GB1585077A - Thermocouple element - Google Patents

Description

(54) IMPROVED THERMOCOUPLE ELEMENT We, THERMAL SYNDICATE LIMITED, a British Company of P.O.Box 6, Neptune Road, Wallsend, Tyne & Wear NE28 6DG, England, do hereby declare the invention, for which we pray that a Patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following Statement: This invention relates to a hot junction element cement for a high temperature thermo- couple measuring circuit.It is well known that the temperatures of molten metal baths can be determined by a thermocouple measuring circuit, which includes a hot junction element which is dipped into the bath, the element incorporating the dissimilar thermocouple wires which meet at the junction and contact members by which the thermocouple wires are electrically connected to the remainder of the measuring circuit. It is conventional to construct such junction elements as cheaply as possible since their service life under the extreme operating conditons is normally not more than a few dips and not infrequently a junction element of this type is used for just one dip and then discarded.

This invention relates to an improved disposable hot junction element which in its preferred embodiments enables reduced quantities of thermocouple wires to be used (and since these are usually made from new expensive metals, a small saving in the quantity used can significantly affect the cost of an element) reduces the response time of the element and reduces the unit cost of the element by reducing the number of reject elements produced.

U.K. Specification 1022172 describes a hot junction element for a high temperature thermocouple which comprises a hollow tube of vitreous silica material, a pair of dissimilar thermocouple wires forming a thermocouple junction sealed within the tube and each connected to a different contact member extending from an end of the tube.

The junction element described in Specification 1022172 leaves the junction of the thermocouple wires exposed and tensions the thermocouple wires within the tube to keep them separated within the tube. This form of construction does not lend itself to automatic production techniques, the manufacture being difficult and resulting in a high proportion of unsaleable junction elements, the cost of which adds to the unit cost of the saleable elements.

French Specification 1,600,350 describes a hot junction element in which the thermocouple wires are disposed loosely within the tube but are separated between the termocouple junction and the contact members by a dimple formed in the tube wall after the wires have been located therein.

To prevent the wires contacting the hot tube wall while the dimple is being formed, electromagnetic forces are utilised to urge the wires outwardly, away from each other.

However even with the wires separated there is a risk of the wires being damaged during this dimple-forming operation resulting in the production of unsaleable elements.

According to one aspect of the invention there is provided a method of manufacturing a hot junction element for a high temperature thermocouple circuit comprising the steps of forming between the ends of a vitreous silica tube a deformed region which includes a melt passage extending across the tube in said region and which leaves passages within the tube on either side of the said region, forming an assembly from two compensating pins and two thermocouple wires with the wires electrically connected at a junction adjacent one end and each electrically connected at the opposite end to a respective pin, inserting the assembly, pins first, into said passages so that the ends of the pins remote from the wires project from one end of the tube and the wires, intermediate the junction and the pins, are separated within the tube by said deformed region, securing the pins within the tube adjacent to said one end of the tube and closing the opposite end of the tube to complete the element.

According to a further aspect of the invention a hot junction element comprises a hollow tube of vitreous silica, a pair of dissimilar thermocouple wires forming a thermocouple junction within the tube and each connected to a different contact member extending from an end of the tube, the thermocouple wires being disposed loosely within the tube and separated between the junction and the contact members by a barrier means and is characterised in that the barrier means is formed, prior to the insertion of the wires'in the tube, by the wall of a melt passage extending across the bore of the tube.

The dissimilar thermocouple wires are conveniently very fine wires of metals or alloys of nickel, chromium, aluminium, tungsten, molybdenum, platinum, rhodium or platinum/ rhodium alloys which can be twisted together or welded to form the thermocouple junction.

The use of a connecting conducting bridge between the ends of the thermocouple wires at the thermocouple junction is not ruled out.

The contact members may be spaced-apart pins projecting through the end of the tube, to which the end of the appropriate thermocouple wire is connected (e.g. crimped or welded). The contact members may also be of dissimilar metals (e.g. copper and copper/ nickel) to act as compensating leads in a manner well known in the art. Further the contact members may be physically distinguishable one from the other (as by length or cross-sectional area) to ensure correct polarity connection to the rest of the measuring circuit.

By making the deformed region into a melt passage the response time of the junction element is reduced.

One embodiment of hot junction in accordance with the invention and a rriethod in which it can be constructed will now be described, by way of example, with reference to the accompanying drawing, in which: Figure 1 is a side elevation of the electrical components of the element prior to insertion in the vitreous silica tube, Figures 2 and 3 are, respectively, a side elevation and an end elevation of the vitreous silica tube used with the components of Figure 1 to make a complete junction element, and Figure 4 is a sectional view of the complete junction element.

