GB2243484A - Thermocouple flame sensing device - Google Patents

Abstract

A thermocouple flame sensing device comprises a first electrical conductor element (25) disposed within a tubular second electrical conductor element (29) and connected together at one end to define a 'hot end' of the device, said elements having connected thereto an internal core lead (23) and an external tubular lead (21) mutually spaced one within the other and extending from said elements to a sealed end forming the 'cold end' of the device, the elements and leads of the device having a dry inert gas e.g. helium confined therebetween to fill an internal space (33) thereby excluding corrosive gases and or vapours. A sheath (30) of silicone resin or aluminium may cover the probe tip and limit CFC corrosion and oxygen ingress. A differential pressure seal P may produce a material flow into the tube to effect a hermetic seal. The device may be non-destructively tested using a helium mass spectrometer leak detector. <IMAGE>

Description

Improvements in or relating to temperatlre probes This invention relates to temperature probes and, more particularly, to improvements in or relating to thermocouples.

Thermocouples are commonly used as a safety device in equipment such as domestic heating boilers which have a pilot flame which is intended to remain lit at all times whilst gas is being delivered to the main burner.

The tip or 'hot end' of the thermocouple is placed directly in or in the region of the pilot flame and, whilst the flame is lit, the thermocouple produces sufficient electro-motive force (e.m.f.) to hold open a pilotvalve which allows gas to flow to the main burner. In the event of the pilot flame extinguishing, the 'hot end' of the thermocouple is no longer heated by the flame, the e.m.f. decreases and the pilot-valve closes, thus cutting-off gas to the main burner and preventing gas leaks.

As it will be understood from the above, failure of a thermocouple prevents the flow of gas to the main burner since the pilot valve remains closed and the burner system cannot function. A major cause of failure of thermocouples is corrosion of the core due to the ingress of oxygen into the thermocouple both via the 'hot end' and, at the other end, the 'cold end'. Furthermore, corrosion of the thermocouple can be caused by the presence in the vicinity of the thermocouple of chloro-fluoro-carbons (CFC's) which react with the thermocouple in the region of the pilot flame.

Various sealing arrangements for thermocouples have been proposed which give a limited protection but act merely to delay the ingress of oxygen and the resulting failure of the thermocouple.

In addition, no practical method exists for non-destructive 100% testing of thermocouples for leakage.

The present invention aims to provide an improved thermocouple flame sensing device capable of being tested non-destructively, and a method of producing the same, which overcomes, or at least mitigates, the above disadvantages.

According to one aspect of the present invention there is provided a thermocouple flame sensing device comprising a first electrical conductor element disposed within a tubular second electrical conductor element so as to define an annular space there between and connected one to the other at one end to make up a thermally responsive probe forming the 'hot end' of the device and capable of developing an e.m.f., said elements respectively having joined thereto an internal core lead and an external tubular lead mutually spaced one within the other and extending from said elements to a sealed end forming the 'cold end' of the device, the said probe and leads having a dry inert gas confined there between to fill the internal space and thereby exclude corrosive gases and or vapours.

Preferably, the temperature probe further comprises a sheath covering the probe tip which prevents or, at least, limits corrosion due to CFC gases and the ingress of oxygen through the probe tip.

Conveniently, the sheath is formed of high temperature silicone resin or silicone resin mix.

Alternatively, the sheath may be formed of aluminium.

Advantageously a differential pressure produces a material flow into the tube which effects a seal at this end of the tube to retain the dry inert gas in the tube.

Preferably, the differential pressure seal is an hermetic seal.

Conveniently, the inert gas is helium.

According to a second aspect of the present invention there is provided a method of forming a thermocouple flame sensing device comprising the steps of placing a first electrical conductor element within a tubular second electrical conductor element so as to define an annular space and connecting the elements one to another at one end to make up a thermally responsive probe forming the 'hot end' of the device and capable of developing an e.m.f.; joining said elements respectively to an internal core lead and an external tubular lead mutually spaced one within the other and extending from said elements to a sealed end forming the 'cold end' of the device, and filling the internal space within the probe and leads with a dry inert gas thereby allowing each device to be non-destructively tested and simultaneously excluding corrosive gases or vapours from the thermocouple flame sensing device.

In order that the present invention may be more easily understood and so that further features thereof may be appreciated, the present invention will now be described, by way of example, with reference to the accompanying figures, in which: Figure 1 is a cross-section of a known thermocouple, and figure 2 is a cross-section of a thermocouple flame sensing device according to one aspect of the present invention.

In these figures, the known thermocouple of figure 1 comprises a metal tube 1 forming one lead of the thermocouple and a wire 3 passing through the tube 1 and forming the other lead of the thermocouple. The wire 3 is welded to a conductor rod 5 within the tube 1 which is, in turn, connected to the tip 9 of the thermocouple via a weld bead 11. The tip 9 is, in turn, welded to the tube 1. The junction of the conductor rod 5 with the tip 9 constitutes the 'hot end' of the thermocouple In the vicinity of the other or 'cold end' of the thermocouple flame sensing device, the wire 3 is surrounded by an insulating sheath 12 to prevent short circuiting of the thermocouple by contact between the tube 1 and the wire 3. The space 13 between the insulating sheath 12 and the tube 1 is evacuated to remove as much oxygen from the thermocouple as possible.

A washer 15 surrounds the wire 3 at the point where the wire 3 exits the tube 1. The washer 15 is held in contact with the tube 1 by a metal button 17 which is soldered into position 19.

The arrows A, B and C in figure 1 identify the major points of entry for oxygen and CFC's into the thermocouple flame sensing device which leads to corrosion.

