GB2212621A - Underwater electric field sensor - Google Patents

Abstract

An insulating, impervious baffle (1) having a central aperture, when placed at right angles to an electric current flux in an electrically conducting medium, for example the sea, will constrain a certain cross-sectional area of current to pass through the aperture. The magnetic field that such a concentrated current generates may be concentrated in a ferromagnetic toroid (2) located around the aperture, and measured by means of fluxgate magnetometer, Hall Effect or direct induction techniques. In the direct induction technique (Fig. 2), current induced in a winding (1) around the toroid (2) is measured by a current to voltage converter (3-5). The sensor may also be used to transmit electric current fields into the conducting medium. <IMAGE>

Description

UNDERWATER ELECTRIC FIELD SENSOR This invention relates to a device for the detection and measurement of electric fields in an electrically conducting medium such as the sea.

Electric fields in the sea are generated by a number of natural and artificial mechanisms, such as lightening strikes, the evaporation of electrically charged water vapour, the electric eel, penetration from the surface by low ' frequency radio waves and electric fields emanating from ships hulls.

Conventionally, such electric fields are measured by means of silver electrodes immersed in the sea, each electrode having a solid electrolyte layer of silver chloride. The potential difference between pairs of such electrodes, in a typical band of frequencies from D.C. up to 1000 Hertz, is a measure of the electric field.

The accuracy, sensitivity and frequency response of such electrodes are adversely affected by their variable electrical resistance and capacitance, and the electrical noise that is present. A further problem is caused by the deterioration of the electrodes in sea water.

According to the present invention, because of the high electrical conductivity of sea water, it is advantageous to measure the current fluxes in the sea and to detect these fluxes by contactless methods. It is a feature of this method that an insulating, impervious baffle having a central aperture, when placed at right angles to an electric current flux in an electrically conducting medium, for example the sea, will constrain a certain cross-sectional area of current to pass through the aperture, where it may be detected and measured by means of the magnetic field that such a current generates. If such a magnetic field is concentrated in a high permeability ferrite toroid, or any other suitable ferromagnetic ring core, it may be measured by means of fluxgate magnetometer, direct induction or Hall Effect techniques.

Conversely, the device may be used with advantage to transmit electric fields into the electrically conducting medium such as the sea.

The advantages of this technique are that there are no electrodes exposed to the corrosive effects -of sea water, that electronic amplifiers have a much lower noise level when they are used to detect electric current rather than potential difference, and that the measurements are direct and unaffected by the electrochemical potentials which are present at each interface in the silver electrode method.

Two specific embodiments of the invention will now be described by way of examples with reference to the accompanying drawings in which: Figure 1 shows in cross section the way in which a uniform electric field and current flux are distorted and concentrated by means of a simple insulating baffle that is fitted with an integral ferrite toroid.

Figure 2 shows in schematic form the way in which the current flux through a ferrite toroid may be measured by direct induction.

Figure 3 shows in schematic form the way in which the current flux through a ferrite toroid may be measured by a fluxgate zagnetometer technique.

Referring to Figure 1, if a uniform electric field (3) of V volts per metre is established in a body of seawater having a volume resistivity of R ohms.metres, a uniform current flux (4) of V/R amps per square metre is established. For a particular geometry of baffle (1) and integral ferrite toroid (2), a characteristic area A of current flux may be collected by the baffle and concentrated through the central aperture of the toroid, where it may be thought of as equivalent to a single turn winding, the value of the current being (A x V)/R. The localised magnetic field that is generated by such a current flux is concentrated in the high permeability ferrite toroid and measured by either of the two techniques mentioned herein.

In the direct induction technique, illustrated schematically in Figure 2, the magnetic field induced in the ferrite toroid (2) may be measured by means of the secondary sense winding (1) of N turns. a current would then be induced in this secondary winding of (A x V)/(N x R) amps which may be measured by current measuring operational amplifier (3) whose sensitivity is defined by the value of the negative feedback resistor (4). The amplifier in this configuration behaves as a current to voltage convertor whose output signal may be measured by the voltmeter (5).

In the fluxgate magnetometer method, illustrated schematically in Figure 3, the ferrite toroid (1) may be excited by means of a sinusoidal current from the generator (3) in a winding (2). The current and the number of turns in the winding should be sufficient to drive the ferrite in and out of magnetic saturation in alternating clockwise and anti-clockwise directions.

The presence of a magnetic field due to the current flux through the toroid will make it easier to saturate the toroid in one direction than the other. The voltage peaks that are developed across the winding when the ferrite toroid achieves saturation may be differentiated by the capacitor (4) and resistor (5) into zero-voltage crossing fluctuations, and converted into a rectangular wave by the comparator (6). The symmetry or otherwise of this waveform may be amplified by the integrator which comprises the resistor (7), capacitor (8) and the operational amplifier (9).

Asymmetries in the waveform are fed through a nulling winding (11) in such a direction as to oppose the original exciting current flux. The current necessary to do this is measured by the ammeter (10) and is a measurement proportional to the exciting current flux.

Claims (6)

1. UNDERWATER ELECTRIC FIELD SENSOR

   CLAIMS 1 An underwater electric field sensor comprising an insulating, impervious baffle having a central aperture about which is positioned a ferromagnetic toroid which assembly, when placed at right angles to an electric current flux in an electrically conducting medium, for example the sea, constrains a certain cross-sectional area of that current flux to be conducted through the aperture, the resultant magnetic field being collected and concentrated as a magnetic flux in the ferromagnetic toroid.
2. 2 An underwater electric field sensor as claimed in Claim 1, whereby the magnetic flux in the ferromagnetic toroid is measured by means of an electric current induced in a toroidal conductor winding about the ferromagnetic toroid, the system in essence comprising a toroidal transformer with the current flux through the central aperture forming a single turn primary transformer winding.
3. 3 An underwater electric field sensor as claimed in Claim 1, whereby the magnetic flux in the toroid is measured by means of a Hall Effect sensor interposed in a gap that is introduced in the ferromagnetic toroid.
4. 4 An underwater electric field sensor as claimed in Claim 1, whereby the magnetic flux in the toroid is measured by means of fluxgate magnetometer techniques.
5. 5 An underwater electric field sensor as claimed in Claim 1 and Claim 2 which may be used to transmit electric current fields into the electrically conducting medium.
6. 6 An underwater electric field sensor substantially as described herein with reference to Figures 1, 2 and 3 of the accompanying drawing.