WO1997037391A1

WO1997037391A1 - Manufacture of mineral insulated thermocouple probes

Info

Publication number: WO1997037391A1
Application number: PCT/GB1997/000849
Authority: WO
Inventors: Mark Liddell; Christopher John Huntley
Original assignee: Bicc Public Limited Company
Priority date: 1996-03-29
Filing date: 1997-03-26
Publication date: 1997-10-09
Also published as: AU2169297A; GB9815821D0; GB2324655B; GB9606630D0; GB2324655A

Abstract

A process for the manufacture of a mineral insulated thermocouple probe (1) of predetermined diameter comprises: (i) cutting a length of metal tube (2) to a predetermined length; (ii) sealing one end of the metal tube, for example by capacitance discharge welding; (iii) joining at least one pair of thermocouple wires (4 and 6) formed from dissimilar metals together to form a hot junction and positioning the wires together so that they are substantially parallel to each other with the hot junction at one end thereof; (iv) locating the thermocouple wires inside the tube (2) together with mineral insulant (8) so that the interior of the tube is substantially filled with the mineral insulant and the thermocouple wires, and the hot junction is located in the region of the sealed end of the tube; (v) reducing the diameter of the tube to form a thermocouple having a sheath of the predetermined diameter; and (vi) sealing the opposite end of the thermocouple (the cold junction). The process has the advantage of speed and relatively low scrap rate.

Description

MANUFACTURE OF MINERAL INSULATED THERMOCOUPLE PROBES

This invention relates to the manufacture of mineral insulated thermocouple probes.

Mineral insulated thermocouple probes typically comprise a length of mineral insulated cable comprising one or more pairs, e.g. two pairs, of thermocouple wires formed from dissimilar metals which are surrounded in a mineral insulant, usually magnesium oxide, and enclosed in a metal sheath. At one end of the probe, called the hot end, which is intended to be located in the region whose temperature is to be measured, the ends of the thermocouple wires are welded or brazed together, and the sheath is sealed from the exterior. At the other end, the thermocouple wires are connected to standard conductors or tails, normally formed from copper, to take the signal to the appropriate location. Such thermocouple probes are manufactured by first forming a length of mineral insulated cable, for example 200m in length, cutting the mineral insulated cable to the appropriate length for the thermocouple (which may be anywhere in the range of from 10cm to 10m), and then forming the hot and the cold junctions of the probe. The hot junction is formed by: cutting back a small length of the conductors (normally by about a length equal to the diameter of the sheath), coupling the ends of the conductors for example by capacitance welding, backfilling the end of the sheath with the mineral insulant, inserting a cap formed from a small disc of the sheath metal, and welding the cap to the end of the sheath. The cold junction is formed by threading an appropriate size pot onto the end of the sheath, stripping back a length of the sheath and removing the mineral insulant, brazing the tails onto the thermocouple wires, crimping or brazing the pot in position on the shield, and filling the pot with an epoxy sealant. The initial mineral insulated cable from which the probe is formed is manufactured by inserting the thermocouple rods into a tube of the sheath material and filling the interior of the tube with mineral insulant to form a preform that typically has a diameter of about 12.5 mm and a length of about 2 m. The preform is then subjected to a number of drawing and annealing operations during which its diameter is reduced to about 0.05 to 0.8 cm, and its length increases approximately in proportion to the ratio of the squares of the start diameter to the finishing diameter.

The conventional process for forming such thermocouple probes has a number of disadvantages: First, the process is relatively slow, taking about 15 to 20 minutes per probe in addition to the time taken to manufacture the initial mineral insulated cable. Secondly, the manufacturing process is a complex, multistep process requiring highly skilled operatives. Thirdly, the process normally generates a relatively high scrap rate, for example because of losses due to the need to reswage one end of the cable periodically during the process and the need to remove test samples at the end of the process. Such reasons will normally account for an increase of about 50% in the level of scrap generated in the process.

According to the present invention there is provided a process for the manufacture of a mineral insulated thermocouple probe of predetermined diameter which comprises: (i) cutting a length of metal tube to a predetermined length;

(ii) sealing one end of the metal tube;

(iii) joining at least one pair of thermocouple wires formed from dissimilar metals together to form a hot junction and positioning the wires together so that they are substantially parallel to each other with the hot junction at one end thereof; (iv) locating the thermocouple wires inside the tube together with mineral insulant so that the interior of the tube is substantially filled with the mineral insulant and the thermocouple wires, and the hot junction is located in the region of the sealed end of the tube; (v) reducing the diameter of the tube to form a thermocouple having a sheath of the predetermined diameter; and

(vi) sealing the opposite end (the cold end) of the thermocouple Steps (i) and (ii) may be conducted in either order and, obviously, step (iii) may be conducted before, after or at the same time as steps (i) and (ii). In addition, step (iv) need not be conducted before step (v), but may be conducted at the same time as step

(v).