Referring to Figure 4, the complete junction element comprises a tube 1 of transparent vitreous silica closed by pinching at 2 and pinched around compensating pins 3 and 4 at the other end. The pins 3 and 4 are slightly flattened over regions 3a and 4a to enhance their securement in the pinched tube.

Thermocouple wires 5 and 6 connect the pins 3 and 4 to a thermocouple junction 7 which is located close to the pinch 2 but within the interior of the tube. Welding is used to bond the wires to each other at the junction 7 and to bond each wire to its pin.

The pins are of different gauge to ensure that the element can only be plugged into its receiving socket (and thus connected into the remainder of the measuring circuit) one way round. A refractory cement 8 fills the lower end of the tube 1 more securely to bond the pins to the tube and to thermally insulate the junctions between the wires and the pins.

To space the wires 5 and 6 apart in the region between the pins and the junction 7, a hole 9 is provided through the tube 1 to define an insulating barrier 9a between the wires 5 and 6.

The element shown in Figure 4 is manufactured from the parts shown in Figures 1 and 2 by lowering the pins 3 and 4 onto the tube 1 from above until the junction 7 is disposed just above (or even resting on) the barrier 9a, pinching the tube lightly around the regions 3a and 4a, injecting the cement 8 into the lower end of the tube and finally pinching the end 2 to close the tube.

The element illustrated in Figure 4 is designed for temperature measurements in the range 5000Cto 18000C. Pin 3 is of 1.42 mm diameter and is made of copper/nickel alloy, pin 4 is of 1.63 mm diameter and is made of copper, wire 5 is of 0.075 mm diameter platinum and wire 6 is 0.075 mm diameter and is a 13% rhodium!87% platinum alloy.

The overall lengiil of the oval cross-section tube 1 is 45 mm and the undistorted bore is 8-9 mm in the direction of the major axis and 2-3 mm in the direction of the minor axis. The hole 9 has a bore of approximately 3 mm and the cement 8 (e.g. Plaster of Paris) extends 3 mm beyond the upper ends of the pins 3 and 4.

The hole allows molten metal to come close to the junction 7 and thereby reduces the time taken for the element to record the true temperature of a melt after having been first dipped into the melt.

The volume within the tube may be sealed but a small gas vent can be provided (to reduce pressure rise on dipping) and this may be incorporated in the cement 8.

When the wires 5 and 6 become very fine, welding them to the more robust pins 3 and 4 may pose problems. Crimping is an alternative fixing means which can be used.

Although the embodiment illustrated employs a tube 1 of oval cross-section as the starting material this is not essential and other cross-sectional shapes can be employed.

WHAT WE CLAIM IS: 1. A hot junction element for a high temperature thermocouple comprising a hollow tube of vitreous silica, a pair of dissimilar thermocouple wires forming a thermocouple junction within the tube and each connected to a different contact member extending from an end of the tube, the thermocouple wires being disposed loosely within the tube and separated between the junction and the contact members by a barrier means, characterised in that the barrier means is formed, prior to the insertion of the wires in the tube, by the wall of a melt passage extending across the bore of the tube.

2. A hot junction element as claimed in Claim 1, in which the contact members are

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

Claims (8)

