Therefore, CTA sensors require velocity response calibration prior to use. Variations in ambient temperature can be compensated by analyzing the temperature measurement voltage.
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