GB2201837A - Thermocouple assembly - Google Patents

Abstract

A thermocouple has an outer metal sleeve 1 with an insulating mineral filling 40 that supports thermocouple junctions 11 to 13. A wire 20 extends from a first terminal 5 to the forward end of the sleeve 1 where it branches into shorter wires 21 to 23 of the same material which extend rearwardly by different distances to the respective junctions 11 to 13 from which wires 31 to 33 of material of the opposite polarity extend in parallel to a second terminal 6. The output across the two terminals is a measure of the average temperature of the region R over which junctions 11 to 13 are located. The different resistance between the two terminals 5 and 6 via the different junctions 11 to 13 that would otherwise be produced, because of the difference in resistivity of the two materials forming each junction, is compensated by varying the diameters of the wires 31 to 33 or 21 to 23. <IMAGE>

Description

THERMOCOUPLE ASSEMBLIES This invention relates to thermocouple assemblies.

The invention is more particularly concerned with thermocouple assemblies having several thermocouple junctions at different locations and connected together to give an average indication of temperature at those locations.

The usual arrangement for such averaging thermocouple assemblies is to employ a separate pair of thermocouple wires for each junction, each pair of wires extending along a respective metal sleeve. A compacted insulating mineral filling in each sleeve insulates the wires from each other rearwardly from the junction, and insulates the wires from the sleeve. At their rear end, each pair of thermocouple wires is connected to two common output terminals which provide the averaged thermocouple assembly output. The sleeves for each junction may be contained within a common outer protective sleeve.

One problem with this arrangement is that the individual sleeves encasing each junction can lead to a relatively bulky and heavy construction.

Also, the junctions are isolated from the environment by two sleeves and their insulating filling which leads to a significant thermal lag. This can be .4 a disadvantage when measuring rapidly changing temperatures. Also, each.

junction requires two wires to extend along the length of the probe to each junction. Because each wire must be insulated from others, this leads to a relatively large diameter of the probe. The wires used in thermocouple junctions can be made from expensive materials so it is desirable to reduce the amount of wire used.

It is an object of the present invention to provide an improved thermocouple assembly.

According to one aspect of the present invention there is provided a thermocouple assembly comprising an outer sleeve within which are contained a plurality of thermocouple junctions, the assembly including a first conductor that extends forwardly from a first terminal, along the length of the outer sleeve to a location towards the forward end of the sleeve, the first conductor being connected to each said junction via respective first wires of a first material the same as said first conductor, the junctions being formed by joining the first wires to respective second wires of a second material of opposite polarity to said first material and of different lengths from each other, the second wires each being joined at their rear end to a second terminal of the assembly, and the cross-sectional area of the first and second wires being selected such that the resistance between the first and second terminals via each junction is substantially equal.

The cross-sectional area of the second wires may differ from one another in such a way that the resistance between the first and second terminals via each junction is substantially equal. The first and second wires may be of circular section. The junctions are preferably supported in the outer sleeve by a filling in the sleeve of a compacted, electrically-insulative powder.

The first or second wires may be of nickel chromium or nickel aluminium.

Preferably the first wire is of nickel chromium and the second wire is of nickel aluminium.

A thermocouple assembly for a gas-turbine engine in accordance with the present invention will now be described, by way of example, with reference to the accompanying drawing, which is a sectional elevation view of the assembly.

The thermocouple assembly comprises an outer metal sleeve 1 that is closed at both its forward and rear ends 2 and 3. The rear end 3 is provided with an annular mounting flange 4 by which the assembly is mounted with the housing (not shown) of a gas-turbine engine so that the forward end 2 projects into the region of the engine the temperature of which is to be measured. The assembly has three thermocouple junctions 11, 12 and 13 contained within it which are connected to two electrical terminals 5 and 6 at the rear end of the assembly and by which external electrical connection of the assembly is made.

The three junctions 11 to 13 are spaced apart from one another over a region R along the length of the sleeve 1 and provide, at the terminals 5 and 6, an output representative of the average temperature within this region R.

