A kit for measuring the permittivity spectrum of bulk and liquid materials and a method for calibrating the multi-rod probe included in this kit

Abstract

The object of the invention is to provide a kit for measuring the permittivity spectrum of bulk and liquid materials, characterized in that it comprises a multi-rod reference probe (1A) consisting of a signal rod (2A) located in the central part of the probe (1A), and ground rods (3A) arranged at equal angular intervals around the signal rod (2A), and a multi-rod target probe (IB) consisting of a signal rod (2B) located in the central part of the probe (IB), and ground rods (3B) arranged at equal angular intervals around the signal rod (2B), and a primary calibration system and a secondary calibration system and a control unit (6), wherein the probes (1A, IB) are connected by a microwave connector (5) to a vector circuit analyzer (4) that is connected to the control unit (6). The object of the invention is also to provide a method for calibrating the multi-rod probe included in this kit.

Description

[0001] A kit for measuring the permittivity spectrum of bulk and liquid materials and a method for calibrating the multi-rod probe included in this kit

[0002] The invention relates to a kit for measuring the permittivity spectrum of bulk and liquid materials and a method for calibrating such a multi-rod probe included in this set.

[0003] Measurement of the permittivity spectrum is the basis for non-invasive methods of determining physical parameters of materials. Such methods are widely used in industry and agriculture in order to quickly assess as well as continuously monitor physical parameters of materials, such as volume humidity or contamination content. One of the methods for measuring such a spectrum consists in measuring the complex reflectance r of a multi-rod probe immersed in the tested material, and then determining the permittivity spectrum by adopting either a simple capacitive model [J. Szerement et al., "A Seven-Rod Dielectric Sensor for Determination of Soil Moisture in Well-Defined Sample Volumes," Sensors, 19, 7, 1646, 2019], or a model using a transmission line [E. Piuzzi, A. Cataldo, G. Cannazza, E. De Benedetto, "An Improved Reflectometric Method for Soil Moisture Measurement Exploiting an Innovative Triple-Short Calibration," in IEEE Transactions on Instrumentation and Measurement, 59, 10, 2747-2754, 2010],

[0004] In the prior art, methods for calibrating a multi-rod probe connected with a vector circuit analyzer are known, which are typically performed using several calibration liquids of known parameters, e.g. water, methanol, isopropanol, or ethanol [W. Skierucha, A. Wilczek, "A FDR Sensor for Measuring Complex Soil Dielectric Permittivity in the 10-500 MHz Frequency Range," Sensors, 10, 4, 3314-3329, 2010; A. Wilczek, A. Szyptowska, W. Skierucha, J. Ciesla, V. Pichler, G. Janik, "Determination of Soil Pore Water Salinity Using an FDR Sensor Working at Various Frequencies up to 500 MHz," Sensors, 12, 8, 10890-10905, 2012; J. Szerement et al., "A Seven-Rod Dielectric Sensor for Determination of Soil Moisture in Well-Defined Sample Volumes," Sensors, 19, 7, 1646, 2019] or by means of a metal element moved along the probe that short circuits its rods [E. Piuzzi, A. Cataldo, G. Cannazza, E. De Benedetto, "An Improved Reflectometric Method for Soil Moisture Measurement Exploiting an Innovative Triple-Short Calibration," in IEEE Transactions on Instrumentation and Measurement, 59, 10, 2747-2754, 2010], The disadvantage of the first method is its time-consuming nature, high dependence of liquid parameters on temperature and impurities, as well as the toxicity of some of the organic liquids used. The disadvantage of the second method is its mechanical complexity, especially when there are more than 3 rods, and its difficulty of ensuring the repeatability of the complex reflectance r for the short-circuit moved along the probe due to the partial wave penetration through this short-circuit caused by diffraction.

[0005] The aim of the invention was to develop a kit for measuring the permittivity spectrum of bulk and liquid materials and a fast and accurate method to calibrate a multi-rod probe connected to a vector circuit analyzer and included in this kit, and in particular the use of calibrated broadband measurements of the permittivity spectrum in the soil moisture and salinity assessment and in other industries for non-invasive evaluation of material parameters.

