GB2184554A - Temperature monitoring devices for use with mineral insulated cables - Google Patents

Abstract

In a temperature monitoring device, each protruding end portion of the wire 1 of a resistance temperature detector, e.g. a platinum wire, is welded to a conductor tail 5 of a metal which will not be deleteriously contaminated by gases given off at high temperatures, e.g. above 250 DEG C, by a mineral insulated electric cable 11. The resistance temperature detector is housed in a fluid-tight enclosure 2 which is made of a material impervious to said gases and from which the conductor tails 5 project. The projecting conductor tails 5 are welded or otherwise electrically connected to the conductors 12 of the mineral insulated cable 11, and the hermetically sealed resistance temperature detector 10 and the exposed welded conductor tails and cable conductors are enclosed within a metal hood 14 filled with compacted powdered aluminium oxide and which is welded or otherwise electrically connected to the metallic sheath 13 of the mineral insulated cable. <IMAGE>

Description

SPECIFICATION An improved temperature monitoring device This invention relates to a temperature monitoring device comprising a temperature resistance detector and, electrically connected to the detector, a mineral insulated electric cable.

In the temperature monitoring device ofthe kind with which the present invention is concerned, the resistance temperature detector comprises a length of wire of metal or metal alloy, usually but not necessarily platinum, wound around an elongate former in at least two coils of opposite hand, one overlying the other, to form a detector element, the element being housed in a sleeve of electrically insulating material with end portions ofthewire protruding from one end ofthe sleeve, and the mineral insulated electric cable comprises at least two elongate conductors embedded in, and electrically insulated from one another and from a surrounding sheath of metal or metal alloy by, compacted mineral insulated powder, e.g.

magnesium oxide, the conductors of the cable being electrically connected to the protruding end portions of wire ofthe resistance temperature detector. A temperature monitoring device ofthe aforesaid kind will, for conven ience, hereinafter be referred to as "ofthe kind described".

Temperature monitoring devices of the kind described are more accurate and morestablethan thermocouple cables when the tem peratu res to be monitored do not exceed around 250"C but we have found that, fortemperatures above this figure, there is an undesirable reduction in accuracy and stability.

It is believed that such reduction in accuracy and stability isto a substantial extent due to contamination ofthe platinum or other metal or metal alloy ofthe wire ofthe resistance temperature detector by gases given off attemperatures above about250 C by the mineral insulation ofthe mineral insulated cable and, when the sheath of the cable is of stainless steel, by the stainless steel.

It is an object of the present invention to provide an improved temperature monitoring device of the kind described which is of high accuracy and stability over a temperature range of -200 C to 600"C and which has a substantially longer usefu I I ife than temperature monitoring devices of the aforesaid kind hitherto proposed and used.

According to the invention, in the improved temperature monitoring device each protruding end portion ofthewire of the resist ance temperature detector is welded or otherwise permanently electrically connected to a conductortail of a metal ormetalalloywhichwill not be deleteriously contaminated by gases given off at high temperatures by the mineral insulated electric cable; the resistance temperature detector is housed in a substantially fluid-tight enclosurewhich is made of a material or materials substantially impervious to said gases and from which the conductortails project; the projecting conductortails are welded or otherwise permanently electrically connected to the conductors ofthe mineral insulated cable; and the hermetically sealed resistance temperature detector and the exposed electrically connected conductor tails and cable conductors are enclosed within a hood of metal or metal alloy which is welded or otherwise permanently electrically connected to the sheath ofthe mineral insulated cable.

Preferably, each of the conductor tails projecting from the hermetically sealed resistance temperature detector is made of nickel; other metals or metal alloys ofwhich the conductortails may be made include silver and constantan.

With a view to reducing the risk of damage to the electrical connections between the conductortails andtheconductorsofthemineral insulated cable as a result of vibration, preferably any space within the hood not occupied by the hermetically sealed resistance temperature detector and by the electrically connected conductor tails and cable conductors isfilled with compacted powdered mineral insulation. Forthis purpose, it is preferred to employ compacted powdered aluminium oxide as this material does not give off contaminating gases at high temperatures.

