GB2278015A

GB2278015A - Sheath for temperature sensor

Info

Publication number: GB2278015A
Application number: GB9409371A
Authority: GB
Inventors: Horbury Winder
Original assignee: Individual
Current assignee: Individual
Priority date: 1993-05-14
Filing date: 1994-05-11
Publication date: 1994-11-16
Also published as: GB9309931D0; GB9409371D0

Abstract

The invention relates to a sheath for temperature sensing means, for sensing, for example, the temperature of molten metal. Known metal sheaths have adequate strength but gaseous products of reaction between the sheath and the molten metal can penetrate through to, and damage, the temperature sensing means. In addition, sheaths frequently need a high length-to-diameter ratio, that is difficult to achieve with conventional casting and mould technology. Ceramic sheaths are also known but they have verv poor mechanical strength when of the required high length-to-diameter ratio. The invention provides a sheath formed from two components, with a bonded or fused interface between the outer surface of the inner component and the inner surface of the outer component, the material of said outer component being such as to provide mechanical strength and protection of a temperature sensing element within the inner component from the environment in which it is placed, and the material of the inner component being such as to provide impermeability. <IMAGE>

Description

TEMPERATURE SENSING MEANS This invention relates to temperature sensing means, and is particularly concerned with means for sensing the temperature of molten metal.

There are various known forms of temperature sensing means for molten metal, such as thermocouples, resistance thermo-devices and radiation monitors. To protect such means against the harsh environment in which they must function, it is usual to locate them within an outer protective sheath, able to provide resistance against abrasion or mechanical attack, and chemical attack. Traditionally sheath materials have been of ceramic or of metal.

Conventional sheath materials can be considered to be impervious in the sense of preventing direct penetration by the molten metal in which the temperature sensing means are immersed, but dependent on the particular molten metal and the particular material of the sheath, a chemical reaction between the sheath and the molten metal can generate gaseous products that can diffuse through the sheath material and adversely affect the temperature sensing means within the sheath. In an attempt to overcome this problem, it has been proposed to provide a double component sheath, with an outer component to provide mechanical strength and protection of the temperature sensing means when immersed in molten metal, and an inner component to protect the thermal components against attack by diffusion of secondary products of reaction between the outer component and the molten metal.

However, such proposals have presented several difficulties. Heat flow or flux across the two components can be seriously impaired even when they are in close proximity, and there is the problem of differential expansion as between the outer and inner components. Temperature sensing means of the type in question frequently have a high length-to-diameter ratio. Major problems are involved in producing a sheath with this criteria with current sand mould technology and coring methods. A ratio of much greater than 2:1 outer to inner diameter ratio is extremely difficult to achieve with sand casting techniques, putting limitations on the minimum inner bore which can be produced. The need for a long, small diameter core to produce a bore of the sheath presents difficult support problems, and an inner bore with an eccentric wall thickness can result, with associated service life problems. Use of refractory sheaths results in fracture of the core during the formation of the inner sheath component, and this plus the support problems mentioned above, and difficulty of removing the core after the sheath has been produced, make them unsuitable.

The object of the invention is to provide a temperature sensing means particularly for molten metal that avoids all of those disadvantages mentioned above.

According to the present invention, a sheath for temperature sensing means comprises two components, with a bonded or fused interface between the outer surface of the inner component and the inner surface of the outer component, the material of said outer component being such as to provide mechanical strength and protection of a temperature sensing element when present within the inner component from the environment in which it is placed, and the material of the inner component being such as to provide impermeability.

Preferably, the material of the inner component is a ceramic, for example a glass ceramic such as quartz, and in particular silica quartz. The advantage here is that such as a silica quartz tube, closed at one end, can readily be produced with any required inner diameter particularly, very small diameter bores. Glass ceramic materials are totally impervious, and have strength characteristics such that small diameter tubes of considerable length can readily serve the purpose of a core for the production of the outer component of the sheath, and the tube left in situ to constitute the inner component of the sheath. Another advantage of glass ceramic materials is their ability to serve as an electrical insulator.

Thus, and for example, the outer component of the sheath may be a cast iron, and the outer sheath cast directly around the glass-ceramic tube, with consequent bonding or fusing of the cast outer sheath component to the glass ceramic tube. Such a construction offers considerable advantages by its avoidance of manufacturing difficulties, and its ability to provide a two-component sheath with any required bore in the inner component. In addition, the outer sheath provides considerable mechanical strength, and the inner component considerable resistance to thermal shock with total avoidance of diffusion through it of any products of reaction between the outer component and the environment in which it is placed. The presence of a fully integrated interface between the two components avoids all complications in terms of differential thermal expansion characteristics between the two components, and in addition any impairing of heat or flux transfer across the two components.

One embodiment of the invention will now be described with reference to the accompanying drawing, which is a sectional side elevation of a sheath in accordance with the invention.

As shown in the drawing, a sheath 1 comprises an outer component 2 of a material to provide high mechanical strength such as, for example, a cast iron, within which is an inner component 3 of a material to provide complete impermeability, such as, for example, a glass ceramic, particularly a quartz, and more particularly, a silica quartz.

By its nature, silica quartz allows the production of tubes of very small internal diameters, and of considerable lengths, e.g. over 1 metre. The temperature and thermal shock resistant characteristics of such as, silica quartz, are such that long and small bore tubes 3, closed at one end, can readily serve the purpose of a core around which the outer component 2 of such as, cast iron, can be directly cast, with a resultant bonding or fusing of the outer component to the inner component.

Thus, with temperature sensing means located in the bore of the inner component 2, the sheath can be inserted into molten metal, and when the cast iron material of the outer component protects the temperature sensing means against molten metal attack, the bonded, or fused, interface between the outer and inner components ensuring that there is no impairing of heat or flux transfer across the two components to the temperature sensing means within the inner component, and silica quartz material of the inner component ensuring that any products of reaction between the outer component and molten metal, cannot penetrate the inner component and damage the temperature sensing means within the inner component.

The further advantage of the invention is that the fused or bonded interface between the inner and outer components avoids all problems that can arise from differential expansion.

Claims

1. A sheath for temperature sensing means comprising two components, with a bonded or fused interface between the outer surface of the inner component and the inner surface of the outer component, the material of said outer component being such as to provide mechanical strength and protection of a temperature sensing element when present within the inner component from the environment in which it is placed, and the material of the inner component being such as to provide impermeability.

2. A sheath as in Claim 1, wherein the material of the inner component is a ceramic.

3. A sheath as in Claim 2, wherein the ceramic is a glass ceramic.

4. A sheath as in Claim 3, wherein the glass ceramic is silica quartz.

5. A sheath as in any of Claims 1 to 4, wherein the material of the outer component is a cast iron.

6. A temperature sensing means comprising a sheath and temperature sensor within the sheath, said sheath being formed from two components, with a bonded or fused interface between the outer surface of the inner component and the inner surface of the outer component, the material of said outer component being such as to provide mechanical strength and protection of a temperature sensing element when present within the inner component from the environment in which it is placed, and the material of the inner component being such as to provide impermeability.

7. A method of producing a sheath as in Claim 1, comprising first forming an inner component as a closed tube of an impervious material, and casting directly on to the inner component the material of the outer component, to form the outer component on the inner component with a fully integrated fused or bonded interface between the two components.

8. A sheath substantially as hereinbefore described with reference to the accompanying drawing.