Stable metal-sheathed thermocouple cable

A thermocouple cable (10) has a positive and negative thermoelement (14) conforming to the standard emf-temperature specification for type K thermocouples, a sheath (12) and compacted ceramic (16) insulating the thermoelements (14) from each other and from the sheath (12). The sheath comprises an oxidation resistant allow having a thermal coefficient substantially the same as that of the negative thermoelement and a melting point in excess of 1300.degree. C.; the sheath preferably being formed from nickel based alloy, such as a nickel-chromium alloy. The positive and negative thermoelements preferably also comprise nickel-based alloys.

This invention relates to a mineral-insulated, metal-sheathed (MIMS) 
thermocouple cable, and to thermocouples made from such cable. 
The manufacture of MIMS cable is well known. The components consist of a 
metallic sheath and two thermoelements (thermocouple conductors) insulated 
from each other, and the sheath, by a compacted ceramic-oxide-insulation 
material. The components are assembled under clean, dry conditions and by 
a process such as drawing, swaging or rolling, the sheath diameter is 
reduced to compact the ceramic and fill the available space. The assembly 
is further reduced in diameter to the desired size, an overall reduction 
in diameter of 10 to 1 being common. Before diameter reduction the 
assembly may be evacuated, annealed and or back-filled with an inert gas. 
Thermocouples are temperature-measuring sensors. They are fabricated from 
MIMS cable by cutting to the required length, welding the thermoelements 
together at one end of the cable to form the `hot junction` and welding 
extension leads to the other. Insulating powder is packed in around the 
hot junction to avoid an air pocket in the completed product, and the MIMS 
section of the thermocouple is then sealed by welding in some sheathing 
alloy over the hot junction and, for example, providing a suitable potting 
compound at the other. 
Conventional base-metal MIMS cables are produced with sheaths of inconel or 
stainless steel and with thermoelements of one of the five 
internationally-standardised thermocouple types (types E,J,K, N and T: 
letter designations of the Instrument Society of America). 
Of the two types suitable for high-temperature use (types N and K) the type 
N alloys have well-defined compositions whereas those for the type K 
alloys are not defined. The main requirement for the positive and negative 
type K thermoelements is that, as a matched pair, the relationship between 
their net emf and temperature should agree with the relevant 
internationally-accepted reference equations (such as BS4937, ASTM E230) 
within defined limits of error. 
The present invention is directed to providing an improved MIMS 
thermocouple cable, and to thermocouples made from such cable. 
A thermocouple cable according to the invention has a positive and a 
negative thermoelement conforming to the standard emf-temperature 
specification for type K thermocouples, a sheath through which the 
thermocouples extend, and compacted ceramic insulating the thermoelements 
from each other and from the sheath; the sheath comprising an 
oxidation-resistant alloy having a thermal coefficient substantially the 
same as that of the negative thermoelement and a melting point in excess 
of 1300.degree. C. 
The alloy of which the sheath is formed may be a nickel based alloy, 
although other alloys such as cobalt based alloys can be used. The nickel 
based alloy preferably is a nickel-chromium alloy, a particularly 
preferred alloy being one containing 13 to 15 wt. % chromium, 1 to 2 wt. % 
silicon with the balance most typically principally comprising nickel. The 
nickel-based alloy preferably contains substantially no manganese; while 
manganese, if present, should be less than 0.1 wt. %. 
The nickel-based alloys for the sheath may contain elements such as 
magnesium to enhance their oxidation resistance, and refractory metals 
such as niobium, tungsten, tantalum or molybdenum to enhance physical 
properties of the sheath, such as its strength. Nicrosil (nominally 14.2 
wt. % silicon and the balance essentially nickel) is an example of a 
suitable alloy for the sheath. 
