GB2210463A

GB2210463A - Electrode assembly

Info

Publication number: GB2210463A
Application number: GB8822660A
Authority: GB
Inventors: Thomas Gunn; Richard John Popplestone
Original assignee: UK Atomic Energy Authority
Current assignee: UK Atomic Energy Authority
Priority date: 1987-10-02
Filing date: 1988-09-27
Publication date: 1989-06-07
Also published as: GB8822660D0

Abstract

A capacitance probe (10) for immersion in a liquid e.g. liquid metal (20) for the purpose of monitoring changes in the liquid level comprises a tube or sheath (12) of temperature- resistant dielectric material, such as a non-porous ceramic tube, enclosing a conductor e.g. in the form of a particulate conductive material (15), such as iron filings, packed into the tube or sheath (12) so as to be free to expand and contract axially and/or radially relative to the sheath. The plates of the capacitance are constituted by the particulate conductor and the liquid metal and the capacitance may form part of an oscillator whose frequency varies with the extent to which the probe is immersed in the liquid metal whereby the oscillator frequency provides a measure of the liquid level. The use of a conductor in the form of a packed particulate filling avoids problems with differential thermal expansion between the dielectric tube and the conductor. Other forms of conductor can be used eg a liquid or a tubular metal liner which can radially expand or is longitudinally split. The internal wall of tube (12) may be metallised. <IMAGE>

Description

Electrode assembly This invention relates to an electrode assembly, particularly a capacitance probe for use in high temperature environments such as in liquid metals (eg sodium) for the purpose of monitoring variations in liquid level within a vessel.

According to the present invention an electrode assembly comprises a substantially liquid-impervious tube or sheath of temperature-resistant dielectric material closed at one end and enclosing an electricallyconductive medium which is free to expand and contract axially and/or radially relative to the sheath in response to changing thermal conditions.

Because the internal conductive medium is free to expand and contract relative to the sheath, the latter is not unduly stressed as a result of differential thermal expansion.

The sheath is conveniently composed of a low porosity recrystallised alumina material or a substantially non-porous ceramic material.

In one embodiment of the invention, the conductive medium comprises a mass of particles of electrically conductive material, eg in the form of iron filings or balls such as those used in ball bearings.

In another embodiment, the conductive medium may comprise an electrically-conductive liquid such as sodium or water.

In yet another embodiment, the conductive medium is in the form of a radially expansible tubular metal liner inserted into the sheath so that the liner expands into contact with the internal wall of the sheath but is free to slide relative to the sheath as a result of differential thermal expansion. The tubular liner may for example be longitudinally split along its entire length to enable'it to be contracted radially and elastically in the course of insertion into the sheath and then allowed to expand into contact with the sheath.

In each of the above-mentioned embodiments, the internal wall surface of the sheath may be metallised, eg by the application of a metal-loaded paint, this being of particular advantage in the case of the particulate filling medium since electrical continuity between the particles is provided by contact between the particles and the applied metal coating or layer as well as by inter-particle contact.

Where the electrode assembly is used as a capacitance probe, the enclosed conductor forms the inner electrode of the probe; the outer "electrode" may be constituted by the medium into which the probe is inserted in use, eg a liquid metal.

The probe may be used in conjunction with suitable electrical circuitry for determining changes in the capacitance of the probe, eg in response to variations in the level of the liquid in which the probe is immersed in use.

The invention includes apparatus for monitoring liquid levels within a vessel comprising an electrode assembly as above for immersion in the liquid, and electrical circuitry for determining changes in the capacitance of the probe in response to variations in the Liquid level.

One embodiment of the invention is illustrated by way of example only in the sole Figure of the accompanying drawing to which reference is now made.

As shown1 the probe 10 comprises a tubular sheath 12 of substantially non-porous ceramic material enclosing a central conductor rod 14 and a filling 16 of tightly packed, finely divided conductive material such as iron filings. The sheath 12 is closed at its lower operative end and at the other end is sealed by a plug 18 through which the conductor 14 extends.

The probe 10 is shown immersed in a pool 20 of liquid metal (eg sodium) enclosed in a tank. The electrodes of the probe 10 are formed by the inner conductor comprising the rod 14 and iron filings packing 16 and the outer conductor comprising the liquid metal 20. Electrical connections can be made to rod 14 and a container for the metal 20. The dielectric of the capacitance probe is constituted by the ceramic sheath 12. The filling or packing maintains electrical contact between the rod 14 and the sheath 12.

It will be seen that the capacitance of the probe 10 will vary with the extent to which the probe 10 is immersed in the liquid metal 20. The capacitance formed by the probe 10 may be incorporated in a high frequency oscillator circuit so that the output frequency of the oscillator circuit varies with the capacitance of the probe 10 and hence provides a measure of the depth of the liquid 20.

Claims

1. An electrode assembly comprising a substantially liquid-impervious tube or sheath of temperature-resistant dielectric material closed at one end and enclosing an electrically conductive medium which is free to expand and contract axially and/or radially relative to the sheath in response to changing thermal conditions.

2. An electrode assembly as claimed in claim 1, in which the medium comprises a packing of particulate material.

3. An electrode assembly as claimed in claim 2, in which the particulate material is iron filings.

4. An electrode assembly as claimed in claim 1, in which the medium comprises an electrically conductive liquid.

5. An electrode assembly as claimed in claim 4, in which the liquid is sodium.

6. An electrode assembly as claimed in claim 4, in which the liquid is water.

7. An electrode assembly as claimed in claim 4, in which the medium is in the form of a radially expansible tubular metal liner inserted into the sheath so that the liner expands into contact with the internal wall of the sheath but is free to slide relative to the sheath as a result of differential thermal expansion.

8. An electrode assembly as claimed in claim 7, in which the liner is longitudinally split along its entire length.

9. An electrode assembly as claimed in any preceding claim, in which the tube or sheath has an internal wall surface which is metallized.

10. An electrode assembly as claimed in claim 9, in which the surface is metallized by application of metal-loaded paint.

11. An electrode assembly as claimed in any preceding claim, in which the tube or sheath is composed of a substantially non-porous ceramic material.

12. An electrode assembly substantially as hereinbefore described with reference to, and as shown in, the accompanying drawinq.

13. Apparatus for monitoring liquid levels within a vessel comprisinq an electrode assembly as claimed in any preceding Claim for immersion in the liquid, and electrical circuitry for determining changes in the capacitance of the probe in response to variations in the liquid level.