Resper meter

Highlights
The electrical RESistivity and dielectric PERmittivity measuring device (RESPER meter) for non-invasive investigation of enviroment is a survey device exploiting the electrical induction by means of a capacitive coupling with media as terrestrial soils and concretes.
RESPER_Fig1a

Figure 1. RESPER meter is characterized by a galvanic contact with the subjacent medium. RESPER is connected to an analogical digital converter (ADC) which samples in phase and quadrature (IQ) mode and is specified by a minimum bit resolution equal to nmin= 12, ensuring measurement inaccuracies below a predefined limit (10%) up to the middle frequency (MF) band. The medium can consist of a large variety of terrestrial soils or concretes. Terrestrial soils: with low or high electrical resistivity, and respectively high or low dielectric permittivity, i.e. (1/σ = 130 Ω•m, ε = 13) or (1/σ = 3000 Ω•m, ε = 4). Concretes: with low or high resistivity, and respectively high or low permittivity, i.e. (1/σ = 4000 Ω•m, ε = 9) or (1/σ = 10000 Ω•m, ε = 4). RESPER performs MF measurements, and the media are analyzed at frequency flow = 3 MHz, apart from soils with low resistivity (fup = 30 MHz). The Like-Bode’s diagrams of inaccuracy Δσ/σ(flow,up, σ, ε) as a function of σ (a), and semi-logarithmic plots of inaccuracy Δε/ε(flow,up, σ, ε) as a function of permittivity ε (b), for both the soils and concretes, are shown.
Fig.1 – RESPER meter is characterized by a galvanic contact with the subjacent medium. RESPER is connected to an analogical digital converter (ADC) which samples in phase and quadrature (IQ) mode and is specified by a minimum bit resolution equal to nmin= 12, ensuring measurement inaccuracies below a predefined limit (10%) up to the middle frequency (MF) band. The medium can consist of a large variety of terrestrial soils or concretes. Terrestrial soils: with low or high electrical resistivity, and respectively high or low dielectric permittivity, i.e. (1/σ = 130 Ω•m, ε = 13) or (1/σ = 3000 Ω•m, ε = 4). Concretes: with low or high resistivity, and respectively high or low permittivity, i.e. (1/σ = 4000 Ω•m, ε = 9) or (1/σ = 10000 Ω•m, ε = 4). RESPER performs MF measurements, and the media are analyzed at frequency flow = 3 MHz, apart from soils with low resistivity (fup = 30 MHz). The Like-Bode’s diagrams of inaccuracy Δσ/σ(flow,up, σ, ε) as a function of σ (A), and semi-logarithmic plots of inaccuracy Δε/ε(flow,up, σ, ε) as a function of permittivity ε (B), for both the soils and concretes, are shown.

Abstract
RESPER meter utilizes a four-electrode probe to inject a radio frequency voltage into a medium and to register an induced current signal. Complex transfer impedance can be determined from a ratio between a potential measured across two electrodes, and an induced current flowing in the medium. Electrical parameters of resistivity and permittivity characterizing the medium can be established from the transfer impedance, using inversion formulas that also take into account the geometric ratio between the distances and position of the electrodes.

RESPER_Fig2a

Fig.2 - Photos of the RESPER meter configured as a multi dipole-dipole array in perspective view (a) and details of its “spring” poles (b).
Fig.2 – Photos of the RESPER meter configured as a multi dipole-dipole array in perspective view (A) and details of its “spring” poles (B).

 

In-depth analysis
RESPER exploits the in-phase and quadrature under sampling technique which, together with some numerical operations performed by a microcontroller, allows the device to attain a required performance. Indeed, it is possible to execute just a number of numerical integrations which, combined with some circuit solutions, can reduce the amplitude and phase errors of the acquired signal. The device can operate at variable frequency, even maintaining a suitable under-sampling frequency to fully exploit the analogical-digital acquisition performance both in velocity and dynamic range.

Fig.3 - The logical scheme of a sampling and holding circuit (S&H), which employs two IQ ADCs, is shown. The frequency f of input signal is forwarded to a 90° phase-shifter. Two chains of identical programmable counters operate a division for the down-sampling factor M. The rate is precisely fS = f/M.
Fig.3 – The logical scheme of a sampling and holding circuit (S&H), which employs two IQ ADCs, is shown. The frequency f of input signal is forwarded to a 90° phase-shifter. Two chains of identical programmable counters operate a division for the down-sampling factor M. The rate is precisely fS = f/M.

 

Fig.4 - Electrical scheme for the analogical part of the measuring device: a signal generator (1), coupled to an amplifier stage, feeds one of two current electrodes (T1). The same current signal, picked to the other electrode (T2), is converted into voltage (2) and then amplified (3). The stage of differentiation for the voltage (4) is preceded by the feedback device in order to compensate the parasite capacities (5). The signal is sent to an ADC and transferred to a personal computer (PC), where it can be properly processed. The electronic circuit is composed primarily from two stages. The first consists of a current-voltage converter followed by a cascade of amplifiers, to amplify the weak currents typical of high transfer impedances and the second consists of a voltage amplifier with a retroactive chain of capacitive compensation. The circuit has been designed to work linearly at low and middle frequencies (LFs-MFs) in the band from direct current (DC) to 2000 kHz. The selected components have been developed specifically for electronic instruments of precision. The circuit techniques adopted for the compensation of the parasite capacities are innovative and allow performing measurements of high impedances. This analogical device is connected to an analogical digital conversion board which contains even a digital analogical converter (DAC) used as a signal generator that, properly projected, can generate a whole series of measurements in an automatic way even at different frequencies for a full analysis.
Fig.4 – Electrical scheme for the analogical part of the measuring device: a signal generator (1), coupled to an amplifier stage, feeds one of two current electrodes (T1). The same current signal, picked to the other electrode (T2), is converted into voltage (2) and then amplified (3). The stage of differentiation for the voltage (4) is preceded by the feedback device in order to compensate the parasite capacities (5). The signal is sent to an ADC and transferred to a personal computer (PC), where it can be properly processed. The electronic circuit is composed primarily from two stages. The first consists of a current-voltage converter followed by a cascade of amplifiers, to amplify the weak currents typical of high transfer impedances and the second consists of a voltage amplifier with a retroactive chain of capacitive compensation. The circuit has been designed to work linearly at low and middle frequencies (LFs-MFs) in the band from direct current (DC) to 2000 kHz. The selected components have been developed specifically for electronic instruments of precision. The circuit techniques adopted for the compensation of the parasite capacities are innovative and allow performing measurements of high impedances. This analogical device is connected to an analogical digital conversion board which contains even a digital analogical converter (DAC) used as a signal generator that, properly projected, can generate a whole series of measurements in an automatic way even at different frequencies for a full analysis.