[0006] The object of this invention is to provide a kit for measuring the permittivity spectrum of bulk and liquid materials, characterized in that it contains a multi-rod reference probe consisting of a signal rod located in the central part of the probe, and ground rods arranged at equal angular intervals around the signal rod, and a multi-rod target probe consisting of a signal rod located in the central part of the probe, and ground rods arranged at equal angular intervals around the signal rod, and a primary calibration system and a secondary calibration system and a control unit, wherein the probes are connected by a microwave connector to a vector circuit analyzer which is connected to the control unit.

[0007] Preferably, the length of the probe rods is in the range from 7 to 30 cm, wherein the length of the reference probe rods is longer than the length of the target probe rods.

[0008] Preferably, the primary calibration system is a measuring cylinder placed on a laboratory scale, filled with a high conductivity liquid, which is connected to the flow pump and the laboratory scale by means of the control unit.

[0009] Preferably, the high conductivity liquid is a concentrated aqueous sodium chloride solution having a conductivity greater than 15 S / m. Preferably, the secondary calibration system is an electronic calibrator.

[0010] The object of the invention is also to provide a method for calibrating a multi-rod probe included in the kit for measuring the permittivity spectrum of bulk and liquid materials according to the invention, comprising the following steps: i) primary calibration and determination of the electronic calibrator

[0011] • placement of a reference probe in the measuring cylinder and introduction of a liquid into the measuring cylinder by means of a flow pump, preferably a concentrated aqueous sodium chloride solution having a conductivity of more than 15 S / m, in equal and known increments, the amount of which is calculated according to the liquid weight gain as read on the laboratory scale, followed by

[0012] • measurement of the raw reflectance Tmof the reference probe for each fixed height of the liquid column, followed by

[0013] • removal of the reference probe from the cylinder, followed by

[0014] • measurement of the raw reflectance Tmof the reference probe located outside the cylinder, followed by

[0015] • determination of the calibration coefficients of the reference probe system connected to the vector circuit analyzer, followed by

[0016] • placement of the electronic calibrator at the end of the reference probe, followed by

[0017] • measurement of the corrected reflectance T for all impedances carried out by the calibrator ii) secondary calibration

[0018] • placement of the electronic calibrator at the end of the target probe, followed by

[0019] • measurement of the raw reflectance rmfor all impedances carried out by the calibrator, followed by

[0020] • determination of calibration coefficients for the target probe system connected to the vector circuit analyzer.

[0021] The advantage of the kit and method according to the invention is the speed and repeatability of the secondary calibration process and higher calibration accuracy compared to the methods known in the state of the art using many organic liquids due to the dependence of the definition of calibration standards only on geometrical parameters (i.e. the height of the liquid column in the primary calibration and the transverse dimensions of the probe determining its reflectance r in the open state). In addition, the advantage of this method is its easy implementation for probes with more than 3 rods.

[0022] The object of the present invention is illustrated by means of figures in which:

[0023] Fig. 1 shows a kit according to the invention for A) performing a primary calibration and B) characterising a calibrator;

[0024] Fig. 2 shows a kit for performing a secondary calibration;

[0025] Fig. 3 shows an exemplary spectrum according to the invention of the real part s' and the imaginary part E" of the permittivity for dry sand and isopropanol.

[0026] The object of the invention is illustrated in the embodiment and in the drawings, showing the kit for primary calibration and characterization of the electronic calibrator, and the kit for performing the secondary calibration.

