Development of an Automatic Linear Calibration System for Hot-Wire Anemometers

Ming-Chung Tsai¹, Jui-Ching Chiu¹, Yu-Ping Kuan¹, Chia-Te Chou¹, Jen-Cheng Chu², Hsin-Yeh Chu²

¹ Graduate Institute of Automation and Control, National Taiwan University of Science and Technology

² eyc-tech

Abstract

The purpose of this study is to develop an automatic linear calibration system for hot-wire anemometers. The system consists of a PC host with a human–machine interface (HMI), a high-speed wind tunnel test chamber with an airflow control module, communication interfaces, a data acquisition module, a linear calibration model, and a linear compensation module implemented in a microcontroller.

To achieve automatic linear calibration and quality control (QC) inspection for hot-wire anemometers, the completed system provides three different operating modes, all integrated into a single HMI for user selection. This allows operators to select the desired operating mode according to specific requirements. The three operating modes include a calibration mode, a quality control mode, and a calibration-then-quality-control mode.

All three modes integrate the sensor’s microcontroller (MSP430) to process voltage signals for calibration and quality inspection. Taking the calibration-then-quality-control mode as an example, a temperature-compensated anemometer is first installed in a wind tunnel test chamber with controllable airflow. The system then automatically performs anemometer calibration by acquiring raw voltage signals at different flow ranges through the data acquisition module. These signals are transmitted to the PC-based calibration system, which automatically analyzes the data and calculates polynomial coefficients. The coefficients are then loaded into the microcontroller via the communication module to complete linearization calibration. 

After linear calibration is completed, the automatic quality inspection function can be applied to acquire QC voltage signals to verify the improvement in linearity, thereby shortening production time. This system can also be used for automated quality inspection and product grading of general anemometers.

Keywords 

Hot-wire anemometer, wind tunnel, quality control inspection, data acquisition, linearization calibration, temperature compensation.

1. Introduction

In agricultural, industrial, and civil production equipment and systems, flow rate or air velocity is a critical sensing parameter. Hot-wire anemometers are widely used across various industries. For example:

(1) In the automotive industry, they are used to measure the intake air volume entering an engine to provide accurate fuel injection control, achieving an optimal air–fuel ratio, improving engine performance, reducing environmental pollution, and saving fuel.

(2) In power plants, they are applied to control the ratio of combustion air supply to coal input in boilers.

(3) In HVAC systems, they are used for optimal control of indoor air quality and ventilation energy efficiency.

(4) In civil and industrial fields, they are applied to flow measurement of combustible gases such as natural gas, liquefied petroleum gas, and coal gas.

(5) In medical applications, respiratory function testing instruments measure parameters such as mid-expiratory flow rate, peak expiratory flow rate, and lung volume to provide medical data for diagnosis and treatment of respiratory diseases [1].

The basic principle of a hot-wire mass flow meter is to heat a thermocouple inside a pipe using an external heat source. Heat is carried away by the fluid flow, and changes in heat (temperature) caused by the flow are measured to reflect the mass flow rate of the fluid. Hot-wire flow meters can also be used to measure gas velocity, commonly referred to as anemometers [2].

Due to slight variations in the size of the thermistors used in hot-wire anemometers, their nonlinear characteristic curves differ slightly from unit to unit. Manual calibration of anemometers is time-consuming, costly, and inconsistent in accuracy and precision. Therefore, replacing manual recording with an automated sensing system can significantly improve measurement accuracy and precision, enhance production efficiency, stabilize product quality, and reduce costs.

2. Principles of Hot-Wire Anemometers and Calibration Models

2.1 Hot-Wire Anemometer

The basic operating principle of a hot-wire anemometer is to place a thin metallic wire in a fluid flow and heat it by passing an electric current through the wire (the hot wire), raising its temperature above that of the surrounding fluid. When the fluid flows past the wire in a perpendicular direction, part of the heat is carried away, causing the wire temperature to decrease. The amount of heat dissipation from the hot wire is related to the flow velocity. This heat loss results in a change in wire resistance, converting the flow velocity signal into an electrical signal. A typical operating principle of a hot-wire anemometer is shown in Figure 3 [3].

The hot-wire anemometer used in this study incorporates a Wheatstone bridge and a power control circuit to maintain the hot wire at a constant temperature. The heat transfer between the hot wire and the surrounding fluid forms a function of flow velocity; however, variations in temperature and pressure can affect this relationship. The temperature compensation circuit automatically increases the current supplied to the hot wire to restore its original temperature and resistance until the Wheatstone bridge is rebalanced.

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