The junctions 11 to 13 are formed and connected with the terminals 5 and 6 in the following way. A single first conductor 20 of a material of one polarity, such as nickel chromium, is welded at its rear end to the terminal 5. The conductor 20 extends to the forward end 2 of the sleeve 1 where it is welded to three shorter wires 21, 22 and 23 of an identical material to that of the first wire but different lengths. The wires 21 to 23 extend rearwardly of the sleeve 1 to respective ones of the junctions 11 to 13 and are therefore each of a different length. The junctions 11 to 13 are formed by welding the wires 21 to 23 to respective rear wires 31 to 33 of a polarity opposite to that of the wires 21 to 23, such as nickel aluminium. Each of the three rear wires 31 to 33 extend in parallel rearwardly of the sleeve and are welded to the other terminal 6.

Because the resistivity of the materials forming the forward and rear wires 21 to 23 and 31 to 33 will in general be different, and because the lengths of the forward and rear wires differ for each junction, the combined resistance of each pair would be different if the wires had the same cross-sectional area. In the present arrangement, the combined resistance of each pair of wires 21 and 31, 22 and 32, and 23 and 33, and hence the resistance between the two terminals 5 and 6 via each junction 11 to 13; are equalised. This is achieved by varying the cross-sectional area of at least one of the wires in each pair from that of the corresponding wires in the other pairs. In general, the wires are of circular section so this is achieved by selecting wires of the appropriate diameter.For example, if the material forming the rear wires 31 to 33 has a higher resistivity than the material forming the forward wires 21 to 23 this would, with equal diameter wires, make the resistance of the pair 21 and 31 higher than that of the other two pairs. To overcome this, the diameter of the wire 31 is selected to be greater than that of the wire 32, and the diameter of the wire 32 is selected to be greater than that of the wire 33. Alternatively, the diameter of wire 21 could be selected to be greater than that of wire 22, and the diameter of wire 22 selected to be greater than that of wire 23. It would also be possible to produce an equal resistance of each pair of wires by selecting different diameters of both wires in each pair.

The wires 20 to 23 and 31 to 33 are supported within the sleeve 1 along a major part of their length by a filling 40 in the sleeve of compacted electrically-insulative mineral powder such as, aluminium oxide. This protects the junctions 11 to 13 from vibration and prevents the wires from contacting each other or the sleeve 1.

Because the junctions 11 to 13 are contained within a common outer sleeve, without respective individual sleeves, as in some previous assemblies, the thermal lag, weight, size and cost of the assembly can be reduced.

Also, it will be seen that the number of wires used is less than with previous assemblies in which each junction is connected to both terminals via respective, individual wires. In the arrangement described, it can be seen that, for three junctions, only four lengths of wire are required to extend along the sleeve. This can lead to a saving in material costs of the assembly and can enable a sleeve of smaller diameter to be used.

It will be appreciated that various modifications could be made. For example the thermocouple junctions could be formed by combinations of any other thermocouple materials, such as platinum and platinum-rhodium or Nicrosil and Nisil.

Any number of junctions could be used arranged in any disposition within the outer sleeve.

Claims (9)

1. A thermocouple assembly comprising an outer sleeve within which are contained a plurality of thermocouple junctions, wherein the assembly includes a first conductor that extends forwardly from a first terminal, along the length of the outer sleeve to a location towards the forward end of the sleeve, wherein the first conductor is connected to each said junction via respective first wires of a first material the same as said first conductor, wherein the junctions are formed by joining the first wires to respective second wires of a second material of opposite polarity to said first material and of different lengths from each other, wherein the said wires are each joined at their rear end to a second terminal of the assembly, and wherein the cross-sectional area of the first and second wires is selected such that the resistance between the first and second terminals via each junction is substantially equal.

2. A thermocouple assembly according to Claim 1, wherein the cross-sectional area of the second wires differ from one another in such a way that the resistance between the first and second terminals via each junction is substantially equal.

3. A thermocouple assembly according to Claim 1 or 2, wherein the first and second wires are of circular section.

4. A thermocouple assembly according to any one of the preceding claims, wherein the said junctions are supported in the outer sleeve by a filling in the sleeve of a compacted, electrically-insulative powder.

5. A thermocouple assembly according to any one of the preceding claims, wherein the first or second wires are of nickel chromium.

6. A thermocouple assembly according to any one of Claims 1 to 4, wherein the first or second wires are of nickel aluminium.

7. A thermocouple assembly according to any one of Claims 1 to 4, wherein the first wires are of nickel chromium and the second wires are of nickel aluminium.

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

9. Any novel feature or combination of features as hereinbefore described.