An
electrochemical hydrophone based on the principles of molecular electronic
transfer (MET) has been described. The paper presents theoretical and
experimental results for the sensitivity and the level of self-noise
determination for the MET hydrophone (METH) in the frequency range of
0.02–200 Hz, which determines the fields of acceptance of the devices being
developed. An experimental model has been developed by using a
force-balancing feedback. Different methods and techniques for its
calibration have been developed. The experimental device with
0.75 mV Pa
At
present, there is a great variety of pressure sensors and the fields of their
application in the world (Sherman and Butler, 2007; Gautschi, 2002). Sensors
are the primary source of information about the processes taking place in the
world ocean (Asolkar et al., 2017; Bradley and Nichols, 2015). Hydrophones
provide navigation and location of vessels (Lasky et al., 2004), fishing
technologies for the fishing industry (Mismund, 1997), scientific research of
the underwater biosphere (Slotte et al., 2004), and work of underwater
acoustic communications (Kopp et al., 2000). Another field of application is
the seismic exploration of minerals; hydrophones are used as part of towed
braids and stationary bottom stations. A significant difference in this
direction of applications is the requirement of high sensitivity in the
ultra-low-frequency range of 0.1–100 Hz, as well as the possibility of
registering extremely weak signals of pressure variation. Previous-generation
technologies, such as piezoelectric (de Medeiros et al., 2015),
electrostrictive, magnetostrictive and electrostatic sensors, could not be so
effective in the range of ultra-low-frequency measurements. In these
circumstances, there is a problem of developing pressure sensors based on the
new technological principles that can meet the growing demands of engineering
and scientific tasks. Among the popular scientific and technical
developments, there are several directions. For example, an electromagnetic
hydrophone consisting of a conventional wire and a magnet can be used to
measure acoustic pressure (Grasland-Mongrain et al., 2012), and another
physical principle is used in photonic hydrophones with a relatively high
sensitivity. They are created based on the interaction of two polarized
lasers from an optical fibre (Liu et al., 2016). Another type of acoustic
sensor can be based on the effect of optical reflection on the fibre end
(Shen et al., 2011), Bragg fibre gratings (Tan et al., 2011) or Fabry–Perot
interferometers (Kim et al., 2014; Ma et al., 2016). The fibre optic
hydrophone has been shown to have a good linear response with a high
sensitivity and a high-pressure resolution. Another direction is the
development of pressure sensors based on the MEMS technology (Xu et al.,
2016). The sensitivity and the bandwidth of such devices can be quite high in
the frequency range of 20 Hz–1 kHz, which is sufficient for underwater
acoustic detection at low frequencies. At the same time, many leading
companies produce hydrophones based on the traditional piezoelectric effect
(available at:
A comparatively new and successfully proven technology based on the molecular electronic transfer (MET) (Hurd and Lane, 1957) in the field of seismology and geophysical research (Deng et al., 2016), navigation and motion control (Zaitsev et al., 2016), earthquake-proof construction (Antonovskaya et al., 2017), and offshore exploration (Agafonov et al., 2017) can be used to develop pressure sensors other than traditional piezoceramic, micromechanical and fibre-optic technologies. Distinctive features of sensors based on the MET are of extremely high sensitivity in the field of low frequencies and low level of self-noise.
The results of the work are presented in three main parts. In the first part, the principles of the device for developing an acoustic pressure sensor based on the molecular–electronic transfer technology are discussed. In the second part, theoretical operational principles are analysed and a theoretical model of noise in the working frequency band is formed; the third part is devoted to the experimental studies of the possibility of creating a MET hydrophone, principles of calibration, sensitivity and self-noise of prototypes. The model of its self-noise is built based on the previous knowledge of the physical processes responsible for noise in the MET sensor systems (Kozlov and Safonov, 2003). Including such physical mechanisms as convective noise (Safonov, 2003), noise caused by the impedance of the transforming element (Shabalina, 2007), hydrodynamic noise and noise due to the geometry of the electronic node (Zaitsev et al., 2015), and noise due to cross axis sensitivity (Zaitsev et al., 2018a) has been considered.
