NBS MONOGRAPH 40

Thermocouple Materials

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

UNITED STATES DEPARTMENT OF COMMERCE • Luther H. Hodges, Secretary

NATIONAL BUREAU OF STANDARDS • A. V. Astin, Director

Thermocouple Materials

F. R. Caldwell

National Bureau of Standards Monograph 40

Issued March 1, 1962

Reprinted with corrections, February 1969

For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 - Price 50 cents
Contents

Page

1. Introduction 1

2. Bare thermocouple wires 2

2.1. Noble metals 2

a. Platinum and platinum-rliodimn alloys 2

b. Iridium versus iridium-rhodium alloys 10

c. Platinum-iridium 15 percent versus palladium 13

d. Platinel thermocouples 16

2.2. Base metals 16

a. ISA type K thermocouples 17

b. Geminol thermocouples 18

c. Iron-constantan thermocouples (ISA types Y and J) 19

d. Copper-constantan thermocouples (ISA type T) 20

e. Chromel-constantan thermocouples 21

f. Properties of base-metal thermocouple materials 21

2.3. Refractory metals 29

2.4. Carbon and carbides 35

3. Ceramic-packed thermocouple stock 37

3.1. Thermocouple wires 37

4. Conclusion 39

5. References 39

6. Appendix 42

ii
Tables

Page

1. Errors due to assuming the temperature of the

reference junction of a Pt-Rh 30% versus Pt-Rh 6% thermocouple to be at 0 °C when

it is at some higher temperature 3

2. Errors due to assuming the temperature of the

reference junction of a Pt-Rh 10% versus Pt thermocouple to be at 0 °C when it is at some higher temperature 3