The substantially fluid-tight enclosure in which the resistance temperature detector is housed preferably comprises a sleeve of ceramic material which is sealed at each of its ends with ceramic cement and, preferably also, at leqstthe ceramic end seals have a coating of a glassfritpvhich may have a thermal co-efficient of expansion Bpproximating to the thermal co-efficients of expansion of the material ofthe ceramic seals and of platinum.

In the simplest form ofthe improved temperature monitoring device, two conductor tails protrude from the hermetically sealed resistance temperature detector of the device and are electrically connected to two conductors of a mineral insulated cable.

Preferably, however, one or each of the conductor tails comprises a length ofwire of nickel or other suitable metal or metal alloy which is folded backon itself intermediate of its ends in the shape of a hairpin, the folded end of the conductor tail being welded or otherwise permanently electrically connected to one end portion of the wire ofthe resistance temperature detector and the two ends of thefoldedconductortail being welded orotherwise permanently electrically connected to two conductors of a mineral insulated cable having three orfourelongateconductors (depending on whether one or each of the conductortails is of hairpin form).

Although the connections between the protruding end portions ofthe wire of the resistance temperature detector and the conductor tails may be at the end of the resistance temperature detector nearerthemineral insulated cable' preferably in order to further reduce risk of any contamination of the resistance temperature detector, these connections are at the end ofthe resistance temperature detector remote from the mineral insulated cable, each conductortail passing through one of a number of circumferentially spaced throughbores in the wall ofthe ceramic sleeve or otherfluid-tight enclosure.

The hood surrounding the hermetically sealed resistance temperature detector may be a single body of metal or metal alloy orthe hood may be a sleeve of metal or metal alloyto one end of which a separatelyformed cap or lid of metal or metal alloy is welded or otherwise permanently secured.

Preferably, the hood has an external diameter which is approximately the same as that of the mineral insulated cable, the end faces of the cable sheath and hood abutting and being welded together. To protect the conductors of the mineral insulated cable during the welding operation, preferably a collar of substantially rigid electrically insulating material, e.g. a ceramic material, having a separatethroughbore for each cable conductor is positioned in the end of the cable so that it underlies the abutting end faces of the sheath and hood.

The mineral insulated cable ofthe improved temperature monitoring device may also include two elongate compensating conductors which are welded or otherwise permanently electrically connected together ata position adjacentthe electrical connections between the conductortails and conductors ofthe mineral insulated cable.

The mineral insulated cable ofthe improved temperature monitoring device may further include at least one positive and at least one negative elongate conductor electrically connected together within the cable or hood to form a hot junction. In somecircumstances,themineral insulated cable may have, at each of at least two positions spaced along the length ofthe cable, a hotjunction atwhich positive and negative conductors ofthe cable are welded or otherwise permanently electrically connected together.

A modified form ofthe improved temperature monitoring device may include at leasttwo hermetically sealed resistance temperature detectors as hereinbefore described at longitudinally spaced positions along the length ofthe hood and, in this case, the mineral insulatedcablewill include sufficient conductors for connection to the conductortails ofthe hermetically sealed resistance temperature detectors and the fluid-tight enclosure ofthe detector nearer the cable will have sufficient throughbores in its wall forpassagetherethrough of the conductortails of both or all of the detectors.

The invention also includes, for use in an improved temperature monitoring device according to the invention, an improved hermetically sealed resistance temperature detector as hereinbefore described.

The invention is further illustrated bya description, byway of example, of a preferred hermetically sealed resistance temperature detector and of a preferred temperature monitoring device incorporating the preferred resistance temperature detector with reference to the accompanying diagrammatic drawings in which Figure lisa side view, partly in section and partly in elevation, drawn on an enlarged scale, ofthe preferred hermetically sealed resistance temperature detector, and Figure2 is a side view, partly in section and partly in elevation, drawn on an enlarged scale, ofthe preferred temperature monitoring device.

