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Mini Inside-Out Nuclear Magnetic Resonance Sensor Design for Soil Moisture Measurements

1
State Key Laboratory of Power Transmission Equipment and System Security and New Technology, Chongqing University, Chongqing 400044, China
2
School of Physics and Electronic Engineering, Chongqing Normal University, Chongqing 401331, China
*
Authors to whom correspondence should be addressed.
Sensors 2019, 19(7), 1682; https://doi.org/10.3390/s19071682
Received: 6 March 2019 / Revised: 1 April 2019 / Accepted: 3 April 2019 / Published: 9 April 2019
(This article belongs to the Special Issue Advanced Sensors in Agriculture)
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Abstract

The improvement of water management in agriculture by exactly detecting moisture parameters of soil is crucial. To investigate this problem, a mini inside-out nuclear magnetic resonance sensor (NMR) was proposed to measure moisture parameters of model soils. This sensor combines three cylindrical magnets that are magnetized in the axial direction and three arc spiral coils of the same size in series. We calculated and optimized the magnet structure by equivalent magnetization to current density. By adjusting the radius and height between the cylinders, a circumferential symmetric constant gradient field (2.28 T/m) was obtained. The NMR sensor was set at 2.424 MHz to measure the water content of sandy soil with small particle diameter and silica sand with large particle diameter. The complete decaying, an NMR signal was analyzed through inverse Laplace transformation and averaged on a T2 space. According to the results, moisture content of the sample is positively correlated with the integral area of T2 spectrum peak (Apeak); T2 of the water in small pores is shorter than that in large pores, because the movement of water molecules are limited by the inner wall of the pores. In the same volume, water in large pore sample is more than that in small pore sample, so Apeak of silica sand is larger than Apeak of sandy soil. Therefore, the sensor is capable of detecting moisture both content and pore size of the sample. This mini sensor (4.0 cm in diameter and 10 cm in length) is portable, and the lowest measurable humidity is 0.38%. Thus, this sensor will allow easy soil moisture measurements on-field in the future. View Full-Text
Keywords: agriculture; inside-out nuclear magnetic resonance; constant gradient field; soil moisture agriculture; inside-out nuclear magnetic resonance; constant gradient field; soil moisture
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This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited (CC BY 4.0).
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Wu, J.; Guo, P.; Shen, S.; He, Y.; Huang, X.; Xu, Z. Mini Inside-Out Nuclear Magnetic Resonance Sensor Design for Soil Moisture Measurements. Sensors 2019, 19, 1682.

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