3. Thermal emf and thermoelectric power of type

R, type S, and Pt-Rh 30% versus Pt-Rh 6%

(30-6) thermocouples 4

4. Grades of platinum 6

5. Tensile strenaiths of platinum and platinumrhodium 10% at room temperature and at 1,000 °C (1,832 °F) 6

6. Tensile strength of platinum-rhodium 10% at elevated temperatures 6

7. Tensile strengths of several platinum-rhodium

alloys at 20 °C (68 °F) 7

8. Proportional limits of platinum and platinumrhodium 10% 7

9. Relative electrical resistivities at temperatures

to 1,500 °C of platinum, Pt-Rh 10%, and

Pt-Rh 13% 7

10. Electrical resistivity at 20 °C of four Pt-Rh

alloys 7

11. Density at 20 °C of some platinum-rhodium

alloys 7

12. Thermal expansion of platinum, Pt-Rh 20%,

and rhodium relative to length at 0 °C

{I,Ho) 8

13. Mean specific heat above 20 °C of platinum 8

14. True specific heats of platinum and rhodium. _ 8

15. Thermal conductivity of platinum 8

16. Total emissivity, e, of platinum 9

17. Spectral emissivities of platinum 9

18. Emissivity and brightness temperature of

platinum at 0.660 n 9

19. Spectral emissivity of platinum in the infrared

at 1,125 °C 9

20. Volatilization of platinum in vacuo 9

21-. Calculated liquidus-solidus temperatures for

three iridium-rhodium alloys 10

22. Thermal emf of iridium versus Pt 27 12

23. Temperature coefficient of electrical resistance

of iridium 12

24. Electrical resistivity of iridium 12

25. Thermal expansion of iridium relative to length

atO°C(Z,/Zo) 12

26. True specific heat of iridium 13

27. Normal spectral emissivities of iridium at 295 °K 13

28. Reflectivity of iridium 13

29. Thermal emf of platinum-iridium 15% and

palladium versus Pt 27 13

30. Ultimate tensile strength of platinum-iridium

alloys, psi 14

31. Tensile strength of commercial palladium at temperatures up to 1,100 °C 14

32. Electrical resistivity and temperature coeffi- cient of resistance of some platinum-iridium

alloys 15

33. Electrical resistivity of palladium at temperatures up to 1,400 °C, and relative resistivity

up to 800 °C 15

34. Coefficient of thermal expansion of the platinum-iridium 20% alloy 15

35. Thermal expansion of palladium 15

36. True and mean specific heats of palladium 15

37. Total emissivity of palladium 16

38. Properties of Platinel 16

39. Calibration of Platinel thermocouples 16

40. ISA types of base-metal thermocouples 17

41. Special tolerances for different sizes of ChromelAlumel thermocouple wire 17

42. Maximum recommended operating temperatures for type K thermocouples 17

43. Changes in the calibration of Chromel-Alumel

(type K) thermocouples heated in air (electric furnace) ^ 18

44. Thermoelectric power of Geminol 18

45. Maximum recommended temperatures for Geminol thermocouples 19

46. Recommended upper temperatures for diiferent

wire sizes, iron-constantan 19

47. Changes in the calibration of iron-constantan

thermocouples heated in air (electric furnace) 20

48. Recommended upper temperatures for different

wire sizes, copper-constantan 20

49. Thermal electromotive force of different sizes

of constantan wire drawn from the same coil (reference material NBS Pt 27) 20

50. Thermoelectric power of the Chromel-constantan thermocouple - _ 21

51. Compositions of type K thermocouples. , 21

52. Some physical properties of type K thermocouple wires 22

53. Resistance of type K thermocouple wires at elevated temperatures 22

54. Resistance and length per pound of several

sizes of Chromel-Alumel and Kanthal wires. 22

55. Thermal coefficient of expansion of Tophel and

J^jg^J 23

56. Specific heat or80'%Ni-20%'Cr]!!]"'"III" 23

57. Thermal conductivity of Tophel and Nial 23

58. Calibrations for Chromel-platinum and Alumelplatinum thermocouples 23

59. Some physical properties of Geminol P and

Geminol N 24

60. Resistance and length per pound of several sizes of Geminol P and Geminol N wires 24

61. Thermal emf of Geminol P and Geminol N

versus Pt 27 24

62. Chemical and spectrographic analyses of iron

samples 24

63. Variation of tensile strength and modulus of

elasticity of Armco iron with temperature 25

64. Results of hardness measurements on samples

of iron wire 25

65. Electrical resistivity of iron relative to the re- sistivity at 0 °C {R,/Ro) 25

66. Thermal expansion of electrolytic iron 26

67. True and mean specific heats of electrolytic

iron 26

68. Thermal conductivity of Armco iron 26

69. Thermal emf of iron versus Pt 27 26

70. Some comparative stress-strain relations of

tough-pitch and oxygen-free copper 27

71. Softening temperature of pure copper containing individual impurity additions 27

72. Some physical properties of copper 27

73. Electrical resistivity of copper relative to the

resistivity at 0 °C {R,/Ro) 28

74. True and mean thermal coefficients of expansion of copper 28

75. True and mean specific heats of copper 28

76. Thermal conductivity of copper 28

77. Thermal emf of copper relative to platinum 28

78. Some properties of constantan 29

79. Thermal emf of constantan versus Pt 27 29

80. Electrical resistivity of constantan (55'Cu-45

Ni) relative to the resistivity at 0 °C {R,/Ro) . 29

81. Physical properties of tungsten 30

82. Chemical properties of tungsten 31

83. Physical properties of molybdenum 31

84. Chemical properties of molybdenum 32

iii
Tables—Continued

Page

85. Thermal emf of the tungsten versus molybdenum thermocouple 32

86. Analysis of rhenium, molybdenum, and tungsten 33

87. Tensile and elongation data for annealed and

cold-rolled rhenium sheet 33

88. Electrical resistivity of rhenium from 20 to 2,400 °C 33

89. Specific heat of rhenium 33

90. Linear thermal expansion of rhenium 33

91. Spectral emissivity of rhenium 34

92. Thermal emfs of W versus Re and Mo versus Re thermocouples [106] 34

93. Thermal emfs of W versus Re and Mo versus Re thermocouples [122] 34

94. Calibration data for tungsten versus rheniumalloy thermocouples and for thermocouples

of two tungsten-rhenium alloys 34

95. Chemical analyses of tungsten-rhenium alloys

and tungsten 34

96. Thermal emf of the tungsten versus iridium

thermocouple 35

97. Thermocouples using carbon and its modifications 36

98. Dimensions and wire sizes of available ceramicpacked stock 38

99. Sheath materials for ceramic-packed thermocouple stock and some of their properties 38

Page

100. Quality of and some thermal data on insulating

materials used in ceramic-packed thermocouple stock 38

101. Comparison of the average coefficients of

thermal expansion of refractory insulating

materials with those of three common

metals 39

102. Electrical resistivities of ceramic insulating

materials 39

1-A. Condensed calibration for the platinum 70%-

rhodium 30% versus platinum 94%-rhodium 6% thermocouple 42

2-A. Condensed reference table for the iridium

40%-rhodium 60% versus iridium thermocouple 43

3-A. Thermal emf of iridium 40%-rhodium 60%

versus copper and of copper versus iridium. 43

4-A. Condensed reference table for the platinum

85%-iridium 15% versus palladium thermocouple 43

5-A. Thermal emf of platinum 85%-iridium 15%

versus copper and of copper versus palladium 43

6-A. Condensed calibration data for Geminol P

and N thermocouple 43

7- A. Calibration data for iridium 60%-rhodium

40% versus iridium thermocouple 43

iv

I
Thermocouple Materials

F. R. Caldwell

Thermocouple materials are considered that are used primarily as immersion temperature sensors in the range from 0 °C up. Included are the conventional thermocouples that