The fundamentals of the technology and the physical principles of the MET sensor devices are discussed in the teaching materials (Lidorenko et al., 1984), while at the same time there are many current reviews on this topic and articles in periodicals (Huang et al., 2013) and patents (Abramovich and Kharlamov, 2003). In the literature, one can find information on the current state of developments based on the MET, development vectors and key achievements of scientific groups. As for the operational principles of the devices based on the MET, they are analysed in detail in Agafonov et al. (2013).
The main element of the MET is the transforming electrode cell, placed in a
concentrated electrolyte solution (Fig. 1). The composition of the solution
is selected in such a way as to enable the reversible electrochemical
oxidation–reduction reaction to proceed on the electrodes. For these
purposes, a so-called iodine iodide electrolyte is most often used. An
example of such an electrolyte is a concentrated (
The reduction of the triiodide on the cathode:
Transforming electrode cell (MET). 1 – channel walls; 2 – electrical package; 3 – electrolyte; 4 – mesh electrodes (external anodes A, internal cathodes K).
Distribution of the electrolyte concentration in the
electrode assembly with mesh permeable electrodes.
The design and basic operational principles of a closed-loop molecular electronic hydrophone (METH) are shown in Fig. 3. It is designed on the basis of the construction of the MET closed-loop seismic accelerometers (Egorov et al., 2017) and the basic principles were approved in Zaitsev et al. (2018b). The sensing element of the MET sensor consists of two pairs of electrodes (cathode–anode) forming a so-called electrical package, as shown in Fig. 1. The electrical package is placed into a channel, bounded by rubber membranes (4) and filled with an electrolyte (7); 1 denotes the external sensor housing. A neodymium magnet (3) is glued to one of the rubber membranes, which can freely move inside the coil (2). The coil (2), in its turn, is rigidly adhered to the upper cover (8) (the insulating part of the volume of air from the external medium), so that the magnet can move inside it under the action of the Lawrence force. Such a simple scheme, on the one hand, makes it possible to introduce closed-loop feedback into the mechanical system, and on the other hand, it allows the hydrophone to self-calibrate.
Constructional parts of the MET hydrophone. 1 – external body; 2 – coil; 3 – magnet; 4 – membranes; 5 – electrical package; 6 – electrical terminals of anodes and cathodes; 7 – electrolyte; 8 – cover with air bubble under it.
The METH signal is the output current from the electrical terminals of the
cathodes. As the two membranes of the sensor could be under different
pressures, tiny pressure variations on the open membrane of the hydrophone
can transform into a flow of working fluid. Further, the signal current
passes through the correction, amplification and filtering circuits and the
output of the electronic board is I
The MET hydrophone electronic circuit.
As was mentioned above, the hydrophone has been constructed based on the
design of the MET closed-loop seismic accelerometer. However, these devices
have significant differences. The main difference is the membrane function.
One of the two membranes separates the electrolyte solution from the air
chamber, where the coil (2) and magnet (3) are located. Thereby the ambient
pressure can be measured, because the pressure in the air chamber changes
when the membrane deforms. Consider a simplified ideal gas model for gas
enclosed between the membrane and the upper cover – 8. The parameter
Substitution of Eq. (4) into Eq. (3) gives the equation of membrane oscillations:
Then, using (3) and (6),
Therefore, the mechanical transfer function is presented by the following
formula:
Spectrum of the mechanical transfer function.
Note that the overall transfer function
Since the MET technology has been studied rather well, let us try to model
the METH self-noise using knowledge of the existing noise mechanism in the
MET. According to Kozlov and Sakharov (1994), at low frequencies the spectral
density of hydrodynamic noise given in units of equal pressure is frequency
independent and presented by the following formula:
METH noise modelling for different pV and
The first question of the research: how to calibrate the MET hydrophone at a very low frequency (0.01–100 Hz). To solve this problem, an experimental calibration set has been designed (Fig. 7).
