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Conf - 650204 - 4
MAR 23 1165
INFLUENCE OF CO-CO2 ENVIRONMENTS ON. THE CALIBRATION
OF CHROMEL-A-ALUMEN LIBRMOCOUPLES
H. E. Mccoy, Jr.
Metals and Ceramics Division
MASTER
INTRODUCTION
Probably no single problem has hindered the work of the metallur-
gist as has his inability to measure temperatures accurately. It is the
purpose of this paper to report observations made of the behavior of
Chromel-Palumel thermocouples in carbon monoxide and carbon dioxide
environments. These tests were min to determine satisfactory techniques
for measuring temperatures in these enviroriments rather than as a basic
thermocouple study. All thermocouples were prepared by normal techniques
with the hot junction being prepared by fusing the two wires with a
Heliarc torch.
RESULTS
Carbon Monoxide Environment
T
A test was run in which the drift rate of a 20-gage (0.032-in. diam)
Chromel-Alumel thermocouple in a carbon monoxide environment was measured.
A stainless steel thermocouple well was sealed inside a refractory tube
through which carbon monoxide was passed. A standard thermocouple was
attached to the outside of the thermocouple well so that it was exposed
to the carbon monoxide environment. The furnace temperature was controlled
on the basis of the thermal output of the exposed thermocouple. The re-
sults of this test are shown in Fig. ll.1. The thermal output of the
thermocouple exposed to carbon monoxide decreased such that after 650 hr
the furnace temperature was actually at 1000°C in order to supply the
controller with a 34.1-mv signa..
The exposed thermocouple was analyzed for carbon after the test and
both wires contained greater than 5 wt % C near the hot junction. The
as-received carbon analyses of the Chromel-P and Alumel wires were 0.024
and 0.033 wt%, respectively.
18171310- 33% -1::20
Small pieces of the thermocouple wires near the hot junction were
examined metallographically. Representative photomicrographs of the as-
received wires are shown in Fig. 11.2 and those of the test wires are
shown in Figs. 11.3 and 11.4. The Alumel wire was internally oxidized
throughout the entire cross section. The Chromel wire was heavily oxi-
dized and a grain-boundary precipitate, probably a chromium carbide, was
fomed.
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of any taforation, apparitu, method, or procon diectoard la we report may not Infringe
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20
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100
200
500
10
Fig. 11.1. Effect of a Carbon Monoxide Environment on the Calibra-
tion of Chromel-Palumel Thermocouples.
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Fig. 11.2. Photomicrographs of As-Received (a) Chromel-P and
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In có. Photographed in the as-polished condition. (a) 100%, (b) 500X.
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Fig. 11.4. Photomicrographs of Alumel Thermocouple Wire Tested in
Photographed in the as-polished condition. (a) 100X. (b) 500X.
INI-MIC-OFFICIAL
ORNI-AE6-OFFIC
11.6
95% CO2-5 vol% CO Environment
Three 24-gage (0.020-in. diam) Chiromel-P-Alumel thermocouples were
tested at approximately 650, 750, and 850°C in flowing 95 vol % CO2-5
vol % Co. The hot junction of each of these thermocouples was mechani-
cally fastened to the hot junction of a calibrated 24-gage (0.020-in.
diam) Pt vs Pt-10% Rh thermocouple. The thermocouples were inserted
different distances into a furnace to obtain the different hot-junction
temperatures. All of the thermocouples were contained in a 2-in. diam
refractory tube through which the gas mixture flowed. The furnace tem-
perature was controlled by a thermocouple placed near the furnace windings.
Comparative thermal electromotive force readings were made of the test
Chromel-P-Alumel and the noble metal thermocouples as a function of time.
The results of the tests are shown graphically in Figs. 11.5, 11.6, and
11.7. Much of the scatter obtained in these measurements resulted from
the furnace tube being moved and thus causing the hot-junction tempera-
tures to change slightly. The thermocouple at 650°C showed an initial
positive calibration shift of about 7°C. The thermal electromotive force
output continued to drift positive and reached a value of 10°C after
2500 hr. At 750°C an initial positive calibration shift equivalent to
about 16°C was observed. The posi rive drift continued to a maximum value
of 19°C in 80 hr. The thermocouple then began to drift negative and indi-
cated a temperature too low by 50°C after 2500 hr. At 850°C the initial
positive shift was not noted, but rather the thermocouple drifted nega-
tively throughout the test. After 2500 hr the indicated temperature was
low by about 120°С. At the end of the 2500-hr test, all thermocouples
were extremely brittle.
