Instrumentation & Measurement Magazine 26-1 - 58

A Seawater Conductivity
Measurement Method Based on
Magnetic Resonance Coupling
Ning Liu, Yuanjie Miao, Tao Wang, Yuhua Wu, and Jianrui Zhao
T
his paper proposes a magnetic resonance coupling
seawater conductivity measurement method with
a dual-coil structure. The passive transceiver coil
is composed of two detection coils and capacitive elements
placed in seawater. The seawater conductivity measurement
is realized by analyzing the relationship between the transmission
loss of the resonant magnetic field between the passive
transceiver coils in the seawater and the seawater conductivity.
In the article, the magnetic resonance coupling measurement
method is established and analyzed, the circuit model of the
magnetic resonance coupling measurement system transmitted
in seawater is established, the experimental device is set
up for laboratory testing, and the corresponding test situation
is simulated by finite element electromagnetic modelling. The
test and simulation results show that the measurement method
is feasible; the system has the best performance at the resonant
frequency, especially for low-conductivity conditions, and better
sensitivity. In addition, the resonant frequency increases,
and the conductivity resolution of the measurement system
for seawater increases.
Conductivity Sensors for Measuring
Salinity
Seawater salinity is an element of ocean hydrological measurements.
Its measurement plays a significant role in marine
scientific research, the development and utilization of marine
resources, and the precise navigation systems of ships [1]. At
present, the salinity of seawater is measured mainly by measuring
the conductivity of seawater and then converting it to
the actual salinity. Conductivity sensors have the advantages
of low delay, high stability, high accuracy, and convenient measurement.
Conductivity sensors have become the primary
method of salinity measurements in seawater [2].
According to their different mechanical structures, current
conductivity sensors used for measuring seawater can be divided
into contact sensors and noncontact sensors. Contact
conductivity sensors, mainly electrode-type seawater conductivity
sensors, are used in seawater conductivity measurements
and can be divided into two-electrode, four-electrode, and
58
six-electrode sensors and other multielectrode forms [1].
An electrode-type conductivity sensor is based on seawater
electrolysis and conducts electricity, and there is a complex
electrochemical system between the electrodes. A pair of the
same electrodes generates the detection current and the voltage
between the measurement electrodes in the two-electrode
structures. The voltage and winding ratio are used to deduce
the seawater resistance between the two voltage electrodes to
obtain the seawater resistance. However, polarization reactions
occur between the two electrodes, and the resistance of
seawater cannot be simply derived from the voltage to current
ratio. Thus, the accuracy of the derived conductivity is poor.
For multi-electrode forms such as four-electrode forms,
the polarization effect is eliminated due to the minimum current
of the internal electrodes [3]. They are more resistant to
the influence of the external environment on the measurement
accuracy of the sensor and have a higher measurement range
[4]. However, for this electrode type, the chemical effect has
not been completely eliminated [5]. A multi-electrode sensor
can mechanically isolate the current and voltage electrodes,
unlike two-electrode sensors. Multi-electrode sensors have a
better shielding effect, yield measurement results that are less
influenced by the external environment, and have better measurement
accuracy. However, the electrode polarization is still
not eliminated, and electrodes are easily corroded in seawater
during long-term operation, which leads to a decrease in measurement
accuracy.
A noncontact inductively coupled seawater conductivity
sensor is based on electromagnetic induction coupling. A
loop composed of seawater couples two coils together, and
then a linear relationship between seawater resistance and
induced electromotive force is derived from the principle of
mutual inductance so that the seawater conductivity is output
according to the output voltage [6]. The metal part of the
inductively coupled seawater conductivity sensor is not in
contact with seawater, and the nonmetallic shell is not easily
corroded. At the same time, the influence of the polarization
effect is avoided. Noncontact inductively coupled seawater
conductivity sensors show long-term high stability, but they
IEEE Instrumentation & Measurement Magazine
1094-6969/23/$25.00©2023IEEE
February 2023
Instrumentation & Measurement Magazine 26-1

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