Principal design and view of the MET hydrophone calibration set. 1 – container with water and hydrophones; 2 – vertical offset generator; 3 – DA converter/digitizer; 4 – the MET hydrophone; 5 – piezoelectric hydrophone BC-311; 6 – open-end pipe with water.
The experimental set consists of a stiff container filled with water (1) with
the MET hydrophones and reference piezoelectric hydrophone on its floor. The
container (1) has an output pipe (6) with an open end. The pipe is raised on
the level of height, so that the water column makes additional pressure
inside the container (1). A tilting calibration platform (2) was used to
change the pressure. Its main part is a platform 300 mm high, 600 mm long
and 400 mm wide. This heavy construction (over 60 kg) is suspended on a
rigid torsion bar. Attention is paid to providing both static and dynamic
stiffness in the operating frequency range. The calibration process is
controlled by a computer with the appropriate software. Sinusoidal
oscillations are set by a 12-bit L-card
digital-to-analogue converter, the
signal from which, passed through a smoothing filter, is fed through a power
amplifier to a pair of powerful (250 W) low-frequency drivers. The speakers
bring the platform into a vibrational–rotational movement, to which they are
connected by rods through the frictionless joints (Fig. 7). The key
principles of work and the practical basis of calibration are described in
Abramovich et al. (1997). The water pipe was fixed on the vibration platform
with the possibility of vertical movement. An eight-channel 16-bit data
acquisition system, NI USB-6215 (Bus-Powered M Series Multifunction DAQ for USB-16-Bit, 2018), (3) was used to collect
the signal from the reference sensors and the output signals of the
hydrophones. This was the striate way to make pressure variations with the
known amplitude at different frequencies. Vertical water column displacement
There are other ways to calibrate the MET hydrophone, and one of the main aims of the present research was to investigate the simplest way of METH calibration. So, we have compared all the techniques and present the results for later discussion.
The second and prior way for closed-loop METH calibration is self-test. The
METH frequency response can be described by the mathematical model (1). The
amplitude and the phase-frequency response of the function
Self-calibration METH
METH
In accordance with the corollary of the stability criterion for the
Nyquist–Mikhailov dynamical system, if the open system with the transfer
function
The third way is to make the strong pressure variations by the vertical water column displacements and find the spectrum ratio of the METH signal and the reference hydrophone signal BC-311 (BC 311 Underwater/threaded hydrophone, 2018); see Fig. 10. The METH and BC-311 were placed close to each other. Using the assembly from Fig. 7, strong shift signals were excited at the resonance frequency of the vibrating table, which were fixed by two closely located hydrophones. The strong signal was registered in the recorded spectrum over a wide frequency band; as can be seen from the analysis of Fig. 10, the red and blue spectra are strongly correlated, practically in the entire frequency band. Assuming a frequency response of the reference hydrophone BC-311 flat, the relationship between the signals from the correlated spectra of the studied METH to the spectrum of BC-311 with a known flat transfer function has been found. As a result, the METH transfer characteristic in the blue graph in Fig. 11 has also been obtained.
Pressure variations at the 24 Hz signal. Red is the MET hydrophone
signal spectrum, and blue is the ZETLab BC-311 hydrophone signal spectrum. Hz
on the
The results for comparison of self-test METH calibration with the calibration
by the calibrating platform and reference hydrophone at low-frequency
amplitude response are given in Fig. 11. The green curve is the METH platform
calibration, the blue curve is the strong signal calibration with BC-311, and
the red curve is the self-test calibration calculated in the
METH self-test calibration (red) with METH calibrating platform
calibration (green) and a strong signal calibration curve (blue). Hz on the
Based on the comparison shown in Fig. 11, we can conclude that all the proposed METH calibration methods are equivalent in terms of the result and give the same sought-after METH frequency response. The technically simplest method of calibration can be used hereafter, which is self-calibration by the coil.
