Thermocouple

Stable high-temperature thermocouple cable

Thermocouple Abstract:
A mineral-insulated metal sheathed cable comprising at least one type K thermoelement and characterized in that the sheath alloy consists essentially of up to about 40 weight-% chromium, up to about 10 weight-% niobium, about 0.5 to about 5.0 weight-% silicon, up to about 0.5 weight-% magnesium, up to about 0.3 weight-% cerium, and the balance nickel.

Thermocouple Claims:
The claims defining the invention are as follows:

1. A mineral-insulated metal sheathed cable comprising at least one type K thermoelement and characterised in that the sheath alloy consists essentially of from about 13.9 weight percent to about 14.5 weight percent chromium, from about 1.3 weight percent to about 1.5 weight percent of silicon, and the balance nickel.

2. A cable according to claim 1 in which the sheath alloy consists essentially of from about 14.05 weight percent to about 14.35 weight percent of chromium, from about 1.35 weight percent to about 1.45 weight percent of silicon, and the balance nickel.

3. A mineral-insulated metal sheathed cable comprising at least one type K thermoelement and characterised in that the sheath alloy consists essentially of up to about 40 weight-% chromium, up to about 10 weight-% niobium, about 0.5 to about 5.0 weight-% silicon, up to about 0.5 weight-% magnesium, up to about 0.3 weight-% cerium, and the balance nickel.

4. A cable according to claim 3 in which the sheath alloy consists essentially of from about 13 weight percent to about 15 weight percent of chromium, from zero to about 10 weight percent of niobium, from about 0.5 weight percent to about 3.5 weight percent of silicon, from zero weight percent to about 0.3 weight percent of magnesium, from zero to about 0.3 weight percent of cerium, and the balance nickel.

5. A cable according to claim 4 in which the sheath alloy consists essentially of from about 13.9 weight percent to about 14.5 weight percent of chromium, from about 1.3 weight percent to about 1.5 weight percent of silicon, from about 1.0 weight percent to about 5.0 weight percent of niobium, from about 0.05 weight percent to about 0.20 weight percent of magnesium, from about zero weight percent to about 0.2 weight percent of cerium, and the balance nickel.

6. A cable according to claim 5, in which the sheath alloy consists essentially of from about 14.05 weight percent to about 14.35 weight percent chromium, from about 1.5 weight percent to about 3.0 weight percent of niobium, from about 1.35 percent weight to about 1.45 percent of silicon, from about 0.10 weight percent to about 0.20 weight percent of magnesium, from zero weight percent to about 0.1 weight percent of cerium and the balance nickel.

7. A cable according to claim 1 in which the sheath alloy consists essentially of 14.2 weight percent chromium, 1.4 weight percent silicon, and the balance nickel.

8. A cable according to claim 3 in which the sheath alloy consists essentially of 14.2 weight percent chromium, 2.5 weight percent niobium, 1.4 weight percent silicon, 0.15 weight percent magnesium, 0.04 weight percent cerium, and the balance nickel.

9. A cable according to claim 1 in which the sheath alloy is Nisil.

10. A cable according to claim 3 in which the sheath alloy is Nicrosil.

11. A cable according to any one of the preceding claims in which the type K thermoelement contains no manganese.

12. A cable according to any one of the preceding claims in which the mineral insulating material is selected from magnesium oxide, beryllium oxide and aluminium oxide.

13. A cable according to any one of the preceding claims in which air in the mineral insulation is replaced by an inert gas.

Thermocouple Description:

This invention relates to mineral-insulated integrally metal-sheathed electrically conductive cable, and devices made therefrom.

The cable of this invention is suitable for use as thermocouple cable and is particularly useful at high temperatures.

The invention utilises nickel-base alloys as sheath alloys, which are used in conjunction with conventional standard nickel-base alloy thermocouples designated type K by various national standards bodies such as the American National Standards Institute, the British Standards Institution, the International Electrotechnical Commission, and other such bodies.

In one aspect the invention provides nickel-base thermocouple cables, and nickel-base thermocouple sensor systems made therefrom, having superior oxidation resistance, greater longevity and greater thermoelectric stability under longer time periods and over a range of higher temperatures up to about 1200.degree. C., than existing type K base-metal cables and sensor systems of the same general kind.

Nickel-base alloys have been used as thermocouples since the early part of this century. The most commonly used thermocouple of this kind is the type K thermocouple. The properties of type K thermocouples are well-known, and are summarized in the following references:

(1) NBS Monograph 125 "Thermocouple Reference Tables Based on the International Practical Temperature Scale (IPTS-68)", U.S. National Bureau of Standards, 1974. Column 1 on page 137 refers to compositional characteristics, while the thermal emf tables start at page 144.