The insulant may be introduced into the tube by any appropriate means. For example, the insulant may be provided in the form of a powder which is blown into the tube at the same time as, or after, the thermocouple wires are inserted. Preferably, however, the insulant is provided in the form of inserts formed from compacted insulant which are introduced into the tube together with the thermocouple wires. The inserts are preferably in the form of circular cylinders that have a diameter slightly less than the bore of the tube to allow the cylinders to be slid along the tube. In this case, the inserts may be provided with two or more throughbores to allow the thermocouple wires to extend through the inserts. Normally, one of the inserts, which is to be located at the hot junction, will have a configuration that is different from the configuration of the other inserts to allow the insert to enclose the junction between the thermocouple wires. For example, part of the insulant located between the throughbores may be removed from one end of the insert, so that, when the insert has been pushed along the thermocouple wires, the junction is located in the region of the centre of the insert.

The process according to the invention has the advantage that it can be performed relatively quickly as compared with the conventional method of forming mineral insulated thermocouples. For example, it is possible to form a thermocouple probe according to the invention in about 5 to 10 minutes without the necessity to manufacture the mineral insulated cable beforehand. Preferably, the diameter of the tube is reduced in step (v) by a single reducing operation only, thereby reducing the time needed for annealing operations in the manufacture of the probes. In order to achieve this, the diameter of the initial tube is preferably no more than 1.5 times that of the final thermocouple sheath diameter, and especially, from 1.2 to 1.3 times that of the final thermocouple sheath diameter. In view of the relatively small reduction in diameter of the tube, and consequently the relatively small increase in its length, it is possible to calculate relatively easily the length of the metal tube that needs to be cut in order to generate a thermocouple probe of defined length. It is found that the length of tube that needs to be cut in step (i), l₀, is given by equation I:

I where: / is

the final length of the thermocouple; d is the final diameter of the thermocouple; d *_n0 is the initial diameter of the tube; and

k is a factor in the range of from 0.6 to 1 , preferably at least 0.8.

Thus, instead of forming a mineral insulated thermocouple probe from a length of mineral insulated cable, according to the invention the thermocouple probe is always treated as a discrete item. The relatively small ratio by which the diameter of the thermocouple is reduced during manufacture as compared with the reduction ratio that is typically employed in the manufacture of the mineral insulated cable precursor (for instance in the range of from 25 to 50%, preferably from 35 to 45%, means that the initial diameter of the thermocouple wires is relatively small. For example, the thermocouple wires will typically have an initial diameter of 0.3 to 1.0mm, which, after reduction, will correspond to a diameter of from 0.2 to 0.7mm. Thus, in contrast with conventional mineral insulated thermocouple probes which require non-standard rods of thermocouple metal, for example of diameter of about 4mm, the initial thermocouple wires will have a diameter that corresponds to those that are employed in other, non mineral insulated, thermocouples which form the bulk of the thermocouple market, with the result that the range of supply of the thermocouple wire is considerably increased. This is particularly important since, in order to manufacture a Class I thermocouple, account has to be taken of the change in emf output which occurs during the processing. This change in emf output is reduced by minimising the number of drawing and annealing cycles.

The difference between the value of k and unity is due to compaction of the mineral insulant and to changes in the wall thickness, and has the disadvantage that the initial length of tube, l_σ, is increased. The particular value of k, however, is dependent on the processing conditions, and may therefore be increased. Thus, while the particular method by which the diameter of the thermocouple is reduced is not critical to the invention, at least in its broadest aspect, and may, for instance, be achieved by passing the thermocouple through a rotary swaging unit, this can lead to a relatively low value of k. Alternative methods, passing it through reduction rollers of decreasing nip size or passing it through a drawing die may lead to higher values of k. In addition, slower reduction speeds may also be employed. The thermocouple can then be passed directly into an annealing stage in which the temperature is maintained, for example, at 900 to 1 100°C for a period of 2 minutes to 1 hour in the case of stainless steels and nickel based alloys.