[0027] The kit for measuring the permittivity spectrum of bulk and liquid materials according to the invention comprises a multi-rod probe 1A consisting of a steel signal rod 2A having a diameter of 3 mm and located in the central part of the probe 1A, and a plurality of steel ground rods 3A, preferably 10, also having a diameter of 3 mm, arranged at equal angular intervals around the signal rod 2A on a circle having a diameter of 30 mm, connected to the vector circuit analyzer 4 via a microwave connector 5, preferably Type-N, and a multi-rod target probe IB consisting of a steel signal rod 2B located in the central part of the probe IB, and ground rods 3B arranged at equal angular intervals around the signal rod 2B, and comprising a system for primary calibration and characterization of an electronic calibrator and a system for secondary calibration and a control unit 6, wherein the vector circuit analyzer 4 is connected to the control unit 6.

[0028] The primary calibration system is a glass measuring cylinder 7 (e.g., having an outer diameter of 190 mm, a height of 280 mm, and a wall thickness of 3 mm) placed on a laboratory scale 9 (e.g., RADWAG APP 30 / c / 2), filled with a liquid 8 having a conductivity of more than 15 S / m, preferably a concentrated aqueous sodium chloride solution, which is connected to the flow pump 10 and the laboratory scale 9 by a control unit 6, preferably a computer. The primary calibration is performed in the system shown in Fig. 1A. In this system, a probe 1A with long rods 2A, 3A (e.g. 150 mm) is used. This calibration uses a movable short circuit implemented in form of a series of liquid columns having high electrical conductivity, which was achieved by means of a concentrated aqueous solution of common salt. A 150 mm probe 1A immersed in such a liquid, as long as the length of the immersed fragment is greater than about 20 mm, is effectively terminated by an impedance having a constant reflectance corresponding to an electrical short circuit. The calibration therefore consists in measuring with a vector circuit analyzer 4 (e.g. Copper Mountain R61) the raw complex reflectance of the probe rmin the frequency range of 100 MHz - 2 GHz for a dozen different heights of the liquid column, e.g. having an increment of 10 mm. The longer the rods of this probe, the smaller the lower limit frequency of the calibration obtained. For the given values, this frequency is approx. 100 MHz.

[0029] The liquid 8 is introduced from the tank 11 by means of a flow pump 10 (e.g. an aquarium pump) and into the measuring cylinder 7 located on the laboratory scale 9. The height of the liquid column 8 is estimated based on the weight gain. Increasing the liquid column 8 has been automated by controlling the pump 10 and the laboratory scale 9 with a computer 6. Then the calibration procedures correct any inaccuracies in the estimation of the liquid column 8 height by simultaneously estimating the liquid density. The calibration is complemented by the measurement of the raw complex reflectance Tmof the 1A probe located outside the measuring cylinder 7 (the so-called open circuit). The reflectance T of the opening is determined based on electromagnetic simulations, e.g. in the HFSS or CST program. This measurement is necessary because we do not know the reflectance from the surface of the liquid, so after calibration with a liquid column of variable height, two (of the three) complex calibration coefficients are known with an accuracy to within a certain multiplier.

[0030] After performing the primary calibration, i.e. determining the calibration coefficients, an electronic calibrator 12 is installed on the rods of the probe 1A and the complex reflectances T of its impedances in the frequency range of 100 MHz - 2 GHz are measured using a vector circuit analyzer, as shown in Fig. IB. The electronic calibrator is implemented as a set of 12 different impedances (at least 3) switched by means of an electronic switch. These impedances can be implemented, for example, as: - short circuit sections of 50 Ohm semi-rigid cable having lengths of 0, 9, 24, 38, 72.5 and 121 mm,

[0031] - open circuit sections of 50 Ohm semi-rigid cable having lengths of 0, 23, 37, 71.5 and 120 mm,

[0032] - 50 Ohm resistor (the so-called matched load).