(2) ASTM Annual Book of Standards, vol. 14.01 (1986): "Analytical Methods--Spectroscopy; Chromatography; Temperature Measurement; Computerized Systems". This document alludes, on page 242, to the compositional characteristics referred to in reference (1) above, and starts the emf tables on page 268.

(3) ISA American National Standard for Temperature Measurement Thermocouples, Ref. MC96.1 (1975). This document, referred to in (2) above, discusses compositions in column 2 of page V and in Table 1, and presents emfs in Tables XV and XVl.

The type K thermocouple is recommended to be used in an air atmosphere. At higher temperatures the type K thermocouple fails because of its relatively poor oxidation resistance. One way in which attempts have been made to overcome this problem is to incorporate the type K thermocouple in a compacted ceramic-insulated thermocouple sensor assembly.

As is well known in the art, a first step in the manufacture of such thermocouple sensors is to produce so-called MIMS (metal-sheathed mineral-insulated) cable, which comprises a sheath containing one or more thermoelement conductor wires electrically insulated from the sheath (and from each other when two or more conductor wires are used) by compacted mineral insulating material.

To make an actual sensor from this cable, the cable is cut and the ends of the conductors are exposed by removing some of the insulation therefrom. The exposed ends of the conductors are then joined to form a thermojunction, which may be accomplished, for example, by crimping and/or welding.

The thermocouple may simply be left exposed for use in appropriate environments, or may be protected by capping the sheath over the thermojunction, with or without insulant.

The MIMS type of thermocouple has come into common use because of certain advantageous features, including

(i) chemical isolation of thermocouple wires from environments that may cause rapid deterioration;

(ii) electrical isolation of thermoelement conductors from sources of interference that may cause spurious signals;

(iii) mechanical protection of the thermocouple from damage due to pressure or shock;

(iv) mechanical flexibility of the assembly, allowing bending in installation; and

(v) simple fabrication of thermocouples.

The sheath can be made from a material which, hopefully, is compatible with the environments and processes in which it is being used. There are numerous commercial suppliers of type K thermocouples in the compacted integral-y metal-sheathed ceramic-insulated forms.

The full potential of the excellent MIMS design concept for type K thermocouples has not been realized. This is due to several factors associated with its development so far:

(i) The common sheath materials for type K MIMS thermocouples--the Inconels (Inconel is a trade mark) and the stainless steels--will not withstand exposure in air for extended periods of time at temperatures much above 1050.degree. C. Most manufacturers of conventional type K MIMS thermocouples prescribed 1100.degree. C. as the maximum operating temperature for their products. Unfortunately in many instances in industrial pyrometry there are specified operating temperatures in the range above 1100.degree. C. for which conventional type K MIMS thermocouples are quite unsuitable.

(ii) The type K thermoelement conductor wires can be contaminated by chemical elements which thermally diffuse through the compacted insulant material from dissimilar sheath alloys such as stainless steel. It has been found that manganese emanating from the sheath, or even from one or both of the thermoelement conductor wires, is particularly potent as a contaminant by cross diffusion between sheath and conductors. The resultant change in the chemical compositions of the type K thermocouple alloys can cause substantial changes in their thermoelectromotive force (thermal emf). These changes in thermal emf are analogous with and algebraically additive to those caused by oxidation.

(iii) The type K thermoelement conductor wires, particularly the electronegative wire, may fail mechanically because of substantial alternating strains imposed during thermal cycling. These strains are caused primarily by longitudinal stresses which arise because of substantially different temperature coefficients of linear expansion of the thermoelements and stainless steel. Some typical average values of the coefficients of expansion are

______________________________________
Component Material .times.10.sup.-6 .degree.C..sup.-1 (1000.degree.
C.)
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sheath stainless steel
21
thermoelements
type K 17
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As a result, there is a need for a new MIMS cable suitable for production of thermocouple sensors which are substantially free of the degradative influences described above and which demonstrate enhanced environmental and thermoelectric stability at temperatures significantly in excess of 100.degree. C.

It is believed, therefore, that a new integrally metal-sheathed mineral-insulated cable, substantially free of degradative influences such as differential thermal stresses, and cross-contamination by diffusion, and demonstrating enhanced resistance to environmental interactions and to drifts in thermal emf at temperatures up to 1200.degree. C. in a variety of different atmospheres would represent an advancement in the art.

