High-Sensitivity “Hot-Wire”-Based Gas Velocity Sensor for Safe Monitoring in Mining Applications

In this paper, building on a design developed from an advanced mathematical model, a practical fiber optic sensor, which is an analog of the familiar “hot-wire” wind velocity monitor, has been developed, as an intrinsically safe sensor device for use on coal mining monitoring applications. The device response time, the dynamic measurement range, the sensitivity, the sensor probe surface heat transfer coefficient, and the effect of the wind (gas) velocity have been investigated in the laboratory and in situ and results reported, enabling the mathematical model used in the design process to be validated. The underpinning optical fiber-based principle used is the shift in the center wavelength of a fiber Bragg grating which is cooled by the gas flowing over it, and the device sensitivity found was determined to be ~1500 pm per unit m/s wind velocity in the range of 0-0.5 m/s, ~330 pm per unit m/s in the range 0.5-2 m/s, and ~50 pm per unit m/s in the range of 2-4.5 m/s. In the “proof of concept” tests carried out, it was found that the greater the surface dissipation factor of the sensor, the shorter was its response time. Furthermore, the response time was seen to decrease as the wind velocity increased, showing a typical value of 6 s, which is suitable for the mining applications envisaged.