**WARNING** start of CLMS field may overlap end of DESC **. tube and separated between the junction and the contact members by a barrier means and is characterised in that the barrier means is formed, prior to the insertion of the wires'in the tube, by the wall of a melt passage extending across the bore of the tube. The dissimilar thermocouple wires are conveniently very fine wires of metals or alloys of nickel, chromium, aluminium, tungsten, molybdenum, platinum, rhodium or platinum/ rhodium alloys which can be twisted together or welded to form the thermocouple junction. The use of a connecting conducting bridge between the ends of the thermocouple wires at the thermocouple junction is not ruled out. The contact members may be spaced-apart pins projecting through the end of the tube, to which the end of the appropriate thermocouple wire is connected (e.g. crimped or welded). The contact members may also be of dissimilar metals (e.g. copper and copper/ nickel) to act as compensating leads in a manner well known in the art. Further the contact members may be physically distinguishable one from the other (as by length or cross-sectional area) to ensure correct polarity connection to the rest of the measuring circuit. By making the deformed region into a melt passage the response time of the junction element is reduced. One embodiment of hot junction in accordance with the invention and a rriethod in which it can be constructed will now be described, by way of example, with reference to the accompanying drawing, in which: Figure 1 is a side elevation of the electrical components of the element prior to insertion in the vitreous silica tube, Figures 2 and 3 are, respectively, a side elevation and an end elevation of the vitreous silica tube used with the components of Figure 1 to make a complete junction element, and Figure 4 is a sectional view of the complete junction element. Referring to Figure 4, the complete junction element comprises a tube 1 of transparent vitreous silica closed by pinching at 2 and pinched around compensating pins 3 and 4 at the other end. The pins 3 and 4 are slightly flattened over regions 3a and 4a to enhance their securement in the pinched tube. Thermocouple wires 5 and 6 connect the pins 3 and 4 to a thermocouple junction 7 which is located close to the pinch 2 but within the interior of the tube. Welding is used to bond the wires to each other at the junction 7 and to bond each wire to its pin. The pins are of different gauge to ensure that the element can only be plugged into its receiving socket (and thus connected into the remainder of the measuring circuit) one way round. A refractory cement 8 fills the lower end of the tube 1 more securely to bond the pins to the tube and to thermally insulate the junctions between the wires and the pins. To space the wires 5 and 6 apart in the region between the pins and the junction 7, a hole 9 is provided through the tube 1 to define an insulating barrier 9a between the wires 5 and 6. The element shown in Figure 4 is manufactured from the parts shown in Figures 1 and 2 by lowering the pins 3 and 4 onto the tube 1 from above until the junction 7 is disposed just above (or even resting on) the barrier 9a, pinching the tube lightly around the regions 3a and 4a, injecting the cement 8 into the lower end of the tube and finally pinching the end 2 to close the tube. The element illustrated in Figure 4 is designed for temperature measurements in the range 5000Cto 18000C. Pin 3 is of 1.42 mm diameter and is made of copper/nickel alloy, pin 4 is of 1.63 mm diameter and is made of copper, wire 5 is of 0.075 mm diameter platinum and wire 6 is 0.075 mm diameter and is a 13% rhodium!87% platinum alloy. The overall lengiil of the oval cross-section tube 1 is 45 mm and the undistorted bore is 8-9 mm in the direction of the major axis and 2-3 mm in the direction of the minor axis. The hole 9 has a bore of approximately 3 mm and the cement 8 (e.g. Plaster of Paris) extends 3 mm beyond the upper ends of the pins 3 and 4. The hole allows molten metal to come close to the junction 7 and thereby reduces the time taken for the element to record the true temperature of a melt after having been first dipped into the melt. The volume within the tube may be sealed but a small gas vent can be provided (to reduce pressure rise on dipping) and this may be incorporated in the cement 8. When the wires 5 and 6 become very fine, welding them to the more robust pins 3 and 4 may pose problems. Crimping is an alternative fixing means which can be used. Although the embodiment illustrated employs a tube 1 of oval cross-section as the starting material this is not essential and other cross-sectional shapes can be employed. WHAT WE CLAIM IS:

1. A hot junction element for a high temperature thermocouple comprising a hollow tube of vitreous silica, a pair of dissimilar thermocouple wires forming a thermocouple junction within the tube and each connected to a different contact member extending from an end of the tube, the thermocouple wires being disposed loosely within the tube and separated between the junction and the contact members by a barrier means, characterised in that the barrier means is formed, prior to the insertion of the wires in the tube, by the wall of a melt passage extending across the bore of the tube.

2. A hot junction element as claimed in Claim 1, in which the contact members are

pins pinched into the tube.

3. A hot junction element as claimed in claim 1 or claim 2, in which the pins are of dissimilar metals and act as compensating leads for the thermocouple wires.

4. A hot junction element as claimed in claim 3, in which the pins are physically distinguishable one from the other.

5. A method of manufacturing a hot junction element for a high temperature thermocouple circuit comprising the steps of forming between the ends of a vitreous silica tube a deformed region which includes a melt passage extending across the tube in said region and which leaves passages within the tube on either side of said region, forming an assembly from two compensating pins and two thermocouple wires with the wires electrically connected at a junction adjacent one end and each electrically connected at the opposite end to a respective pin, inserting the assembly, pins first, into said passages so that the ends of the pins remote from the wires project from one end of the tube and the wires, intermediate the junction and the pins, are separated within the tube by said deformed region, securing the pins within the tube adjacent to said one end of the tube and closing the opposite end of the tube to complete the element.

6. A method as claimed in claim 5, in which the pins are secured in place by pinching the tube around them and allowing a refractory cement to set in the tube around the pins and around a few millimetres of the wires closest to the pins.

7. A method of manufacturing a hot junction element for a high temperature thermocouple substantially as hereinbefore described with reference to the accompanying drawing.

8. A hot junction element for a high temperature thermocouple substantially as hereinbefore described with reference to, and as illustrated in, the accompanying drawing.