...
-
Samples were cut from the thermocouples for carbon analyses. The
samples were physically located over the distance from 1 to 2 in. from
the hot junction. The carbon analyses of these specimens are given in
Table il.l. Since the samples were small and were severely oxidized, it
is felt that the smell apparent changes in carbon content are not signif-
icant.
H
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Metallographic samples were taken from the thermocouples which
included the hot junction and about 1 in, of each wire. Representative
photomicrographs are shown in Figs. 11.8 through 11.13. At 650°C both
wires were oxidized appreciably. The Alunel wire was uniformly oxidized
internally to a depth of about 3.5 mils (Fig. 11.9). The Chromel-P wire
in general looked quite good except for oxidation in isolated areas such
as that shown in Fig. 11.8. The depth of this attack extended to 5 mils.
At 750°C the Chromel-P wire was internally oxidized throughout (Fig. 11.10).
The Alumel wire was uniformly oxidized to a depth of 1.5 mils (Fig. 21.?1).
At 850°C both the Chromel-P and the Alumel wires were internally oxidized
throughout (Figs. 11.12 and 11.13).
ORNI - AIC - OFFICIAL
ORNI - AC - OFFICIAL
11.7
OINL-ALGO
Table 11.1.
Carbon Analyses of Chromel-P and Alume). Thermocouple Wires
Exposed to 95 CO2-5 CO Mixture for 2500 hr.
Temperature
of Exposure
Wire
Designation
Carbon
Content
(wt %)
As-received
As-received
650°C
650°C
750°C
750°C
850°C
850°C
Chromel-P
Alumel
Chromel-P
Alumel
Chromel-P
Alumel
Chromel-P
Alumel
0.022
0.015
0.009
0.017
0.017
0.012
0.009
0.016
CO2 Environment
Three 20-gage (0.032-in. diam) Chromel-P-Alumel thermocouples were
run 1000 hr in a carbon dioxide environment. The thermocouples were
attached to small stainless steel tabs which were the intended test pieces.
Although electromotive force readings were not made as a function of time,
the electromotive force output of each thermocouple was checked at the
initiation and at the end of the 1000-hr exposure. The thermocouples were
brittle after test. Small specimens were cut near the hot junction for
carbon analyses. The available data on these thermocouples are given in
Table 11.2.
Table 11.2. Data on Chromel-P-Alumel Thermocouples
Exposed to Flowing CO2 for 1000 hr
Original
Temperature
Indicated
Temperature
after 1000 hr
Wire
Designation
Carbon
Content
(wt %)
U
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As-received
As-received
704°C
704°C
815°C
815°C
927°C
927°C
Chromel-P
Alumel
Chromel-P
Alumel
Chromel-P
Alumel
Chromel-P
Alumel
698°C
698°C
723°C
723°C
804°C
804°C
0.006
0.008
0.013
0.014
0.020
0.018
0.003
0.011
.
ORNI - ÅEC-OFFICIAL
In a test similar to the above is was found that in 257 hr the out-
put of three thermocouples drifted such that the indicated temperatures
changed from 833 to 753°C, from 722 to 714°C, and from 620 to 611°C. NO
further examination of these thermocouples was made.
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100
200
500
1000
2000
4000
Fig. 11.5. Behavior of Chromel-P-Alumel Thermocouples in CO 2-5 vol % co at 650°C.
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1000
2000
4000
Fi{. 11.6. Behavior of Chromel-?-Alumcl Thermocouple in CO2-'; vol % co at 750°C.
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100
200
500
1000
2000
4000
Fig. 11.7. Behavior of Chromel-P-Alumel Thermocouples in CO2--5
vol % CO at 850°C.
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ORNI - AC -0F?ICIAL
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ORHL-ALC - Ornici,
11.17
L-
DISCUSSION OF RESULTS.
