Nuclear Magnetic Resonance Sensor Research Articles

A novel temperature drift compensation method based on LSTM for NMR sensor

Nuclear Magnetic Resonance (NMR) sensors have become one of the best choices for angular velocity sensors in future Inertial Navigation Systems (INS) due to their small size and high precision. The performance of NMR sensors can be affected by temperature changes, resulting in temperature drift. Therefore, temperature compensation is essential. Accurate temperature compensation is challenging due to the lack of direct temperature observations in the core sensitive area (cell) and the complex temperature drift model. Considering the time correlation of temperatures inside and outside the cell, a new temperature drift compensation method based on the Long-Short Term Memory (LSTM) network is proposed. The memory characteristics of the LSTM enable it to fully utilize current and previous data to learn complex models. The main contributions of this study are as follows: (1) the mechanism of temperature drift in NMR sensors was analyzed; (2) a multi-time scale input–output model for temperature changes, temperature change rates, and ambient temperature versus sensor output bias was established; (3) a temperature drift model identification method based on LSTM was designed. Experimental results show that, compared to traditional polynomial fitting and memory-free methods, the proposed method improves temperature compensation accuracy by 26.0% to 47.0%. This method effectively reduces the adverse impact of temperature changes on the NMR sensor.

A Novel Design of a Unilateral Nuclear Magnetic Resonance Sensor for Soil Moisture Detection Based on a Simplified Analytical Model

A Novel Design of a Unilateral Nuclear Magnetic Resonance Sensor for Soil Moisture Detection Based on a Simplified Analytical Model

The coupling effect of temperature and Rb-Xe in NMR sensor

The nuclear magnetic resonance (NMR) sensor is the atomic sensor which has the advantages of high accuracy, high sensitivity, small size, etc., and has broad application prospects in the field of inertial navigation. Temperature is one of the important parameters that determine the ultimate performance of NMR sensor including temperature fluctuations, temperature gradients, and temperature uniformity. Among them, temperature fluctuations give rise to the resonance frequency detuning, which not only shifts the carrier frequency, but also affects the alkali metal magnetic field perceived by the Xe nuclei. The accuracy and sensitivity of NMR sensor were limited as consequence of temperature fluctuations on account of its temperature sensitivity. In this paper, we have demonstrated a model of the coupling effect of temperature and Rb-Xe to analyze the influence of temperature fluctuations on the resonance frequency detuning of Xe nuclei. On the one hand, this model has analyzed the resonance frequency detuning caused by the Rb-Xe coupling effect in the NMR sensor demodulation system, whose fitting degree between theoretical analysis and experimental results is achieved 99.78%. On the other hand, it has analyzed the resonance frequency detuning caused by the magnetic field generated by the alkali metal atoms, which is associated with temperature fluctuations and experienced by Xe nuclei. The fitting degree of theoretical model of the coupling effect of temperature and Rb-Xe is achieved 99.87%. These works have provided a valuable scheme for enhancing the ultimate performance of NMR sensor and future applications.

Accurate temperature measurement of miniaturization NMR sensor in vacuum environment

—The nuclear magnetic resonance (NMR) sensor is one of the core sensors in the inertial-grade navigation system, which has the advantages of higher precision and smaller volume. The vapor cell is the sensitive unit of NMR sensor, which is easily affected by temperature. In the process of miniaturization, NMR sensor needs to be reduced in size, which leads to poor thermal insulation performance. Thus, the heat transfer increases between cell and atmosphere environment, and the temperature uniformity of cell gets worse. The average temperature of cell is far from the operating temperature we require. In order to accurately measure the temperature of cell, a vacuum chamber is designed for NMR sensor. The vacuum chamber can provide a vacuum environment with 2.2 × 10−2Pa to isolate NMR sensor from the influence of atmosphere environment temperature. A fitting curve model is established to analyze the actual temperature in atmosphere by measuring the spin polarizability of alkali metal atoms. With this model, the temperature measurement error decreased by 2.45%, and temperature control will also be more accurate. Furthermore, the temperature uniformity simulated by ANSYS has improved by 83.33% in vacuum. It's important to provide a vacuum environment for the cell in miniaturization NMR sensor. If not, the fitting curve model also guarantees the accurate of temperature measurement and control.

