Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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This invention relates to a hot junction element
for a high temperature thermocouple 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 thermo- -
couple 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 conditions 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 very 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. Patent Specification 1,022,172 of Thermal
Syndicate Limited, filed June 22, 1961 describes a hot junction
element for a high temperature thermocouple which comprises
a hollow tube of vitreous silica material, a pair of dis-
similar thermocouple wires forming a thermocouple junction
sealed within the tube and each connected to a different con-
tact member extending from an end of the tube.
The junction element described in Specification
1,022,172 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 con-
struction does not lend itself to automatic production
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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 Patent Specification 1,600,350 of Meci Materiel
Electrique de Controle et Industriel, filed December 31, 1968,
describes a hot junction element in which the thermocouple wires
are dispose~d loosely within the tube but are separated between
the thermocouple 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
utilized 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 said region, forming an assembly from two com-
pensating pins and two thermocouple wires with the wires elec-
trically connected at a junction adjacent one end and each elec-
trically 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 -~
-- 2 --
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silica, a pair of dissimilar thermocouple wires forming a
thermocouple junction wit~nthe 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 formed from the tube wall and is characteri- ;-
zed in that the barrier means is formed, prior to the inser-
tion of the wires in the tube, by the wall of a melt passage
extending across the bore of the tube.
10The 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 pro-
jecting through the end of the tube, to which the end of the
appropriate thermocouple wire is connected (e.'g. crimped or
20 weldedl. 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 element in accordance
with the invention and a method 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 eIectrical com-
ponents of the element prior to insertion in the vitreous
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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 junc-
tion 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 be-
tween 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 into the tube 1 from above until the junction 7 is disposed
just above (or even resting on) the barrier 9a, pinching the
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tube lightly around the regions 3_ 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 500C to 1800C. 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 of 0.075 mm diameter
and is a 13% rhodium/87% platinum alloy. The overall length
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 3mm 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. Crim-
ping 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.
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