In accordance with the above results, full correspondence of all types of calibration techniques can be observed. But it is more convenient to use sensors with a flat frequency response. To do so, special electronic nominal have been found (according to the scheme in Fig. 4). To make the flat-frequency response from 0.02 to 200 Hz, specific circuit parameters have been chosen. The experimental results of METH sensitivity with closed-loop feedback are shown in Fig. 12. Self-test calibration has been made under water.
This way, the use of absolute values of water column surface S, the values of
precise displacement sensor sensitivity for the calibration platform, the
sensitivity of the reference hydrophone BC-311, and the METH sensitivity have
been measured in the frequency range from 0.02 to 200 Hz, and its absolute
value is 0.75 mV Pa
METH closed-loop feedback self-test calibration (blue curve) with
BC-311 sensitivity level (red curve). Hz – on the
Thus, the present study demonstrated the possibility of creating a hydrophone with a high sensitivity in the region of ultra-low acoustic frequencies, and experimentally proved the identity of the calibrations by different schemes.
For experimental noise measurements, two METHs with an identical sensitivity
of 0.75 mV Pa
Installation for measuring the intrinsic noise of molecular–electronic hydrophones. 1 – unbundled seismic foundation, 2 – thick-walled metal tank with water, 3 – foam cap, 4 – foam base, 5 tested molecular and electronic hydrophones, 6 – data acquisition system, 7 – laptop with special software.
The METH's signal power spectral densities are shown in Fig. 14. We research only a quiet period of the night's recording. In Fig. 14, the red and blue colours show power spectral densities of the two identical METHs that are placed coaxially and close to each other, while the noise power spectral density of the data acquisition system in units of pressure is green. The violet curve represents the non-correlative part of the observed METH's night period signals that mean the self-noise of the studied METH, and it was calculated according to the equation of Egorov et al. (2017):
METH night test. Red and blue are the PSD of close METHs, green is
the ADC self-noise, purple is the uncorrelated part corresponding to METH
self-noise, black is sea state zero, and grey
is the TC4035 Teledyne Reson hydrophone. Hz –
Actually, there should be two self-noise curves since the self-noises of the two devices were not exactly the same, but according to Egorov et al. (2017) Eq. (14) works only in assumption of an equivalent-level self-noise of two correlated sensors.
The comparison of the theoretical and experimental results shows their close correspondence, despite the high signal level in the frequency range of 1–40 Hz, which may mean an insufficiently quiet place and environment for a noise test. That result limits the top METH noise level and could be reduced in the next studies.
The main result of the research is the theoretical and experimental model of
METH, methods and the technique for low-frequency calibration, and the
experimental structure of closed-loop
feedback METH with a 0.02–200 Hz flat-frequency response with a sensitivity
level of 0.75 mV Pa
Another significant conclusion, according to Eq. (8), represents the mechanical part of the transfer function of the MET hydrophone; the mass of the electrolyte is cancelled out in Eq. (8). The physical mechanisms of noise presented in the theoretical section are not affected by the mass and overall dimensions of the hydrophone. So, it seems possible to significantly reduce the overall dimensions of the MET hydrophone. The level of self-noise and sensitivity of the hydrophone will not be significantly affected by the mass of the electrolyte.
The underlying measurement data are not publicly available and can be requested from the authors if required.
DLZ – obtaining experimental data and developing the experimental set-up; SYA – developing the structure scheme of METH; MAR – calculating the theoretical dependence of the METH transfer function and METH self-noise; IE – getting experimental data; EVE and VMA – making a sufficient contribution in discussing the results and conclusion.
The authors declare that they have no conflict of interest.
This work was supported by the Russian Ministry of Education and Science state assignment under grant 3.3197.2017. Edited by: Andreas Schütze Reviewed by: two anonymous referees