The positive and negative thermoelements are to be of alloys conforming to 
the standard emf-temperature specification of type K thermocouples. The 
thermoelements preferably are of respective alloys such that they conform 
to such specification to within .+-.0.75% of temperature, most preferably 
to within .+-.0.375% of temperature. The thermoelements should be of 
alloys meeting above 1300.degree. C.; the alloys preferably comprising 
nickel-based alloys. In the case of the positive thermoelement, the alloy 
preferably comprises 9 to 10 wt. % chromium, with the balance 
substantially comprising nickel, although it may contain up to 1 wt. % 
silicon and small quantities of other conventional alloy additions. 
The negative thermoelement preferably contains 1 to 3 wt. % silicon in 
nickel. However, it typically will also contain other elements, preferably 
totalling less than 6 wt. %, such as man9anese, cobalt, aluminium and 
copper. Such other elements preferably are added as required to adjust the 
emf-temperature relationship, of the positive thermoelement relative to 
the negative, to conform to the standard emf-temperature specification for 
type K thermocouples. 
The insulating ceramic may be of any suitable refractory oxide or 
combination of oxides. Examples of suitable oxides are those of magnesium, 
aluminium and beryllium. 
Advantages of the Invention 
Thermocouples fabricated from cable according to the invention, such as by 
the fabrication procedure outlined above, are a considerable improvement 
over bare-wire type K thermocouples and those produced from conventional 
base-metal MIMS cables. They have excellent stability, both thermoelectric 
and mechanical, at high temperatures and the invention is an advance for 
the following reasons. Having the thermolements within a MIMS system 
avoids instabilities and premature failure at high temperatures (beyond 
about 900.degree. C.), due to oxidation, that occur in `bare-wire` 
thermocouples. The use of the sheath alloy specified for the invention 
avoids the main cause of thermoelectric instability in conventional MIMS 
systems, that due to the migration of Mn from the sheath. Also, use of the 
sheath of the invention avoids mechanical failures that occur in 
conventional MIMS systems because of the difference in thermal expansion 
between that of the sheath and that of the negative thermoelement. 
Further, the sheath specified of the invention is more resistant to 
oxidation and can be used in air at higher temperatures and for longer 
periods than the conventional sheathing alloys inconel and stainless 
steel. Finally, the only standardised base-metal thermocouples with a 
practical life beyond 1000.degree. C. are the type K and type N 
thermocouples. Within a MIMS system having the sheath of the invention, 
the type K thermoelements of the invention are more thermoelectrically 
stable than the type N and are less likely to fail because of any mixmatch 
in thermal expansion coefficient. Also, type K thermocouples, including 
those made from the cable of the invention, are of greater practical value 
because there are two orders of magnitude more instruments, and associated 
items of equipment, available in the type K calibration than in the type 
N. 
Development of the Invention

Our research into the thermoelectric behaviour of conventional nickel-based 
MIMS thermocouples showed that the main cause of instability at high 
temperatures was the migration of Mn from the sheath to the 
thermoelements. As a first step in developing a new, more stable MIMS 
system the alloy Nicrosil was considered by us as a sheath. Our subsequent 
work confirmed the importance of avoiding Mn. High-nickel alloys such as 
Nicrosil have the added advantage of having a similar thermal expansion 
coefficient to that of the negative type N and type K thermoelements, both 
being high-nickel alloys. Relative to these thermoelements, the thermal 
expansion of stainless steel or inconel on heating to 900.degree. C. is 
0.2 to 0.4% which is large compared to that of Nicrosil, 0.05%. 
Differential thermal expansion causes mechanical failure during thermal 
cycling in type K and type N MIMS thermocouples having sheaths of inconel 
or stainless steel. 
Nicrosil-sheathed MIMS thermocouples, comprising three (K1, K2 and K3) type 
K thermocouples according to the invention and one type N thermocouple, 
each as described in Table 1, have been studied. What follows is a brief 
account of this work. 
Table 1. Composition of thermoelements studied within Nicrosil-sheathed 
MIMS thermocouples, in weight percent. 
