GB2129210A - Thermocouples - Google Patents

Abstract

A thermocouple junction is made in a thermocouple type mineral insulated cable such that open circuit tails (10a, 11a) extend beyond the junction (12). <IMAGE>

Description

SPECIFICATION Thermocouples This invention relates to thermocouples.

It is known to form a thermocouple junction by taking a thermocouple type mineral insulated cable having an outer sheath, a pair of alloy wires, and mineral insulation; cutting the cable and exposing the alloy wire pair; welding the cut ends of the alloy wire pair; and closing the cut cable off with a sealing end cap.

Such junctions are found to fail when subjected to thermal cycling in excess of 4500C. Such failures become more common when the cable has a diameter of less than 1 mm and is thought to be stimulated by ambient air reaching the finished junction, the air travelling down newly cut cable unexpectedly quickly.

It is an object of the present invention to provide new methods of forming thermocouple junctions in thermocouple type mineral insulated cable which have improved reliability.

According to the present invention a thermocouple junction is formed in thermocouple type mineral insulated cable by forming a junction having open circuit tails extending beyond the junction. Preferably the tails have an initial length of at least 200 times the diameter of the cable but they can be cropped as the last act of their manufacture.

The tailed junction may be made before forming the cable or it may be made in the formed cable using known techniques.

The invention will now be described further with reference to the accompanying drawings in which: Figure 1 shows, in section, the formation of a tailed junction before forming the cable, and Figure 2 shows in section the formation of a tailed junction after forming the cable.

In Figure 1 a pair of alloy nickel/ chromium:nickel aluminium thermocouple wires 10, 11 are joined at a junction 12 and beyond which extends a pair of open tails 1 Oa, 1 1 a. Both the wires and the tails are insulated from each other with insulating beads 13, except at the junctions 12, and the beads and wires are then enclosed in a metal sheath 14. The diameter of the cable so formed is reduced by swaging or drawing through a die. This also compacts the insulation provided by the beads 13. With the cable having a diameter of 1 mm the length of the tails is chosen to be 250mm but these are subsequently cropped.

In Figure 2 a known form of 1 mm diameter thermocouple type mineral insulated cable 20 having a pair of wires 21, 22 and insulation 23 is cut and the end closed off by a cap 24 leaving the wires 21,22 in open circuit. The cable is then subjected to a temperature of 10000C in a narrow zone 25 situated about 250mm from the cap 24 and a voltage pulse is applied to the wires.

This creates a thermocouple junction 26, by diffusion of the metal of the wires into the insulation, with tails 21 a, 22a.

The cables formed as above can be sealed at their connecting ends with epoxy seals in the known manner.

Claims

1. A thermocouple junction in a thermocouple type mineral insulated cable having open circuit tails extending beyond the junction.

2. A method of making a thermocouple junction as claimed in claim 1 comprising making a junction between a pair of thermocouple wires such that the tails extend beyond the junction, insulating the wires and tails with beads in a sheath, and reducing the diameter of the sheath with simultaneous compaction of the beads and cropping the tails.

3. A method of making a thermocouple junction as claimed in claim 1 comprising taking a thermocouple type mineral insulated cable, heating said cable at a zone remote from the end and subjecting the wires of the cable to a voltage pulse to create a junction at the heated zone with tails extending beyond the junction and cropping the tails.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (3)

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Thermocouples This invention relates to thermocouples. It is known to form a thermocouple junction by taking a thermocouple type mineral insulated cable having an outer sheath, a pair of alloy wires, and mineral insulation; cutting the cable and exposing the alloy wire pair; welding the cut ends of the alloy wire pair; and closing the cut cable off with a sealing end cap. Such junctions are found to fail when subjected to thermal cycling in excess of 4500C. Such failures become more common when the cable has a diameter of less than 1 mm and is thought to be stimulated by ambient air reaching the finished junction, the air travelling down newly cut cable unexpectedly quickly. It is an object of the present invention to provide new methods of forming thermocouple junctions in thermocouple type mineral insulated cable which have improved reliability. According to the present invention a thermocouple junction is formed in thermocouple type mineral insulated cable by forming a junction having open circuit tails extending beyond the junction. Preferably the tails have an initial length of at least 200 times the diameter of the cable but they can be cropped as the last act of their manufacture. The tailed junction may be made before forming the cable or it may be made in the formed cable using known techniques. The invention will now be described further with reference to the accompanying drawings in which: Figure 1 shows, in section, the formation of a tailed junction before forming the cable, and Figure 2 shows in section the formation of a tailed junction after forming the cable. In Figure 1 a pair of alloy nickel/ chromium:nickel aluminium thermocouple wires 10, 11 are joined at a junction 12 and beyond which extends a pair of open tails 1 Oa, 1 1 a. Both the wires and the tails are insulated from each other with insulating beads 13, except at the junctions 12, and the beads and wires are then enclosed in a metal sheath 14. The diameter of the cable so formed is reduced by swaging or drawing through a die. This also compacts the insulation provided by the beads 13. With the cable having a diameter of 1 mm the length of the tails is chosen to be 250mm but these are subsequently cropped. In Figure 2 a known form of 1 mm diameter thermocouple type mineral insulated cable 20 having a pair of wires 21, 22 and insulation 23 is cut and the end closed off by a cap 24 leaving the wires 21,22 in open circuit. The cable is then subjected to a temperature of 10000C in a narrow zone 25 situated about 250mm from the cap 24 and a voltage pulse is applied to the wires. This creates a thermocouple junction 26, by diffusion of the metal of the wires into the insulation, with tails 21 a, 22a. The cables formed as above can be sealed at their connecting ends with epoxy seals in the known manner. Claims

1. A thermocouple junction in a thermocouple type mineral insulated cable having open circuit tails extending beyond the junction.

2. A method of making a thermocouple junction as claimed in claim 1 comprising making a junction between a pair of thermocouple wires such that the tails extend beyond the junction, insulating the wires and tails with beads in a sheath, and reducing the diameter of the sheath with simultaneous compaction of the beads and cropping the tails.

3. A method of making a thermocouple junction as claimed in claim 1 comprising taking a thermocouple type mineral insulated cable, heating said cable at a zone remote from the end and subjecting the wires of the cable to a voltage pulse to create a junction at the heated zone with tails extending beyond the junction and cropping the tails.