Ringing cancellation in Carr-Purcell-Meiboom-Gill-type sequences

We propose a four-step phase cycling scheme that can cancel ringings from both the excitation and the refocusing pulses in Carr-Purcell-Meiboom-Gill (CPMG) measurements. It allows accurate measurements of the first echo with short echo spacing tE, yielding improved precision of porosity, permeability, and fluid type identification in porous media study, at a cost of ∼25% signal reduction. In our example, we achieved tE = 100 μs on a 6.8 MHz unilateral NMR sensor without using a Q-switch. These methods can be integrated in all nuclear magnetic resonance (NMR) protocols using CPMG, such as relaxation (T1, T2) and diffusion measurements, and do not require additional hardware (e.g., Q-switches) to cancel ringing (although it can be used with a Q-switch to further reduce residual ringing).

Evaluation of a hydrogen signal detection method using a compact NMR sensor for the measurement of ion concentrations in culture medium

AbstractIn recent years, the demand for highly nutritious and functional vegetables has been increasing in the agricultural sector. In order to cultivate highly functional vegetables, it is necessary to adjust the nutrients in the soil, such as potassium ions, to an optimal state. Therefore, there is a need for a sensor that can measure soil ion concentration in real time in the field. In this report, we aim to develop an ion concentration sensor using Nuclear Magnetic Resonance (NMR). Since the sensor observes atomic specific behavior, it has the potential to measure the concentration of each ion species in soil where multiple ions exist. In this study, we aim to realize compact NMR and to establish a method for NMR measurement of water ahead of ion measurement. Using an NMR measurement system of our own design, we measured water and air. As a result, NMR signal spectra were obtained only when measuring water under resonance conditions, and resonance frequency of hydrogen atoms were successfully measured. We have established an NMR signal measurement method for measuring ion concentrations using a small NMR sensor that can be installed in the agriculture.

Optimization of Inside-Out Nuclear Magnetic Resonance Sensor With Logging-While-Drilling Tool Specification and Experimental Validation

The sensor determines the basic performance of the nuclear magnetic resonance (NMR) logging-while-drilling (LWD) system. However, the electromagnetic calculation and structure optimization of the sensor are complicated and difficult. In this study, we proposed an effective design method for inside-out NMR LWD sensor from the perspective of electromagnetic filed. First, a region of interest (ROI) was set first, the static magnetic field strength range in ROI corresponding to the ideal operating frequency bandwidth was defined as sensitive region, and the magnet system was optimized efficiently by introducing uniform design method to maximize the sensitive region. Different simplified finite element models were established to calculate different electromagnetic parameters of the RF coil system. The inside-out NMR LWD sensor was processed. Static magnetic field measurement result show that the proposed static magnetic field optimization method can effectively obtain the sensitive region with ideal radial position and strength, and is superior to the traditional optimization method. The NMR signal detection ability was tested by importing the Carr–Purcell–Meiboom–Gill (CPMG) sequence and phase-alternate pulse sequence technology. In particular, a porous and radially layered measurement device was designed, the signal sensitive region was quantitatively measured based on changes in sample porosity, and the width and height of signal sensitive region are 50 mm and 90 mm, respectively. The differences between static magnetic field sensitive region and signal sensitive region were analyzed, and suggestions for improving the sensor optimization method were provided at last.

Dedicated NMR sensor to analyze relaxation and diffusion in liquids and its application to characterize lubricants.

A dedicated nuclear magnetic resonance (NMR) sensor was designed for the analysis of liquids. The magnets are arranged in a V shape, creating a spatially dependent magnetic field in the gap. Measurements of samples with diverse diameters are possible underdefined magnetic field gradients at a given position. The magnet thus combines properties of single sided NMR with high static magnetic field gradients and classical time domain (TD) devices with lower to almost zero gradients. The sensor can easily be adapted to the requirements of the considered investigation; probes can be customized. On the example of lubricants and their aging, the added value and applicability of this sensor in quality control are highlighted in this publication. Relaxation and diffusion were measured by Carr-Purcell-Meiboom-Gill (CPMG) while varying the echo time τe and quantified via numerical modeling. Especially, relaxation shows a high sensitivity towards aging of lubricants such as particulate abrasion and changes in molecular dynamics induced, for example, by additive depletion. The applicability of this NMR sensor in quality control is demonstrated on the example of engine and transmission oils as well as of lubrication greases.