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Thermo- Positive 
Type couple thermoelement 
Negative thermoelement 
______________________________________ 
N N Ni 14.0 Cr 1.5 Si 
Ni 4.4 Si 0.1 Mg 
K K1 Ni 9.2 Cr 0.5 Si 
Ni 1.6 Si 1.6 Mn 1.3 Al 0.6 Co 
K K2 Ni 9.7 Cr 0.3 Si 
Ni 2.0 Si 0.4 Mn 0.0 Al 0.4 Co 
K K3 Ni 9.1 Cr 0.4 Si 
NI 2.4 Si 0.0 Mn 2.2 Cu 1.0 Co 
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It is the integrated effect of local changes in Seebeck coefficient along 
the length of a thermocouple that dictates the extent of drift in its 
signal and thus of the temperature error it produces. For this reason the 
changes in Seebeck coefficient that occur at temperatures up to 
1200.degree. C. were measured as a function of time for the various 
thermoelements. Each thermocouple was heated either isothermally or in a 
`gradient-annealing` furnace, the temperature within the latter being 
roughly linear with distance along much of its length. The resultant 
change in Seebeck coefficient, as a function of position along the 
thermocouple, was obtained with a horizontal, motor-driven `scanning 
furnace` moving at 0.125 mm s.sup.-1 . This furnace had a wire-wound 
tubular muffle, 1.2 m long, containing a 20 mm ID earthed Inconel tube, it 
had a relatively steep temperature-gradient region at the entrance port 
and its temperature was uniform at 500 .degree. C. to 5.degree. C. along 
most of its length. At any given time during a scan the emf produced by 
the temperature step, from ambient to 500.degree. C., occurred over a 
distance of 120 mm. If the thermocouple had just been heated in the 
gradient-annealing furnace this length of it would correspond to a range 
of annealing temperatures of typically .+-.30.degree. C. about T.sub.o at 
its centre. Hence the emf measured at this immersion in the scanning 
furnace would reflect the average effect of annealing at T.sub.o 
.+-.30.degree. C. 
The movement of the scanning furnace and the electrical measurements were 
controlled by a Hewlett Packard (HP) 86B computer interfaced to a HP 3456A 
DMM, having 0.1 .mu.V resolution, and a low-thermal (0.1 .mu.V) two-pole 
scanner. The cold junction ends of all thermocouples were connected to a 
set of temperature-monitored terminals at 20.degree. C. uniform to 
.+-.0.01.degree. C. 
During the scan the tip temperature of each thermocouple was obtained with 
a Pt10%Rh versus Pt reference thermocouple whose tip was wire-wrapped to 
that of the thermocouple under test with Nichrome wire. The reference 
thermocouple has 0.5 mm diameter wires in a 1060 mm long twin-bore 
insulator of recrystallised alumina and its Seebeck coefficient varied by 
less than .+-.0.02% along its length. Values of test thermocouple tip 
temperature (about 500.degree. C.) were converted to values of emf, 
E.sub.o, for each thermoelement relative to platinum. From measurements 
taken during the scan, the computer produced a thermoelectric signature 
for each thermoelement as a plot of E-E.sub.o against position along the 
specimen. E is the output of the thermoelement relative to the platinum 
leg of the reference thermocouple and corrected for the cold-junction 
temperature. 
E was developed in the 480.degree. C. temperature step at the entrance port 
of the scanning furnace and any difference, .delta.E, between repeated 
signatures represents a change in Seebeck coefficient of: 
EQU .delta.S=.delta.E/480 
Here it is assumed that .delta.S is independent of temperature to at least 
500.degree. C. This was found to be so for changes produced by 200 h at 
350,700 and 1100.degree. C. The repeatability of the sCanning facility is 
such that changes in signature of 10 .mu.V (0.02 .mu.VK.sup.-1) or more 
are significant. 
Studies were made of both reversible and irreversible changes in Seebeck 
coefficient produced by heating for periods of 0.5 to 300 h. Preliminary 
measurements revealed that significant reversible changes did not occur 
above 1000.degree. C. and that when such changes did occur, at lower 
temperatures, a heating at 1000.degree. to 1100.degree. C. reversed the 
process. There was no noticeable difference in the effects of heating 
periods of 5 and 60 minutes. After each such heating the specimen was `air 
quenched` by withdrawing it in about 2 s from the furnace and holding it 
stationary in air at 20.degree. C. until cool. Hence a 10 minute heating 
at about 1020.degree. C. followed by an air quench was chosen as the 
defined `recovery anneal`. 