Turning to figure 2, there is shown a cross-section of a thermocouple flame sensing device according to one aspect of the present invention, in which a tube 21 and wire 23 form the two leads of the thermocouple. The tube 21 and wire 23 are preferably formed of copper, however they may be formed of different or dissimilar metals. The wire 23 is welded to a conductor rod 25 within the tube 21. The conductor rod 25 is preferably formed of constantan and is, in turn, welded to the tip 29 via a weld bead 31 at the 'hot end' of the thermocouple flame sensing device. The tip 29 is, in turn, welded to the tube 21.

A sheath 30 is formed over and preferably completely encloses the 'hot end' of the thermocouple flame sensing device. The sheath 30 is formed of a high temperature silicone resin/resin mix which offers protection to the 'hot end' of the thermocouple flame sensing device from both the ingress of oxygen and CFC's.

At the 'cold end' of the thermocouple flame sensing device, an insulating sheath 32 similar to that of figure 1 surrounds the wire 23. An inert gas, preferably Helium, fills the space 33 between the insulating sheath 32 and the tube 21. The inert gas is retained in the tube 21 by a pressure seal P which may be of any of the following: molten thermoplastics, Araldite (rut) (TM), apiezon,',Q compound (TM), epoxy resins, hydrocarbon sealant, any suitable elastomer mixture or any suitable hermetic seal.

A washer 35 abuts the tube 21 and is retained by a metal button 37 which is soldered into position 39.

As can be clearly seen from the thermocouple flame sensing device of figure 2, the major points of entry for oxygen and CFC's into the thermocouple flame sensing device are reliably sealed thus providing a protection for the thermocouple against oxygen and CFC corrosion.

A method of forming a thermocouple flame sensing device as described above which allows non-destructive testing will now be described. The constantan rod 25 is placed within the tip 29 and the two elements are connected via the weld bead 31 to form the thermally responsive 'hot end' of the device.

An annular space 33 is defined between the two elements. Rod 25 is welded to wire 23 and tip 29 is welded to the external tubular lead 21. The annular space extends between the leads 21 and 23 which form the 'cold end' of the device.

The 'hot end' of the thermocouple is then treated with silicone resin mixed with aluminium powder or other suitable material, or coated with aluminium to produce the sheath 30 covering the 'hot end' thus providing a barrier against the ingress of oxygen and CFC's into the thermocouple flame sensing device.

The tube 21 is then evacuated and flushed one or more times with dry nitrogen or an inert gas. This step seeks to remove as much oxygen and water vapour from the space 33 between the insulated wire 23 and the tube 21 as possible.

The tube 21 is evacuated once more and filled with a dry inert gas. Helium is preferred as this facilitates non destructive bulk testing of the finished product. This reduces considerably the oxygen and water vapour levels in the space 33 within the thermocouple.

The 'cold end' of the thermocouple is then sealed by a flow of a suitable sealant P under differential pressure.

A washer 35 abuts the tube 21 and is retained by a metal button 37 which is soldered into position 39.

Thermocouple flame sensing devices produced as described above overcome the problems associated with prior art thermocouple flame sensing devices by reducing the ingress of oxygen and harmful CFC's into the thermocouple and thus delaying the subsequent corrosion of the thermocouple. The corrosion produced by CFC's from without at the weld bead tip is also minimised.

The thermocouples thus described may be 100% tested non-destructively by the use of a helium mass spectrometer according to a method known per se which will detect any helium leaking and, thus, predict the probable failure of any of the thermocouple flame sensing devices.

Claims (1)

1 A thermocouple flame sensing device comprising a first electrical conductor element disposed within a tubular second electrical conductor element so as to define an annular space there between and connected one to the other at one end to make up a thermally responsive probe forming the 'hot end' of the device and capable of developing an e.m.f., said elements respectively having joined thereto an internal core lead and an external tubular lead mutually spaced one within the other and extending from said elements to a sealed end forming the 'cold end' of the device, the said probe and leads having a dry inert gas confined there between to fill the internal space and thereby exclude corrosive gases and or vapours.

2. A thermocouple flame sensing device according to Claim 1 wherein the temperature probe further comprises a sheath covering the probe tip which prevents or, at least, limits corrosion due to CFC gases and the ingress of oxygen through the probe tip.

3. A thermocouple flame sensing device according to Claim 2, wherein the sheath is formed of high temperature silicone resin or silicone resin mix.

4. A thermocouple flame sensing device according to Claim 2, wherein the sheath is formed of aluminium.

5. A thermocouple flame sensing device according to any one of Claims 1 - 4, wherein a differential pressure seal produces a material flow into the tube which effects a seal at this end of the tube to retain the dry inert gas in the tube.

6. A thermocouple flame sensing device according to Claim 5, wherein the differential pressure seal is an hermetic seal.

7. A thermocouple flame sensing device according to any one of Claims 1 - 6, wherein the inert gas is helium.

8. A method of forming a thermocouple flame sensing device comprising the steps of placing a first electrical conductor element within a tubular second electrical conductor element so as to define an annular space and connecting the elements one to another at one end to make up a thermally responsive probe forming the 'hot end' of the device and capable of developing an e.m.f.; joining said elements respectively to an internal core lead and an external tubular lead mutually spaced one within the other and extending from said elements to a sealed end forming the 'cold end' of the device, and filling the internal space within the probe and leads with a dry inert gas thereby allowing each device to be nondestructively tested and simultaneously excluding corrosive gases or vapours from the thermocouple flame sensing device.

10. A thermocouple flame sensing device substantially as herein before described with reference to and as shown in Figure 2 of the accompanying drawings.

11. A method of forming a thermocouple flame sensing device substantially as hereinbefore described.