have survived since the beginnings of the art of thermoelectric temperature measurement, newer noble metal thermocouples, and thermocouples of refractory metals for use in the- extreme range for immersed sensors. Thermocouples for thermoelectric generators are not

considered, nor are the types commonly used chiefly in radiation receivers such as those

containing antimony, bismuth, and their alloys. Because of the wide use and increasing

popularity of ceramic-packed thermocouples in metal sheaths, they are included.

Limitations of the thermocouple wires are given as to range, stability, environment

including atmosphere, magnitude of thermoelectric emf, and accuracy of commercially available materials of standard and extra quality. In addition, properties of the separate

elements that are pertinent to the selection or use of thermocouples have been compiled.

Among these are: chemical behavior, mechanical properties, specific heat, density, thermal

conductivity, thermal coefficient of expansion, emissivity, electrical resistivity, and magnetic

and catalytic properties. In the case of the ceramic-paclced thermocouples the following properties are presented:

temperature range of the sheath, mechanical properties of the sheath, kinds of packed insulation, resistance between thermocouple wires and between wires and sheath, minimum bending

radius of the packed stock, gas-tightness of the packed insulation, and types of measuring

junctions available, i.e., grounded, ungrounded, bare, totally enclosed, stagnation mounting,

etc. Not all of the above information is presented for all thermocouples, but all that is readily available in the general literature, catalogs, and by private communication is included.

Limitations on use of thermocouples normally are given in the text, and properties of the

materials generally are presented in tables.

1. Introduction

Of the infinite combinations of metals, alloys,

semiconductors, and nonmetals that can be combined to form thermocouples, only a few have

found use as temperature sensors. The thermocouples that have been in common use for fifty

years and more are still predominant, although

some special-purpose thermocouples have appeared

from time to time. Because of the needs of the

jet-engine, reactor, and rocket technologies in the

higher temperature ranges, however, new thermocouples are being developed to meet these recent

requirements.

The purpose of this paper is to present information on materials that are used from 0 °C up in

thermocouples intended primarily as immersed

temperature sensors, and to give so far as is practical properties of these materials that might

affect their use in temperature measurement.

This information is provided with the hope that

the advantages and limitations given for the

various thermocouples and materials will be of

value to those selecting sensors for particular

uses, and that the effort now required in finding

needed properties will be reduced. The material

included is from many sources including catalogs,

personal communications on work done at various

laboratories, and the general literature. Thermoelectric generators are not considered, nor are thermocouples included of the types commonly

* This paper was presented at the Fourth Symposium on Temperature,

Its Measurement and Control in Science and Industry; Columbus, Ohio; March 27-31, 1961; sponsored by the American Institute of Physics, the Instrument Society of America, and the National Bureau of Standards. The proceedings will be putiished in book form by the Beinhold Publishing

Corporation, New "Sorl!, N, Y.

used in radiation receivers, such as those of

antimony, bismuth and their alloys.

Originally this paper was planned to cover only

bare, unmounted thermocouple elements, but

because of the wide and increasing use of ceramicpacked thermocouples, the latter have been

included. Because the same wires are used in

both kinds of thermocouples, emphasis on the

ceramic-packed type will be directed chiefly

toward properties of the sheath and insulation.

Justification for inclusion of these properties of

the ceramic-packed thermocouples and not of

insulators for the bare-wire thermocouples may be

thought by some to be tenuous, but it arises from

consideration of the packed stock along with bare

wires as thermocouple raw material.

Among factors to be considered in selection of a thermocouple as an immersed temperature sensor

for a particular use may be for the thermocouple

combination:

Temperature-cmf relationship

Temperature range of use Diameters of wires available, and

For the separate thermocouple elements:

Melting point

Effects of environment on thermoelectric and

mechanical properties

Chemical properties and behavior

Mechanical properties

Specific heat

Density

Thermal conductivity

Thermal coefficient of expansion

Emissivity, spectral and total, of bright and

oxidized surfaces

1
Electrical resistivity

Temperature coefficient of resistance, and

Magnetic properties.

Not all of the properties are available for all thermocouples presented, but all that are readily

accessible will be given.