[0033] The secondary calibration system is shown in Fig. 2. In this system, a target probe IB is used, which typically has shorter rods 2B, 3B (e.g., 75 mm) having the same amount of rods of the same diameters and the same spacing as in case of the reference probe 1A. An electronic calibrator 12 is provided on the probe IB. The control unit 6 switches over the impedances of the calibrator and measures the raw reflectance Tmof the target probe for each of them, using a vector circuit analyzer (e.g., Copper Mountain R61) in the frequency range of 100 MHz - 2 GHz. These measurements are then used to determine the calibration coefficients for the connection of the target probe IB with the vector circuit analyzer. These coefficients enable the correction of further reflectance measurements taken with the target probe. The target probe is then placed in the tested material, and basing on corrected measurements of its reflectance, the permittivity spectrum of the material is determined using a transmission line model [E. Piuzzi, A. Cataldo, G. Cannazza, E. De Benedetto, "An Improved Reflectometric Method for Soil Moisture Measurement Exploiting an Innovative Triple-Short Calibration," in IEEE Transactions on Instrumentation and Measurement, 59, 10, 2747-2754, 2010], An exemplary spectrum of the real part s' and the imaginary part E" of the permittivity for dry sand and isopropanol is shown, respectively, in Fig. 3A and Fig. 3B.

[0034] It should be emphasized that a great advantage of this method is that the electronic calibrator 12 can be characterized once (although this procedure should be repeated every few months due to the ageing of the components) and then used repeatedly for rapid (electronic standard switching) calibration of the target probe IB. Another advantage of the secondary calibration is the ability to calibrate short-rod probes that could not be calibrated by the primary method because of the too small range of changes in the liquid column height limiting the calibration frequency band.

Claims

Patent Claims1. A kit for measuring the permittivity spectrum of bulk and liquid materials, characterized in that it includes a multi-rod reference probe (1A) consisting of a signal rod (2A) located in the central part of the probe (1A), and ground rods (3A) arranged at equal angular intervals around the signal rod (2A), and a multi-rod target probe (IB) consisting of a signal rod (2B) located in the central part of the probe (IB), and ground rods (3B) spaced at equal angular intervals around the signal rod (2B), and a primary calibration system and a secondary calibration system and a control unit (6), wherein the probes (1A, IB) are coupled by a microwave connector (5) to a vector circuit analyzer (4) that is coupled to the control unit (6).

2. The kit according to claim 1, characterized in that the length of the probe (1A, IB) rods is in the range of 7 to 30 cm, wherein the length of the reference probe (1A) rods is greater than the length of the target probe (IB) rods.

3. The kit according to claim 1, characterized in that the primary calibration system is a measuring cylinder (7) placed on a laboratory scale (9), filled with a highly conductive liquid (8), which is connected to a flow pump (10) and a laboratory scale (9) by means of a control unit (6).

4. The kit according to claim 3, characterized in that the liquid (8) is a concentrated aqueous sodium chloride solution having a conductivity greater than 15 S / m.

5. The kit according to claim 1, characterized in that the secondary calibration system is an electronic calibrator (12).

6. A method for calibrating a multi-rod probe comprised in the kit for measuring the permittivity spectrum of bulk and liquid materials as defined in any one of claims 1 to 5, characterized in that it comprises the following steps:i) primary calibration and characterization of the electronic calibrator• placement of a reference probe (1 A) in the measuring cylinder (7) and introduction of a liquid (8) into the measuring cylinder (7) by means of a flow pump (10), preferably a concentrated aqueous sodium chloride solution having a conductivity of more than 15 S / m, in equal and known increments, the amount of which is calculated according to the liquid weight gain as read on the laboratory scale (9), followed by• measurement of the raw reflectance rmof the reference probe for each fixed height of the liquid column, followed by• removal of the reference probe ( 1 A) from the measuring cylinder (7), followed by• measurement of the raw reflectance rmof the reference probe (1A) located outside the cylinder (7), followed by• determination of the calibration coefficients of the reference probe system (1A) connected to the vector circuit analyzer, followed by• placement of the electronic calibrator (12) at the end of the reference probe (1A), followed by• measurement of the corrected reflectance r for all impedances carried out by the electronic calibrator (12) ii) secondary calibration• placement of the electronic calibrator (12) at the end of the target probe (IB), followed by• measurement of the raw reflectance rmfor all impedances carried out by the electronic calibrator (12), followed by• determination of the calibration coefficients of the target probe system (12) connected to the vector circuit analyzer (4).

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