After connecting the tails, the cold end of the thermocouple is sealed against moisture ingress. This may be achieved by means of a suitable thermosetting polymer e.g. epoxy, or it may be provided by a glass seal. For example, the insert that is to be located at the cold junction may be formed from a crushable glass instead of from the insulant, and may be crushed during the diameter reduction step before being fired in the annealing step. One process in accordance with the present invention will now be described by way of example with reference to the accompanying drawings, in which

Figure 1 shows schematically a metal tube used for the thermocouple before and after one end thereof is welded to form the hot end; Figure 2 shows schematically assembly of the thermocouple wires and mineral insulant inserts during the process according to the invention;

Figure 3 shows assembly of the wires and inserts with the tube;

Figure 4 shows the diameter reduction step of the thermocouple; and

Figure 5 shows various inserts that can be employed in the process according to the invention

Referring to the accompanying drawings, in order to form a mineral insulated thermocouple probe 1 , a portion of stainless steel tube 2 is first cut to the appropriate length, and the inside of the tube is cleaned. The length of the tube is calculated from the intended final length of the thermocouple probe using equation I using the value for k appropriate to the processing conditions. Thus, to form a thermocouple of lm length and 3mm diameter, a 4mm diameter tube may be cut to a length of 0.77m. After the tube has been cleaned, one end of it is closed by capacitance discharge welding. At the same time, two thermocouple wires 4 and 6 formed from the appropriate material, for example from nickel chromium and nickel-aluminium alloys in the case of a type K thermocouple, each of about 0.77m in length and about 0.6mm diameter, are butt welded together and then bent so that they are parallel to each other with a separation of about 1mm and with the butt weld at one end thereof.

After welding and bending of the thermocouple wires, a number of cylindrical inserts 8 as shown in figure 5 and formed from magnesium oxide (which have previously been stored in an oven in order to prevent absoφtion of moisture) are located on the wires and pushed along the wires until they extend along the entire length thereof. All the inserts with the exception of the first insert are cylindrical having a diameter of about 3.5mm and have either two parallel throughbores 10 and 12 (figure 5a) or four parallel throughbores (figure 5b) extending from one end of the insert to the other end in order to accommodate the thermocouple wires. The first insert, as shown in figures 5c and 5d, has the same general form, but that part of the magnesium oxide 14 between the throughbores 10 and 12 at one end of the insert has been removed over about 40% of the length of the insert to form a region that will enclose the junction between the wires. If desired, a small pellet or quantity of powder 16 formed from magnesium oxide can be inserted into the tube in order to provide insulation in the tip region of the probe, and the thermocouple wires and inserts are inserted into the tube as shown in figure 3. The end of the tube is then nipped down in order to prevent the inserts being pushed out of the tube when it is reduced.

The assembly is then passed to a rotary swager 18 as shown in figure 4 which will reduce the diameter of the tube in one pass from 4mm to 3mm, and after reduction, is annealed. Following the annealing step, the assembly is cleaned and degreased, copper (or other metal) tail leads with the appropriate colour insulation are brazed onto the wires and the cold end of the assembly is sealed against moisture ingress by for example an epoxy resin.

The finished unit is then calibrated to ensure the emf output meets the required specifications, for example IEC 584 and IEC 1515.

Claims

Claims:

1. A process for the manufacture of a mineral insulated thermocouple probe of predetermined diameter which comprises:

(i) cutting a length of metal tube to a predetermined length;

(ii) sealing one end of the metal tube; (iii) joining at least one pair of thermocouple wires formed from dissimilar metals together to form a hot junction and positioning the wires together so that they are substantially parallel to each other with the hot junction at one end thereof;

(iv) locating the thermocouple wires inside the tube together with mineral insulant so that the interior of the tube is substantially filled with the mineral insulant and the thermocouple wires, and the hot junction is located in the region of the sealed end of the tube;

(v) reducing the diameter of the tube to form a thermocouple having a sheath of the predetermined diameter; and

(vi) sealing the opposite end of the thermocouple.

2. A process as claimed in claim 1 , wherein the insulant is provided as a plurality of inserts formed from compacted insulant, which are introduced into the tube together with the thermocouple wires.

3. A process as claimed in claim 2, wherein the inserts are in the form of circular cylinders which are provided with a plurality of throughbores that allow the thermocouple wires to extend therethrough.

4. A process as claimed in claim 2 or claim 3, wherein one insert which is to be located at the hot junction has a configuration that is different from the configuration of the other inserts and which allows the insert to enclose the junction.

5. A process as claimed in any one of claims 1 to 4, wherein the diameter of the tube is reduced in step (v) by a single reducing operation only.

6. A process as claimed in claim 5, wherein the diameter of the initial tube is no more than twice the diameter of the final thermocouple sheath.

7. A process as claimed in any one of claims 1 to 6, which includes an annealing step conducted after step (v).

8. A process as claimed in claim 7, wherein the opposite end of the thermocouple is sealed by introducing an insert formed from crushable glass, crushing the insert during step (v), and fusing the crushed insert during the annealing step.

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