Referring to Figure 1,the preferred hermetically sealed resistance temperature detector comprises a detector element (not shown) consisting of a length 1 of platinum wire wound around an Wlongateformer in two coils of opposite hand, one overlying the other, and a sleeve (not shown) of electrically insulating material in which the detector element is housed and from one end ofwhich end portions of the platinum wire protrude. The sleeve of electrically insulating material in which the detector element is housed is itself housed in a fluid-tight enclosure 2 of ceramic material which is impervious to gases given off at temperatures above about 250"C by the mineral insulation of a mineral insulated cable.Each protruding end portion ofthe platinum wire 1 is welded to the folded end 4 of a length 3 of nickel wire which is folded back on itself intermediate its ends in the shape of a hairpin to form two conductortails 5.

Each conductor tail 5 passes through one offour circumferentially spaced throughbores (not shown) in the wall of the ceramic sleeve 2 so thatthe conductor tails protrude from the resistance temperature detector at an end remote from that at which the folded end 4 of the nickel wire 3 iswelded to the protruding ends ofthe platinum wire 1. The welded connections between the protruding ends of the platinum wire 1 and thefolded ends 4ofthe nickel wires 3 constituting the conductortails 5 are encapsulated in a body 6 of ceramic cement which overlies the end face of and seals the bore of the ceramic sleeve 2 at that end of the sleeve.At the other end of the ceramic sleeve 2, 0 body7 of ceramic cement overlies the end face of and seals the bore of the ceramic sleeve, the conductor tails S protruding from the seal. Each oftheceramic ideals 6 and 7 has an overall coating of a glass fritwhich has a thermal co-efficient of expansion approximating to the thermal co-efficients of expansion of the ceramic material of the seals and of platinum.

In the preferred temperature monitoring device shown in Figure 2, the fourconductortails Sofa preferred hermetically sealed resistance temperature detector 10, as shown in Figure 1,are welded to the four conductors 12 of a mineral insulated electric cable 11. The resistance temperature detector 10 and the exposed electrically connected conductortails 5 and cable conductors 12 are enclosed within a metal hood 14comprising a metal sleeve 15 which, at one of its ends, abuts and is welded to the end face ofthe cable sheath 13 and which, at the other of its ends, is closed by a separately formed metal lid 16 welded to the sleeve.

To protecttheconductors 12 of the mineral insulated cable 11 during welding ofthe sleeve 15 to the cable sheath 13, a collar 18 of a ceramic material, having a separate throughbore for each cable conductor, is positioned in the end of the cable so that it underlies the abutting end faces ofthe sheath and sleeve. Any space within the hood 14 not occupied by the resistance temperature detector 10 and by the electrically connected conductor tails 5 and cable conductors 12 isfilled with compacted powdered aluminium oxide 19.

The preferred temperature monitoring device of the present invention is of high accuracy and stability over a temperature range of -200"C to 600"C and has a substantially longer useful life than temperature monitoring devices ofthe aforesaid kind hitherto proposed and used.

Claims (18)

1. Atemperaturemonitoring device ofthe kind described, wherein each protruding end portion of the wire of the resistance temperature detector is welded or otherwise permanently electrically connected to a conductortail of a metal or metal alloy which will not be deleteriouslycontaminated by gases given off at high temperatures bythe mineral insulated electric cable; the resistance temperature detector is housed in a substantially fluid-tight enclosure which is made of a material or materials substantially impervious to said gases and from which the conductortails project; the projecting conductor tails are welded or otherwise permanently electrically connected to the conductors ofthe mineral insulated cable; and the hermetically sealed resistance temperature detector and the exposed electrically connected conductor tails and cable conductors are enclosed within a hood of metal or metal alloywhich is welded or otherwise permanently electrically connected to the sheath ofthe mineral insulated cable.

2. A temperature monitoring device as claimed in Claim 1,wherein any space within the hood not occupied by the hermetically sealed resistance temperature detector and by the electrically connected conductortails and cable conductors is filled with compacted powdered mineral insulation.

3. Atemperature monitoring device as claimed in Claim 2, wherein the compacted powdered mineral insulation within the hood is compacted powdered aluminium oxide.

