Optical humidity sensors have evolved through decades of research and development, constantly adapting to new demands and challenges. The continuous growth is supported by the emergence of a variety of optical fibers and functional materials, in addition to the adaptation of different sensing mechanisms and optical techniques. This review attempts to cover the majority of optical humidity sensors reported to date, highlight trends in design and performance, and discuss the challenges of different applications.

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.

Accurate estimate the time difference of the leakage negative pressure wave signals reached both sides of the sensor at the two sides of the leakage point is the key to accurate location of the leakage point. Based on the theoretical analysis of cross-correlation time delay estimation algorithm and characteristics of negative pressure wave signal, a cross-correlation delay estimation method for pipeline leakage negative pressure wave signal is proposed. Through wavelet transform, signal conversion and cross correlation function analysis of discrete negative pressure wave signal sequence obtained by high-speed sampling, the time difference of the negative pressure wave signal monitored by the sensors on both sides of the leakage point can be accurately obtained. In the laboratory’s 102.8 meters pipeline, leakage was simulated through the valve, the time delay estimation and leak location experiment of the sensor at the two ends of the pipeline were analyzed, and compared with the wavelet analysis method. The experimental results show under the condition that the leakage is 10%, 5% and 2% of the total pipeline volume, the cross-correlation analysis algorithm has a positioning error interval of 0.97%-1.46%, which is better than the current positioning error 0.48%-2.43% of the wavelet analysis algorithm. At the same time, it avoids the influence of different wavelet types and analysis scales in the wavelet analysis algorithm on positioning accuracy, and is more suitable for pipeline leakage monitoring system to real-time online monitoring needs, has a wide application value.

A high-sensitivity optical fiber hydrogen sensor based on Pt/WO₃ nanoparticles has been fabricated and also characterized. To create the sensor, a metal film was first deposited on the surface of a Fiber Bragg Grating (FBG) by using an electroplating method and this film was then covered with a layer of Glass Fiber (GF). Subsequently, a Pt/WO₃ nano-hydrogen sensitive material was coated on the surface of metal film and the GF. Scanning electron microscopy (SEM) characterization of the device used to inspect the surface revealed that the nanoscale Pt/WO₃ material forms an irregularly stacked porous morphology. This loose and porous structure provides abundant active sites for hydrogen absorption, thereby facilitating rapid response of the sensor to the hydrogen gas present. The sensing performance was evaluated using a custom-built experimental system. Results obtained demonstrate that the sensor developed exhibited a sensitivity of 0.225 pm/ppm within the hydrogen concentration range of 0–9960 ppm, with a lowest detection limit of 13.2 ppm seen. Excellent linearity and a strong capability to deal with extraneous effects were also observed. In tests carried out, the sensors were shown to be capable of dealing well with external temperature changes and vibration tests conducted (on an SA-JZ010 vibration table) under conditions of vibration/acceleration at 50 m/s² at 10 Hz and 150 Hz confirmed its outstanding stability.