Development of an Automatic Linear Calibration System for Hot-wire Air Velocity Sensors

Abstract: The objective of this study is to develop an automatic linear calibration system for hot-wire air velocity sensors. The system consists of a PC host with human–machine interface (HMI), a high-speed wind tunnel test chamber and airflow control module, a communication interface, a data acquisition module, a linear calibration model, and a microcontroller-based linear compensation module. To realize automatic linear calibration and quality control testing of hot-wire air velocity sensors, the completed system integrates three functional modes into a single user interface. This allows operators to select the desired operation mode according to their requirements. The three functional modes include calibration mode, quality control mode, and a combined calibration-and-QC mode. All three modes utilize the microcontroller chip (MSP430) embedded in the sensor to process voltage signals for calibration and quality control operations. Taking the calibration-and-QC mode as an example, the temperature-compensated air velocity sensor is first installed inside a controllable wind tunnel test chamber. The system then automatically performs the anemometer calibration procedure. The data acquisition module measures the raw voltage signals of airflow velocity across different measurement ranges, and transfers them to the PC-based calibration system for automatic detection and calculation of polynomial coefficients. These coefficients are then downloaded to the microcontroller through the communication module, thereby completing the linearization calibration. After the linear calibration process is completed, the automatic quality control testing function can be used to capture QC voltage signals from the anemometer to verify the improvement in linearization performance. This approach significantly reduces production time. The system can also be used for automated QC inspection and product classification for general air velocity sensors.


Keywords: hot-wire air velocity sensing, wind tunnel, quality control inspection, data acquisition, linear calibration, temperature compensation.


1. Introduction
In agricultural, industrial, and general production systems, flow rate or air velocity is an important sensing parameter. Hot-wire anemometers are widely used in many industries. For example: (1) In the automotive industry, they are used to measure the intake air volume entering an engine, providing accurate fuel injection control. This enables more precise air–fuel ratios, improves engine efficiency, reduces environmental pollution, and saves fuel. (2) In power plants, they are used for air supply control in boilers. (3) In HVAC systems, they are used to control indoor air quality and optimize ventilation energy efficiency. (4) In industrial and commercial applications, they are used to measure the flow of combustible gases such as natural gas, liquefied petroleum gas, and coal gas. (5) In medical applications, respiratory diagnostic instruments measure parameters such as mean and peak expiratory flow and respiratory volume to provide clinical data for diagnosis and treatment of respiratory diseases [1].

The basic principle of a hot-wire mass flow meter is based on heating a thermocouple using an external heat source. The heat is carried away by the flowing fluid, and the mass flow rate is determined by measuring the temperature change caused by the fluid flow. Hot-wire flow meters can also be used to measure gas velocity, commonly referred to as hot-wire anemometers [2].

Due to slight variations in the thermistors used in hot-wire anemometers, their nonlinear characteristic curves may differ slightly. Manual calibration of air velocity sensors is time-consuming and costly, and often results in inconsistent accuracy and precision. Therefore, replacing manual recording with an automated sensing system can improve measurement accuracy and precision, increase production efficiency, stabilize product quality, and reduce operational costs.

2. Principle and Calibration Model of Hot-wire Anemometers

2.1 Hot-wire Anemometer

The basic principle of a hot-wire anemometer is to place a thin metal wire in a fluid flow. The wire is heated by electrical current (the hot wire), raising its temperature above that of the surrounding fluid. When fluid flows across the wire, a portion of the heat is carried away, causing the wire temperature to decrease. The rate of heat dissipation in the airflow is related to the flow velocity. The temperature change of the hot wire leads to a change in electrical resistance, which converts the airflow velocity signal into an electrical signal. A typical working principle of a hot-wire anemometer is shown in Fig. 3 [3].

The hot-wire anemometer used in this study includes a Wheatstone bridge circuit and a power controller to maintain constant temperature operation. The heat transfer interaction between the hot wire and the surrounding fluid forms a function of flow velocity, while variations in temperature and pressure can influence the measurement. The temperature compensation circuit automatically increases the current supplied to the hot wire to restore its original temperature and resistance value until the Wheatstone bridge returns to equilibrium.

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