Abstract
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.
Highlights
Soil moisture plays an important role in the soil system, which mainly contains soil minerals, moisture and air [1]
In this work, based on the inherent high water affinity of PMMA material, we experimentally demonstrate a sensor probe with polymer optical fiber Bragg grating (POFBG) encapsulated inside the stainless-steel package
For different soil moisture contents (SMCs) from 0% to 40%, temperature characterization of the sensor probe was implemented for one cycle between 15 ◦ C and 35 ◦ C with a step of 5 ◦ C and a duration of 4 h
Summary
Soil moisture plays an important role in the soil system, which mainly contains soil minerals, moisture and air [1]. A silica fiber Bragg grating (FBG)-based sensor including resistance wires for heating was characterized for indirect volumetric SMC measurement showing an average measurement accuracy of 1.5% This is not a pure optical fiber sensor, and it can be interfered by electromagnetic fields [13]. Though PMMA-based POFBGs have been widely characterized for both humidity and temperature measurement [23], it is more important for them to be investigated in practical applications based on their unique properties, such as large water affinity [15] and large thermal-optic coefficient [16]. In this work, based on the inherent high water affinity of PMMA material, we experimentally demonstrate a sensor probe with POFBG encapsulated inside the stainless-steel package Both SMC and temperature characterization are carried out, and the sensor shows a sensitivity of. The proposed sensor probe, featuring compact size, good reversibility and stability, is appealing for distributed and remote soil condition sensing applications
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