Optically pumped NMR oscillator based on 131Xe nuclear spins

A 131Xe nuclear magnetic resonance (NMR) oscillator can be used in the measurement of rotation rates, CPT and Lorentz violation tests, etc. To improve the measurement precision of devices based on a 131Xe NMR oscillator, its characteristics need to be fully understood. Under the conditions where the Zeeman interaction is much larger than the quadrupolar interaction, the characteristics of the 131Xe NMR oscillator involving the magnetic resonance, free induction decay, and closed-loop oscillation are investigated both experimentally and theoretically. The main findings are as follows. The 131Xe NMR oscillator consists of six oscillators, three of which can be directly observed by a magnetometer. When the polarization of the 131Xe spin ensemble can be described by a spin temperature, the ensemble exhibits both spin orientation and spin alignment. The spin alignment breaks the symmetry of the three main oscillators. The free induction decay signal of 131Xe depends on parameters such as the spin alignment and the driving magnetic field, which make the measurement of the relaxation time difficult. In the closed-loop mode under self-excitation, the 131Xe NMR oscillator may oscillate with more than one frequency at certain feedback gain and phase. If the quadrupole splitting is much smaller than the spin relaxation rate, then the 131Xe oscillator can be described by the Bloch equations, and the 131Xe oscillator will have a large amplitude. The oscillation frequency of the closed-loop oscillator depends on the quadrupole splitting, polarization, and various relaxation times, which should be considered in designing a high-precision NMR sensor. The results are significant for optimizing and improving the performance of the 131Xe NMR oscillator as a sensor for precision measurement.

A Mobile NMR Sensor and Relaxometric Method to Non-destructively Monitor Water and Dry Matter Content in Plants.

Water content (WC) and dry matter content (DMC) are some of the most basic parameters to describe plant growth and yield, but are exceptionally difficult to measure non-invasively. Nuclear Magnetic Resonance (NMR) relaxometry may fill this methodological gap. It allows non-invasive detection of protons in liquids and solids, and on the basis of these measures, can be used to quantify liquid and dry matter contents of seeds and plants. Unfortunately, most existing NMR relaxometers are large, unwieldy and not suitable to measure intact plants or to be used under field conditions. In addition, currently the appropriate NMR relaxometric methods are poorly suited for non-expert use. We here present a novel approach to overcome these drawbacks. We demonstrate that a basic NMR relaxometer with the capability to accept intact plants, in combination with straightforward NMR and data processing methods, can be used as an NMR plant sensor to continuously, quantitatively and non-invasively monitor changes in WC and DMC. This can be done in vivo, in situ, and with high temporal resolution. The method is validated by showing that measured liquid and solid proton densities accurately reflect WC and DMC of reference samples. The NMR plant sensor is demonstrated in an experimental context by monitoring WC of rice leaves under osmotic stress, and by measuring the dynamics of water and dry matter accumulation during seed filling in a developing wheat ear. It is further demonstrated how the method can be used to estimate leaf water potential on the basis of changes in leaf water content.

Hyperpolarized 89Y-EDTMP complex as a chemical shift-based NMR sensor for pH at the physiological range

Yttrium (III) complexes are interesting due to the similarity of their chemistry with gadolinium complexes that are used as contrast agents in nuclear magnetic resonance (NMR) spectroscopy or imaging (MRI). While most of the paramagnetic Gd3+-based MRI contrast agents are T1 or T2 relaxation-based sensors such as Gd3+-complexes for zinc or pH detection, a number of diamagnetic Y3+-complexes rely on changes in the chemical shift for potential quantitative MRI in biological milieu. 89Y, however, is a challenging nucleus to work with in conventional NMR or MRI due to its inherently low sensitivity and relatively long T1 relaxation time. This insensitivity problem in 89Y-based complexes can be circumvented with the use of dissolution dynamic nuclear polarization (DNP) which allows for several thousand-fold enhancement of the NMR or MRI signal relative to thermal equilibrium signal. Herein, we report on the feasibility of using hyperpolarized 89Y-complexes with phosphonated open-chain ligands, 89Y-EDTMP and 89Y-DTPMP, as potential chemical shift-based pH NMR sensors. Our DNP-NMR data show that hyperpolarized 89Y-DTPMP has an apparent pKa ~ 7.01 with a 4 ppm-wide chemical shift dispersion with the signal disappearing at pH below 6.2. On the other hand, pH titration data on hyperpolarized 89Y-EDTMP show that it has an apparent pKa of pH 6.7 and a 16-ppm wide chemical shift dispersion at pH 5–9 range. In comparison, the previously reported hyperpolarized pH NMR sensor 89Y-DOTP has a pKa of 7.64 and ~ 10-ppm wide chemical shift dispersion at pH 4–9 range. Overall, our data suggest that hyperpolarized 89Y-EDTMP is better than hyperpolarized 89Y-DOTP in terms of pH sensing capability at the physiological range.

