A compact magnetic resonance system with inside-out sensor for soil moisture measurement

Accurate measurement of soil moisture parameters plays a vital role in promoting agricultural production. Nuclear magnetic resonance (NMR) measuring instruments have become increasingly compact, especially in low field applications. However, NMR soil moisture measurement instruments still have problems with portability. We report on a compact NMR measuring instrument that focuses on the circuit's design and construction. The instrument includes an inside-out sensor, NMR spectrometer, radio frequency power amplifier, low-noise preamplifier, and duplexer. They are integrated into a slim 3D-printed cylinder. The integrated NMR instrument is portable and user-friendly and can reduce the influence of nonideal factors, such as transmission line effects and electromagnetic interference. We conducted different soil moisture experiments with the prototype using Carr-Purcell-Meiboom-Gill pulse sequence to obtain the corresponding transverse relaxation time (T (2)) decay curves. Then, the T (2) 1D spectrum distributions were obtained by inverse Laplace transformation. Finally, the results reveal that the area of long T (2) component decreases as the soil moisture content decreases.