Abstract
The most widely used technique for measuring capacitive impedances (or complex electrical permittivity) is to apply a frequency signal to the sensor and measure the amplitude and phase of the output signal. The technique, although efficient, involves high-speed circuits for phase measurement, especially when the medium under test has high conductivity. This paper presents a sensor to measure complex electrical permittivity based on an alternative approach to amplitude and phase measurement: The application of two distinct frequencies using a current-to-voltage converter circuit based in a transimpedance amplifier, and an 8-bit microcontroller. Since there is no need for phase measurement and the applied frequency is lower compared to the standard method, the circuit presents less complexity and cost than the traditional technique. The main advance presented in this work is the use of mathematical modeling of the frequency response of the circuit to make it possible for measuring the dielectric constant using a lower frequency than the higher cut-off frequency of the system, even when the medium under test has high conductivity (tested up to 1220 μS/cm). The proposed system caused a maximum error of 0.6% for the measurement of electrical conductivity and 2% for the relative dielectric constant, considering measurement ranges from 0 to 1220 μS/cm and from 1 to 80, respectively.
Highlights
Material impedance measurements range from medical applications such as disease diagnosis [1] to agricultural applications, such as estimating soil electrical parameters to automate irrigation systems [2,3,4,5,6,7,8], optimize water resources [9,10,11], biomass flow sensing [12], or detect food quality properties
In [2], we presented a low-cost system to measure the complex electrical permittivity of the soil using lower frequencies compared to the standard method (100 kHz and 5 MHz), aiming to automate irrigation systems
The main advance reported in this work is the use of the frequency response mathematical
Summary
Material impedance measurements range from medical applications such as disease diagnosis [1] to agricultural applications, such as estimating soil electrical parameters to automate irrigation systems [2,3,4,5,6,7,8], optimize water resources [9,10,11], biomass flow sensing [12], or detect food quality properties. The main difficulty encountered when measuring the complex electrical permittivity of materials is to separately evaluate the effects of the conductivity (σ) and the relative dielectric constant (ε) of the medium under test, especially when the conductivity is high. The separation of σ and ε effects is not a problem, as the material under test is not too conductive (up to 10 μS) [13,14,15,16,17]. Many works show studies about the development of circuits for measuring lossy capacitive sensors [18,19,20], but for applications which don’t have frequency dependence and/or the conductance is not too high (up to 145 μS), such as for measuring gas concentration, humidity, flow, and pressure.
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