Since the gradient-annealing furnace covers only a limited range of 
temperature, about 350.degree. C. for a peak annealing temperature of 
1000.degree. C., the 200.degree. to 1200.degree. C. interval considered in 
this study was examined in four overlapping zones. For each zone a 
different set of specimens was used and each set consisted of three MIMS 
specimens, one of each of the three diameters 1.5, 3 and 6 mm. This 
enabled any diameter-dependence to be observed. 
To distinguish between reversible and irreversible changes the following 
sequence was followed for each specimen: 
(a) recovery anneal 
(b) scan 
(c) recovery anneal 
(d) gradient anneal (Period t.sub.o) 
(e) scan 
(f) repeat of steps (a) to (e) with different t.sub.o. 
The effect of heating for a period t.sub.o at any temperature T.sub.o, 
within the range of the gradient-annealing furnace, was indicated by the 
difference between the signatures obtained before and after the gradient 
anneal. This change has two components, the irreversible and the 
reversible. The irreversible component was determined from the signature 
measured after the subsequent recovery anneal and was cumulative. The 
reversible component was then calculated by substraction. 
Thus, it was found that changes in coefficient are of two types. The first 
is a reversible one i.e. after the Seebeck coefficient changes on heating, 
it will return to its former value on a brief heating at a higher 
temperature (such as 1000.degree. C.) and the process is repeatable. This 
process is a consequence of changes throughout the alloy structure and 
thus occurs equally in bare-wire and MIMS thermocouples. Further, the 
change in coefficient is independent of thermoelement diameter. The second 
type of change is irreversible and occurs because of changes at the 
surface of each thermoelement. It is therefore diameter dependent. Its 
cause and degree of change is different from those that would have 
occurred had the wires been exposed to air, i.e. had they been a bare-wire 
thermocouple. Since there is no relationship between the two sets of 
processes the behaviour of a particular pair of thermoelements in a MIMS 
system is not predictable from data obtained for the same thermoelements 
as bare wires exposed to air. 
The reversible changes in Seebeck coefficient for each thermocouple pair 
tested is indicated in FIG. 2 and clearly the change in each of the 3 type 
K thermocouples is similar yet considerably less than that in the type N 
thermocouple. It was also found that reversible change at any one 
temperature occurs rapidly at first and then levels off in the long term. 
For example, there is little change beyond that at 200 hours and half this 
change occurs in less than 10 hours. 
In the MIMS configuration, irreversible change in the coefficient is 
minimised if the manganese content of the sheath is negligible, as is the 
case for a Nicrosil sheath. Indeed it was found that the irreversible 
change in 200 hours in all 4 thermocouple examples studied (Table 1) was 
insignificant below 800.degree. C. and less than about 0.2 .mu.V/K at 
1100.degree. C. Beyond this temperature the change becomes increasingly 
negative reaching -0.9.+-.2 .mu.V/K at 1200.degree. C. for all specimens. 
The variation in behaviour between the three type K thermocouples was 
similar to the difference between their mean behaviour and that of the 
type N thermocouple. Hence the irreversible changes occurring in the type 
K and type N thermocouples are not significantly different. 
The integrated effect of reversible and irreversible changes in the Seebeck 
coefficient is evident in the in situ drift of a thermocouple. This is the 
drift in signal that occurs for a thermocouple whilst the temperature of 
its tip and its temperature profile are held fixed. FIG. 3 shows the in 
situ drift in 200 hours at any given temperature for the 4 examples. For 
temperatures to about 1000.degree. C. the main contributor to drift is the 
reversible effect and hence the type K combinations performed better than 
the type N. Furthermore, for periods of use beyond 200 hours the 
contribution from reversible change in the coefficient will change little 
and it is only at temperatures approaching 1200.degree. C. that the 
long-term irreversible changes would have a significant effect. For 
example, for type N thermoelements in a Nicrosil MIMS sheath at 
1100.degree. C. the in situ drift for 2000 hours was not significantly 
different from that at 200 hours. There was a barely-measurable decrease 
in signal in the long term, as expected, because of irreversible changes 
occurring in that part of the thermocouple at 100.degree. to 1100.degree. 
