College of Science, Shenyang Aerospace University, Shenyang 110136, China.
Research Center for Advanced Optics and Photoelectronics, Department of Physics, College of Science, Shantou University, Shantou 515063, Guangdong, China.
Sensors (Basel). 2021 Oct 20;21(21):6946. doi: 10.3390/s21216946.
Soil moisture measurement is very important for soil system monitoring. Compared to the traditional thermo-gravimetric technique, which is time-consuming and can be only performed in labs, the optic-fiber technique has unique advantages, such as small size, remote application in fields, fast response time and immunity to electromagnetic fields. In this paper, the soil moisture is measured by using a polymer optical fiber Bragg grating (POFBG) probe with a packaged dimension of 40 mm × 15 mm × 8 mm. Due to the intrinsic water-absorbing property of poly (methyl methacrylate) (PMMA), optical fiber Bragg gratings based on PMMA have been widely investigated for humidity measurement. Taking advantage of this, a sensor based on the POFBG is investigated to verify the soil condition. The POFBG is protectively integrated inside a stainless-steel package. A window is opened with a thin polypropylene mat as a filter, which allows the air to go through but prevents the soil from going inside to pollute the POFBG. The sensor probe is embedded in soils with different gravimetric soil moisture contents (SMCs) ranging from 0% to 40% and, then, insulated by polyethylene films to minimize the impact from the external environment, showing an average temperature cross sensitivity of -0.080 nm/°C. For a constant temperature, an exponential relationship between the Bragg wavelength and the SMC is obtained. For the SMCs between 8% and 24%, linear relationships are presented showing a temperature-corresponded sensitivity between 0.011 nm/% and 0.018 nm/%. The maximal sensitivity is calculated to be 0.018 nm/% at 20 °C, which is 28 times as high as that in the previous work. For the SMC over 24%, the sensor becomes insensitive because of humidity saturation in the cavity of the sensor probe. Though temperature cross sensitivity is problematic for SMC measurement, the influence could be eliminated by integrating another humidity-insensitive temperature sensor, such as a silica FBG temperature sensor.
土壤湿度测量对于土壤系统监测非常重要。与传统的耗时且只能在实验室进行的热重量技术相比，光纤技术具有独特的优势，例如尺寸小、可在现场远程应用、响应时间快且抗电磁场干扰。本文使用尺寸为 40mm×15mm×8mm 的聚合物光纤布拉格光栅（POFBG）探头测量土壤湿度。由于聚甲基丙烯酸甲酯（PMMA）的固有吸水性，基于 PMMA 的光纤布拉格光栅已被广泛研究用于湿度测量。利用这一点，研究了一种基于 POFBG 的传感器来验证土壤状况。POFBG 被保护性地集成在不锈钢封装内。一个窗口开有一个薄的聚丙烯垫作为过滤器，允许空气通过但防止土壤进入污染 POFBG。传感器探头被嵌入到不同重量土壤湿度含量（SMCs）为 0%到 40%的土壤中，然后用聚乙烯薄膜绝缘以最小化外部环境的影响，表现出平均温度交叉灵敏度为-0.080nm/°C。对于恒定温度，获得了布拉格波长与 SMC 之间的指数关系。对于 SMC 在 8%到 24%之间的情况，呈现出线性关系，表现出 0.011nm/%到 0.018nm/%的温度对应灵敏度。在 20°C 时，最大灵敏度计算为 0.018nm/%，是之前工作的 28 倍。对于 SMC 超过 24%的情况，由于传感器探头腔体内的湿度饱和，传感器变得不灵敏。尽管温度交叉灵敏度对 SMC 测量有问题，但可以通过集成另一个对湿度不敏感的温度传感器，如硅光纤布拉格光栅温度传感器，来消除其影响。