OBJECTS AND SUMMARY OF THE INVENTION

It is one of the objects of this invention to provide an integrally metal-sheathed mineral-insulated (MIMS) type K thermocouple cable and sensor which is thermoelectrically stable up to 1200.degree. C. It is a further object of this invention to provide an integral compacted base-metal thermocouple cable and sensor which are highly oxidation resistant up to 1200.degree. C.

These and other objects of this invention are achieved by the use of certain specific alloys, and certain other compositional variants of these alloys, as MIMS sheath materials. We have now surprisingly found that the use of the type N alloys Nicrosil and Nisil and compositional variants thereof as the sheath material provides an unexpected benefit to the performance of the type K thermoelements. The specific compositions and properties of the type N alloys Nicrosil and Nisil are well known and are summarized in the following reference:--NBS Monograph 161 "The Nicrosil versus Nisil Thermocouple, Properties and Thermoelectric Reference Data", U.S. National Bureau of Standards, 1968.

The invention accordingly provides a mineral-insulated metal sheathed cable comprising at least one type K thermoelement and characterised in that the sheath alloy is of the following composition: up to about 40 weight-% chromium, up to about 10 weight-% niobium, about 0.5 to 5.0 weight-% silicon, up to about 0.5 weight-% magnesium, up to about 0.3 weight-% cerium, and the balance nickel (apart from metallurgically acceptable levels of impurities).

It will be clearly understood that the sheath alloy includes within its scope Nicrosil and Nisil, and the alloys disclosed in Australian Patent Application Nos. 41675/85 and 62404/86 by the present applicant.

A preferred sheath alloy of this invention consists essentially of from about 13 weight percent to about 15 weight percent of chromium, from zero to about 10 weight percent of niobium, from about 0.5 weight percent to about 3.5 weight percent of silicon, from zero weight percent to about 0.3 weight percent of magnesium, from zero to about 0.3 weight percent of cerium, and the balance nickel.

The preferred type K thermocouple conductors for the MIMS able of this invention are those commercial varieties which are available which contain no manganese in their chemical compositions.

Preferred refractory mineral-insulating materials for the MIMS thermocouple cable include magnesium oxide, aluminium oxide, beryllium oxide and other suitable refractory oxides.

Preferably, in order to reduce the oxidation of the type K thermoelement alloys, air is removed from the interstices of the mineral-insulate powder grains and replaced by an inert gas such as argon and nitrogen.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

For a better understanding of the present invention together with other and further objects, advantages, and capabilities thereof, reference is made to the following disclosure, and to the accompanying drawings, in which:

FIG. 1 represents a typical MIMS cable containing two conductor wires; and

FIG. 2 represents the relative oxidation resistance of nickel-chromium binary alloys

The structure of a typical conventional MIMS thermocouple is illustrated in FIG. 1, showing an integral sheath 1, usually made from stainless steel or Inconel, containing mineral insulation 2 which surrounds thermoelement conductor wires 3. The mineral insulation material is usually magnesium oxide, and the thermoelement wires are usually type K alloy.

The integral base-metal thermocouple cable of the present invention has excellent oxidation resistance and thermoelectric stability at temperatures up to 1200.degree. C.

It has been found that the alloys of this invention change very little both in thermal emf output and degree of oxidation even after exposure at 1200.degree. C. When compared with the conventional thermoalloys of type K and sheath alloys of Inconel and stainless steel, which materials are conventionally used in existing integral compacted thermocouple sensors, the integral compacted thermocouple sensor of this invention incorporating the type K thermoelements and sheath alloys described above shows markedly improved thermoelectric and environmental stability, to a degree hitherto unattainable with conventionally-used type K integrally metal-sheathed mineral-insulated thermocouples.

The sheath alloys to be incorporated in this invention have preferred compositions

(i) from about 13.9 weight percent to about 14.5 weight percent chromium, from about 1.3 weight percent to about 1.5 weight percent of silicon, and the balance nickel, or more preferably

(ii) from about 14.05 weight percent to about 14.35 weight percent of chromium, from about 1.35 weight percent to about 1.45 weight percent of silicon, and the balance nickel;

(iii) from about 13.9 weight percent to about 14.5 weight percent of chromium, from about 1.0 weight percent to about 5.0 weight percent of niobium, from about 0.05 weight percent to about 0.20 weight percent of magnesium, from about zero weight percent to about 0.2 weight percent of cerium, and the balance nickel, or more preferably

(iv) from about 14.05 weight percent to about 14.35 weight percent chromium, from about 1.5 weight percent to about 3.0 weight percent of niobium, from about 1.35 percent weight to about 1.45 percent of silicon, from about 0.10 weight percent to about 0.20 weight percent of magnesium, from zero weight percent to about 0.1 weight percent of cerium and the balance nickel.