YEN
Appreciable negative drifts have been observed for Chromel-P-Alumel
thermocouples in various CO-CO2 environments. These drifts are consider-
ably larger than those observed by Potts and McElroyd for Chromel-P-Alumel
thermocouples in air. It is felt that the behavior of these thermocouples
in CO-CO2 environments can be explained in terms of chemical reactions
which tend to carburize and oxidize the components of the thermocouple.
As discussed by McElroy the thermal electromotive force of a Chromel-p-
Alumel thermocouple goes through a maximum with respect to chromium con-
tent. Since the composition of Chromel-P has been chosen to give the
maximum thermal electromotive force, any change in the chromium in solution
in the alloy decreases the thermocouple output. Carburization and oxidation
would both remove chromium from solid solution and cause the thermocouple
to drift negative. Pure carbon monoxide will not oxidize nickel at elevated
temperatures but would readily oxidize the alloying elements present in
Chromel-P and Alumel such as chromium, aluminum, manganese, and silicon.
Carbon dioxide would be oxidizing to all elements present.3 The oxidation
studies by Potts and McElroyt showed that the Alumel wire was oxidized the
most heavily in air. The photomicrographs in Figs. 11.3, 11.4, and l1.gam
11.13 show that this is also the trend in CO-CO2 environments although not
to the extent observed in air. Due to the strong tendency of chromium to
form a carbide, carburization is primarily a problem with the Chromel-P
wire. If the carbide formed is assumed to be of the Cr4C type, 0.1 wt C
can react with about 1.74 wt o Cr. Hence, the chromium concentration in
solution would be reduced and the thermocouple would drift negative. Chem-
ical analyses have indicated a substantial increase in the carbon content
of the thermocouples exposed to pure carbon monoxide but indicated no in-
crease in carbon content as a result of exposure to carbon dioxide and the
CO2-5 vol % CO mixture. At least two possible explanations can be given
for the initial positive årifts observed in the thermocouples in the CO2-5
Vol % CO environment. One possibility is that the chromium content of the
Chromel-P lay somewhat above the composition which gives the maximum ther-
mal electromotive force. Another possible explanation is that the effects
07 cold working were being removed by annealing. The latter process has
been shown to lead to positive drifts in calibration.
Although it is known that the loss of soluble chromium decreases the
output of these thermocouples, it is quite possible that the loss of other
alloying elements may significantly alter the calibration of Chromel-P-
Alumel thermocouples. However, data are not available which allow an esti-
mate of the magnitudes of such changes.
1J. F. Potts, Jr., and D. L. McElroy, Thermocouple Research to 1000°C
Final Report, Nov. 1, 1957 through June 30, 1959, ORNL-2773 (Jan. 16, 1961).
2D. L. McElroy, Progress Report I, Thermocouple Research Report for
the Period Nov. 1, 1956 to Oct. 31, 1957, ORNL-2467 (March 5, 1958).
3L. S. Darken and R. W. Gurry, Physical Chemistry of Metals, p 349,
McGraw-Hill, New York, 1953.
ORNL-AIC-OFFICIAL
DRNI - AC - OFFICIAL
ORMI-ACC-OITIGIA
11.18
CONCLUSIONS
It has been shown that the thermal output of Chromel-P-Alumel ther-
mocouples decreases as a result of exposure to CO-CO2 mixtures at elevated
temperatures. These calibration changes are significantly-greater than
those observed for these thermocouple materials in air. In CO2-rich mix-
tures the negative drift is felt to be due primarily to the loss of chromium
I'rom solution in the Chromel-P wire as a result of oxidation. In co-rich:
mixtures carburization in addition to oxidation removes chromium from
solution thus causing the thermal electromotive force of the thermocouple
to decrease.
ORNL - AEC - OFFICIAL
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ILUXHALL
DATE FILMED
5 / 18 /65
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LEGAL NOTICE
This report was prepared as an account of Government sponsored work. Neither the United
States, nor the Commission, nor any person acting on behalf of the Commission:
A. Makes any warranty or representation, expressed or implied, with respect to the accu-
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END
+"