4. Atemperature monitoring device as claimed in any one of the preceding Claims, wherein each ofthe conductortails projecting from the hermetically sealed resistance temperature detector is made of nickel.

5. Atemperature monitoring device as claimed in any one of the preceding Claims, wherein the substantially fluid-tight enclosure in which the resistance temperature detector is housed comprises a sleeve of ceramic material which is sealed at each of its ends with ceramic cement.

6. Atemperature monitoring device as claimed in Claim 5, wherein at leastthe ceramic end seals ofthe fluid-tight enclosure have a coating of a glassfrit.

7. Atemperature monitoring device as claimed in any one ofthe preceding Claims, wherein one or each ofthe conductor tails comprises a length of wire which is folded back on itself intermediate of its ends in the shape of a hairpin, the folded end ofthe conductortail being welded or otherwise permanently electrically connected to one end portion ofthe wire of the resistance temperature detector and the two ends of the folded conductor tail being welded or otherwise permanently electrically connected to two conductors of a mineral insulated cable having at leastthree elongate conductors.

8. Atemperature monitoring device as claimed in any one of the preceding Claims, wherein the connections between the protruding end portions of the wire of the resistance temperature detector and the conductor tails are at the end ofthe resistance temperature detector remote from the mineral insulated cable, each conductor tail passing through one of a number of circumferentially spaced throughbores in the wall ofthefluid-tightenclosure.

9. Atemperature monitoring device as claimed in any one of the preceding Claims, wherein at least two hermetically sealed resistance temperature detectors are located at longitudinally spaced positions along the length ofthe hood and the mineral insulated cable includes sufficient conductorsforconnectiontotheconductortailsof the hermetically sealed resistance temperature detectors and the fluid-tight enclosure of the detector nearerthe cable has sufficient throughbores in its wall for passage therethrough of the conductortails of both or all of the detectors.

10. Atemperature monitoring device as claimed in any one ofthe preceding Claims, wherein the hood is a sleeve of metal or metal alloy to one end of which a separately formed cap or lid of metal or metal alloy is welded or otherwise permanently secured.

11. Atemperature monitoring device as claimed in any one of the preceding Claims, wherein the hood has an external diameterwhich is approximatelythesameasthatofthemineral insulated cable, the end faces of the cable sheath and hood abutting and being welded together.

12. Atemperature monitoring device as claimed in Claim 1 1,wherein a collar of substantially rigid electrically insulating material having a separate throughborefor each cable conductor is positioned in the end of the cable so that it underlies the abutting end faces of the sheath and hood.

13. Atemperature monitoring device as claimed in any one of the preceding Claims, wherein the mineral insulated cable also includes two elongate compensating conductors which are welded or otherwise permanently electrically connected together at a position adjacent the electrical connections between the conductor tails and conductors of the mineral insulated cable.

14. Atemperature monitoring deviceasclaimed in any one ofthe preceding Claims, wherein the mineral insulated cable further includes at least one positive and at least one negative elongate conductor electrically connected together within the cable or hood to form a hotjunction.

15. Atemperature monitoring device as claimed in Claim 14,wherein the mineral insulated cable has, at each of at least two positions spaced along the length of the cable, a hot junction at which positive and negative conductors of the cable are welded or otherwise permanently electrically connected together.

16. For use in a temperature monitoring device according to anyone ofthe preceding Claims, a hermetically sealed resistance temperature detector wherein each protruding end portion ofthe wire of the detector is welded or otherwise permanently electrically connected to a conductor tail of a metal or metal alloy which will not be deleteriously contaminated bygasesgiven offathigh temperatures by a mineral insulated electric cable and wherein the detector is housed in a substantially fluid-tight enclosure which is made of a material or materials substantially impervious to said gases and from which the conductor tails project.

17. For use in atemperature monitoring device according to any one of Claims 1 to 15, a hermetically sealed resistance temperature detector substantially as hereinbefore described with reference to and as shown in Figure 1 ofthe accompanying drawings.

18. Atemperature monitoring device substantially as hereinbefore described with reference to and as shown in Figure 2 ofthe accompanying drawings.