The Research on Corona Aging Silicone Rubber Materials’ NMR Characteristics

Nuclear Magnetic Resonance (NMR) detection is a non-destructive testing method that can be used to detect the aging state of organic materials. A unilateral NMR sensor is used in this study to detect the aging state of the corona aging samples of silicone rubber materials after 100-hour’s corona aging under different humidity. At the same time, a comparative analysis is made between the NMR results and the conventional methods results such as Fourier Transform Infrared Spectroscopy (FTIR) and static contact angle test. The results show that with the effect of corona discharge, the polymer structure of the silicone rubber materials change to a certain extent. These changes are shown as the effective transverse relaxation time $T_{\mathrm {2eff}}$ of aged sample decreases in the NMR detection. The results of NMR and FTIR both show that the molecular crosslinking density of silicon rubber materials increases, and the activity of the H-containing group decreases. With the increase of relative ambient humidity, $\Delta T_{\mathrm {2eff}}$ gradually increase, and it means the corona aging state is becoming much severer. The results of NMR are consistent with the results of FTIR and contact angle tests. The $T_{\mathrm {2eff}}$ of the same sample tested at different time does not have significantly changes. This indicates that the NMR characteristic of silicon rubber materials after corona are irreversible. The NMR method can test the permanent damage of silicon rubber materials caused by corona. Compared with the static contact angle test, the NMR method can analyze silicon rubber materials from the molecular structure level, and it has good portability and orientation. Meantime, NMR method can achieve a non-destructive aging state detection. Hence the NMR method will provide reference for the operation and maintenance of the silicone rubber composite insulator.

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

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.

Flexible automation with compact NMR spectroscopy for continuous production of pharmaceuticals

Modular plants using intensified continuous processes represent an appealing concept for the production of pharmaceuticals. It can improve quality, safety, sustainability, and profitability compared to batch processes; besides, it enables plug-and-produce reconfiguration for fast product changes. To facilitate this flexibility by real-time quality control, we developed a solution that can be adapted quickly to new processes and is based on a compact nuclear magnetic resonance (NMR) spectrometer. The NMR sensor is a benchtop device enhanced to the requirements of automated chemical production including robust evaluation of sensor data. Beyond monitoring the product quality, online NMR data was used in a new iterative optimization approach to maximize the plant profit and served as a reliable reference for the calibration of a near-infrared (NIR) spectrometer. The overall approach was demonstrated on a commercial-scale pilot plant using a metal-organic reaction with pharmaceutical relevance.Graphical abstract

Novel Unilateral NMR Sensor for Assessing the Aging Status of Silicone Rubber Insulator

The aging of a silicone rubber insulator threatens the safety of the power grid. An accurate estimation of the insulator aging status is necessary for taking precautions against insulation failure. In this paper, we propose a novel unilateral nuclear magnetic resonance (NMR) method to quantify insulator degradation attributed to aging. First, a unilateral NMR sensor was designed specifically for transverse relaxation time (T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) measurement. The sensor static field possesses a constant gradient along the vertical direction to facilitate depth-selected measurements. Second, a step motor system was employed to control the penetration depth of the measurement area precisely. At each depth, the transverse relaxation decay curves were obtained with the sensor by using the Carr-Purcell-Meiboom-Gill sequence. Third, a data-processing method based on fast Fourier transform and band-pass filter was utilized. With this method, the transverse relaxation information of the layer with the desired thickness was extracted. Finally, 1-D T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> distributions of each layer were obtained by applying inverse Laplace transformation on the filtered transverse relaxation decay curves. The 1-D depth profiles taken from three kinds of 220-kV silicone rubber insulators with different service times were compared. The results demonstrate that the insulator with a longer service time is characterized by a shorter relaxation time. Moreover, an increase in the penetration depth into the insulators clearly results in an increase in T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2long-mean</sub> . The potential applications of the unilateral NMR method in electrical engineering include the nondestructive examination for defect detection and the aging evaluation of the insulating material.

V-shaped portable NMR sensor with a deep penetration depth and its application in assessing the aging level of turbine oils in power stations

This paper presents a simple design for a nuclear magnetic resonance (NMR) sensor and its application for detecting turbine oil in two power stations. The magnet, which is composed of two identical cuboid permanent magnet blocks and two iron shimming sheets of a special shape, generates a relatively homogeneous static magnetic B0 field (∼ 364 ppm) on a 10 × 10 mm 2 region of interest. In the vertical direction, it produces a constant gradient of ∼15.5 T/m within 5 mm which allows for self-diffusion measurements and 1 dimensional imaging. This NMR sensor can produce a deep penetration depth into the samples with relatively strong and uniform B0 strength. The NMR sensor was specially designed for assessing the aging status of turbine oil in a power station, and a group of turbine oil of different service times was measured. Results demonstrate that an increase in the service time of the turbine oils clearly decreases transverse relaxation time T2. This portable sensor can be used in fields and provide quantitative evaluation parameters of the aging status of turbine oil during periodic inspection.

