Mildew is an important issue in grain storage, due to the very expensive losses that can arise due to grain spoilage by mildew. Monitoring the conditions which allow mildew to develop is important and current monitoring technology cannot readily, or safely, be used for internal humidity monitoring of the very large quantities of grain kept in storage. In this work, a quasi-distributed, tailored fiber optic temperature and humidity sensor system suitable for use in the large granaries has been developed. Dust from the grain is a major issue and the sensor system copes well with this. Various sensor designs have been evaluated and building on prior work, a design with a humidity sensitivity of 5.8pm/%RH, a measurement error of 2%RH, and response time of 4.3 minutes, which is more than adequate for the applications discussed has been developed. Field-testing of the sensor system was carried out at a major storage facility in China, monitoring over an extended period of 4 months, giving results consistent with the outputs from other point sensors currently used in the granary. The fiber optic sensor system developed shows the additional advantages that as a quasi-distributed system, it can be used to record the temperature and humidity distribution across a longitudinal section of the large size of the grain pile, in real time, reflecting the changes in the internal moisture content. The results obtained showed the sensor system has broad market application prospects in this and other important areas of agricultural and food storage.

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 has 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 ~1500pm per unit m/s wind velocity (in the range of 0 - 0.5 m/s), ~330pm per unit m/s in the range 0.5-2 m/s and ~50pm per unit m/s in the range of 2.0-4.5 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. Further, the response time was seen to decrease as the wind velocity increased, showing a typical value of 6s, which is suitable for the mining applications envisaged.

In this paper, we present a dual-component detection photoacoustic spectroscopy (PAS) sensor based on a YY-type photoacoustic cell (PAC), which is designed for the simultaneous detection of two trace gases. As the core component of the detection system, the YY-type PAC is designed by utilizing 3D printing technology. It incorporates two cylindrical resonators of different sizes and a truncated cone-shaped buffer chamber. The two resonators are arranged in parallel and connected to the microphone via a dedicated sound channel. Each resonator has distinct acoustic and optical properties, enabling it to generate photoacoustic (PA) signals independently. A single microphone is positioned between the two resonators in the YY-type PAC, allowing for the simultaneous measurement of PA signals from both resonators. Additionally, we employ the thermos viscous acoustics (TVA) module of COMSOL Multiphysics software to simulate the designed PAC, optimizing its parameters for improved performance. The high-performance YY-type PAC is designed with resonant frequencies of 3.9 kHz and 5 kHz. In our dual-component PAS sensor, two distributed feedback (DFB) lasers with wavelengths of 1653 nm and 1531.12 nm are employed alongside a lock-in amplifier and a YY-type PAC to enable the high-sensitivity detection of methane (CH ₄ ) and acetylene (C ₂ H ₂ ) gases. This setup achieves minimum detection limits (MDL) of 0.52 ppm for CH ₄ and 1.57 ppm for C ₂ H ₂ , respectively. The YY-type PAC introduced in this study integrates the benefits of simultaneous dual-component detection, ease of integration, low gas consumption, and miniaturization.

Combining the benefits of photothermal conversion in a doped optical fiber with the principle of Fiber Bragg Grating (FBG)-based temperature measurement, a new optical fiber method for the measurement of moisture content has been developed and its performance is reported. This novel all-fiber approach shows the important characteristics of fast response, miniaturization in the design and low power consumption. In this work, a 1480nm pump laser has been used to heat the cobalt-doped fiber used, which has been embedded in the sample under investigation, where the sensor temperature change was obtained by using a FBG written into the cobalt-doped fiber. The fast, in-situ measurement of moisture content developed takes advantage of the linear relationship between the temperature change characteristics experienced during the heating process and the moisture content. To calibrate and characterize the sensor developed, a variable moisture-content test platform was established, and experiments were conducted to investigate the performance of this optical fiber approach. The all-fiber moisture sensor developed in this way has been evaluated by measuring the moisture content of samples of soil and pulverized coal and the data obtained were in good agreement with the results obtained from using the more conventional (and slow) drying method, with a maximum error of just over 1% and a rapid, 2s, optimum measurement time achieved.

This paper introduces a light-induced thermoe-lastic spectroscopy (LITES) system based on a vacu-um-packaged miniature quartz tuning fork (VP-MQTF) for the simultaneous detection of acetylene (C₂H₂) and methane (CH₄). The VP-MQTF stands out due to its compact size and high Q-factor. Its small dimensions enable an enhanced thermoelastic response, leading to stronger signals in the LITES system. Additionally, the vacuum-packaged design of the miniature quartz tuning fork not only achieves a higher Q-factor but also mitigates the issue of frequency drift commonly observed in traditional quartz tuning forks (QTFs) after prolonged exposure to air. By employing the VP-MQTF as a detector, the LITES system successfully demonstrated the detection of C₂H₂ and CH₄. Experimental results re-vealed minimum detection limits (MDLs) of 672 ppb for C₂H₂ and 947 ppb for CH₄. Furthermore, the system exhibited excellent linearity in detecting both gases, with correlation coefficients (R²) of 0.99854 for C₂H₂ and 0.99782 for CH₄.