Instrumentation & Measurement Magazine 26-1 - 63
Fig. 8. Different frequency offset measurement results. (a) Current
measurement fitting curve; (b) Voltage measurement result.
Fig. 9. Different resonance frequencies. (a) Current result; (b) Voltage result.
and ±30 kHz (385.5 kHz and 443.5 kHz). The sensitivity of the
measurement system is the best at the resonant frequency, and
the signal decline is significant. With the increase in the deviation
frequency, the curve gradually shifts downwards, and the
detection sensitivity gradually decreases. The conductivity of
the system gradually decreases. This is due to the measurement
system's frequency deviation, which reduces the degree
of coupling between the coils. The signal is more difficult to
transmit in the system, which causes the current and voltage
signals to drop.
To further study the sensitivity of the signal, we studied the
drop rate ratio ΔT. It is defined as the drop rate percentage to
the total range, as:
100%
T
T
TT
max min
max
(12)
In the resonant state, the seawater conductivity changed from
34.5 ms/cm to 65.5 ms/cm, and the voltage signal decreased
from 590 mv to 157 mv with a decrease ratio ΔT of 73.4%; the
frequency shifts were ±15 kHz (400.5 kHz and 428.5 kHz)
and ±30 kHz (385.5 kHz and 443.5 kHz) with their ΔT of:
February 2023
54.7%, 62.4%, 45.2% and 35%, respectively. It also has higher
resolution at low seawater conductivity, where the voltage
measurement signal amplitude drop-off percentage is 49.2%
in resonant state at seawater conductivity of 34.5 ms/cm to
44.5 ms/cm, and the drop-off percentage range is 22% to 35%
at resonant frequency offset ±30 kHz.
Measurement Results at Different Resonance
Frequencies
Fig. 9 shows the measurement data of the S-S structure system
at different resonance frequencies. The experiment
measures the data at resonance frequencies of 228 kHz, 338
kHz, and 413.5 kHz. It can be seen that, as the resonance frequency
increases, the current signal and the voltage signal
shift downward as a whole. In addition, there is no change in
the downward trend as the conductivity increases.
As the resonant frequency increases,
TT T , the
3 21
signal decline ratio gradually increases, and the signal sensitivity
increases. As the frequency increases, the induction lost
to seawater gradually increases, making it easier to identify
the change in seawater conductivity. Similarly, as the system's
sensitivity increases, the obstructive effect of seawater
IEEE Instrumentation & Measurement Magazine
63
Instrumentation & Measurement Magazine 26-1
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