Optimal Design of a Portable Arc-Shaped NMR Sensor and Its Application in the Aging-Level Detection of Silicone Rubber Insulator

Silicone rubber insulators (SRIs) are widely used in power systems because of their excellent hydrophobicity, high mechanical strength, and light weight; however, detecting SRI degradation remains problematic. Many studies have focused on SRI detection, but reports on rod-sheath detection are limited. We designed a novel nuclear magnetic resonance sensor that could be used to detect the sheath of arc-shaped SRI rods. First, the magnetic structure that generates the main magnetic field (B 0) was designed and optimized, and B 0 was designed purposely for an arc-shaped distribution that can match the sheath of SRI rod well. Second, an arc-shaped spiral radio-frequency coil was designed and optimized. Third, three SRIs serving on a 220-kV power transmission line for different time points were measured using this sensor. The transverse-relaxation decay curves were obtained using the Carr–Purcell–Meiboom–Gill (CPMG) sequence. Inverse Laplace transformation was then used to obtain the T 2 distribution from the CPMG decay curves. T 2 distribution demonstrated that the long component of T 2 decreased with increased serving time. Therefore, the T 2,long mean obtained from the T 2 distribution can be used as an index to reflect the aging level of the sheath of SRI rod.

A portable NMR sensor to measure dynamic changes in the amount of water in living stems or fruit and its potential to measure sap flow

Nuclear magnetic resonance (NMR) and NMR imaging (magnetic resonance imaging) offer the possibility to quantitatively and non-invasively measure the presence and movement of water. Unfortunately, traditional NMR hardware is expensive, poorly suited for plants, and because of its bulk and complexity, not suitable for use in the field. But does it need to be? We here explore how novel, small-scale portable NMR devices can be used as a flow sensor to directly measure xylem sap flow in a poplar tree (Populus nigra L.), or in a dendrometer-like fashion to measure dynamic changes in the absolute water content of fruit or stems. For the latter purpose we monitored the diurnal pattern of growth, expansion and shrinkage in a model fruit (bean pod, Phaseolus vulgaris L.) and in the stem of an oak tree (Quercus robur L.). We compared changes in absolute stem water content, as measured by the NMR sensor, against stem diameter variations as measured by a set of conventional point dendrometers, to test how well the sensitivities of the two methods compare and to investigate how well diurnal changes in trunk absolute water content correlate with the concomitant diurnal variations in stem diameter. Our results confirm the existence of a strong correlation between the two parameters, but also suggest that dynamic changes in oak stem water content could be larger than is apparent on the basis of the stem diameter variation alone.

Moisture profiles of the upper soil layer during evaporation monitored by NMR

AbstractNear‐surface soil moisture profiles contain important information about the evaporation process from a bare soil. In this study, we demonstrated that such profiles could be monitored noninvasively and with high spatial resolution using Nuclear Magnetic Resonance (NMR). Soil moisture profiles were measured in a column exposed to evaporation for a period of 67 days using a stationary Magnetic Resonance Imaging (MRI) high field scanner and a unilateral NMR sensor. The column was packed with medium sand and initially saturated. Two distinct shapes of soil moisture profiles that are characteristic for stage I (evaporation rate is controlled by atmospheric demand) and stage II (evaporation rate is controlled by the porous medium) of the evaporation process were followed by both MRI and unilateral NMR. During stage I, an approximately uniform decrease of soil moisture over time was monitored, whereas during stage II, S‐shaped moisture profiles developed which receded progressively into the soil column. These promising results and the specific design of the unilateral NMR system make it very well suited for determining soil moisture profiles in the field.

Unbiased Electro-Optic Waveguide as a Sensitive Nuclear Magnetic Resonance Sensor

A pigtailed Ti:LiNbO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> waveguide is here associated to a specific nuclear magnetic resonant coil to perform a low invasive magnetic field measurement. The developed device exploits a passive electro-optic transduction between the measured magnetic field and polarization state modulation of a laser probe beam. Because of the use of integrated optics, the coil electromotive force induces a dramatically enhanced electric field, thus leading to sensitivity improvement. A minimum detectable magnetic field lower than 60 fT. Hz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1/2</sup> is achieved at the resonant frequency of 128 MHz. A dynamic range exceeding 100 dB is experimentally demonstrated.