C. 
Hence, the in situ drift for all 4 types of specimen (Table 1) over long 
periods of time, such as 2000 h or so, is little different from that 
indicated in FIG. 3 for 200 h. Below 1000.degree. C. the drift would be 
marginally higher than indicated and beyond 1000.degree. C. it would be 
less and eventually become negative. Overall, the type K varieties perform 
better than the type N (FIG. 3) and for this reason they were selected as 
thermoelements for this invention. The Invention compared with 
Conventional Thermocouples 
For variable-immersion applications at high temperatures requiring a 
short-term use of small-diameter probes, the conventional alternatives 
are: 
(a) 24 AWG (0.5 mm wires) bare-wire thermocouples in woven insulation or 
ceramic beads. For comparison purposes the type N thermocouple was chosen 
for this study because it is the most stable base-metal type for bare-wire 
use in air. 
(b) MIMS probes with 1.5 mm diameter sheaths. Samples of the 
stainless-steel-sheathed type K MIMS system were chosen to represent this 
group since it is the most commonly used MIMS system. 
Errors at high temperatures are greatest when probes are moved to a lesser 
immersion depth and are proportional to the changes in Seebeck coefficient 
that occur on heating at these temperatures, e.g. 64 h at 1100.degree. C. 
These changes were measured by the above mentioned method and the results 
are given in Table 2. The changes found for examples of this invention 
(1.5 mm OD Nicrosil sheath and the thermoelements K1, K2 and K3) are an 
order of magnitude smaller than those for the conventional probes). Table 
2. Change in Seebeck coefficient of small-diameter probes (described 
above) for 64 h of heating at 1100.degree. C. 
______________________________________ 
Change in Coefficient 
Probe .mu.V/K 
______________________________________ 
Bare-wire type N 3.3 
Stainless-steel-sheathed type K MIMS 
-3.2 
Examples of invention: 
K1 0.04 
K2 -0.40 
K3 
______________________________________ 
The most common use of thermocouples is in long-term fixed-immersion 
applications. In such cases, probe (and wire) diameters are larger for 
longer life and to minimise in situ drift at high temperatures. The most 
commonly used is the bare-wire type K thermocouple with each wire 1.5 to 
3.3 mm in diameter. With insulating beads and a separate protection sheath 
the overall probe diameter becomes 12 to 25 mm. 
Some high-temperature in situ-drift data for such probes are as follows: 
(a) 15 AWG (1.5 mm wires) thermocouples at 1000.degree. C. drifted 
6.degree. C. in 300 h and 20.degree. C. in 5000 h. 
(b) 8 AWG (3.3 mm wires) thermocouples at 1100.degree. C. drifted 
12.degree. C. in 800 h. 
(c) 8 AWG (3.3 mm wires) thermocouples at 1200.degree. C. drifted 
27.degree. C. in 700 h. 
(d) 15 AWG (1.5 mm wires) thermocouples at 1200.degree. C. drifted 
66.degree. C. in about 500 h and mechanically failed by 1000 h. 
(e) high-temperature drift rates for bare-wire thermocouples vary 
considerably because of differences in composition and thus differences in 
the oxidation mechanisms. 
In the Nicrosil-sheathed MIMS system, type K thermoelements specified for 
the invention will suffer less drift. For example, at 1100.degree. C. the 
change over a 2000 h period would reach a maximum of about 3.degree. C. in 
200 to 500 h and then decrease slightly. The overall change would not 
exceed 5.degree. C. Notice also that the probe diameter for this example 
is only 6mm (FIG. 3), 2 to 4 times smaller than conventional probes.