In the case of the ceramic-packed stock the

foregoing properties are just as important as for

bare thermocouples; and in addition, some other

factors must be taken into account, such as: Temperature range of the sheath

Mechanical properties of the sheath

Kinds of packed insulation

Resistance between wires, and between wires

and sheath

Minimum bending radius of the packed stock

Gas-tightness of the packed insulation, and

Types of measuring junctions available, i.e. grounded, ungrounded, bare, totally en-- closed, stagnation mounting, etc. References are made by numbers in the text to

corresponding numbers of entries in the Bibliography Section. No attempt has been made to

provide complete coverage of the literature, and

omission of a title from the list of references is not intended as an implication of any inadequacy

of the paper. References have been chosen in

some cases which give broader coverage and thus

provide, it is believed, greater consistency between

results than might be found in similar data on the

same subject from several sources. Some sacrifice

of accuracy might occur occasionally because of

such choices; but since many of the data are found without much, if any, information on how

they were obtained, evaluation of experimental

results is difficult in many cases. The exhaustive work of Potts and McElroy

[77] ' unfortunately was received after this paper

was nearly completed, and time did not permit

revision to include or even properly refer to their

results. It is, however, an extremely fertile source of information on base-metal thermocouples, especially ISA type K, and is recommended as excellent reference material.

No history of the thermocouple art is presented.

This is considered superfluous here, because such

information has been published many times in

recent years, and may be expected to continue to

appear in introductions of papers of the future.

It is assumed throughout this paper that the

reader is familiar with the basic principles of

thermoelectric thermometry to the extent, at

least, that he can intelligently apply them to

practice.

Temperatures referred to throughout the discussions that follow are, unless otherwise indicated,

in degrees C of the International Temperature

Scale of 1948. In all cases where equations

containing temperature are taken from a reference,

they are unchanged; no adjustments have been

made in them to account for the difference between

the 1948 temperature scale and that used at the

time of the particular work cited. In some cases, though, slight adjustments have been made on

tabulated data to put them on the 1948 scale.

Generally the information is given in the same

units as those used by the authors, although

conversion to more familiar units has been made

in some cases. This may lead to some confusion,

but this is thought to be only a minor disadvantage

to those having interests that involve the data

presented. The electromotive forces (emfs) are

in terms of the absolute volt.

2. Bare Thermocouple Wires

The materials considered in this section are, for

convenience, classified as follows:

1. Noble Metals

2. Base Metals

3. Refractory Metals

4. Carbon and Carbides

The first two items represent the vast majority

of thermocouples now in use, and much information on them is available. Fewer data are available on the third item, but a considerable effort is being exerted in the field of refractory metals to

meet the high-temperature needs of recent technological developments. Relatively little work

has been done on materials of the fourth item,

but some thermocouples employing graphite,

carbon and carbides are used. Sufficient promise

has been shown by these materials though, to

make them worthy of inclusion.

Discussions of various thermocouple materials

foUow.

2.1. Noble Metals

a. Platinum and Platinum-Rhodium Alloys

The oldest and most used noble-metal thermocouple is the platinum-rhodium ^ versus platinum

thermocouple. Both the 10 percent (ISA type S) [1] and 13 percent (ISA type R) [1] rhodium-platinum

positive elements are used extensively, and the

platinum-rhodium 10 percent versus platinum

thermocouple continues to serve as a means of

realization of and interpolation on the International Temperature Scale of 1948. More space

will be devoted to these thermocouples for two

reasons. First, they, in defining the temperature

scale, are of more basic importance than are others,

and secondly, more information is available on

them than on most others.

Thermocouples having both legs of alloys of

platinum and rhodium have been proposed, and

some of these combinations are tabulated below.

' Figures in brackets indicate the literature references on page 39.

' When mentioning a thermocouple, the positive element is given first. This is in line with an apparent effort to standardize nomenclature among

users of tliermocouples.

2
Positive wire Negative wire

Platinum-rhodium

13 percent.

Platinum-rhodium

20 percent.

Platinum-rhodium

30 percent.

Platinum-rhodium

40 percent.

Platinum-rhodium

1 percent.

Platinum-rhodium

5 percent.

Platinum-rhodium

6 percent.

Platinum-rhodium

20 percent.

Advantages of the all-alloy thermocouples are

said to be greater stability, a somewhat greater

temperature range of use, and increased stiffness

with greater resistance to mechanical deterioration. The most prominent of the tabulated combinations appears to be the platinum-rhodium 30

percent versus platinum-rhodium 6 percent (30-6)

thermocouple, and it will be discussed in some

detail.

The range of thermocouples of type S or R

is limited by the melting point of platinum,

1769 °C [2,3], while the temperature of the 30-6

thermocouple must be kept below the liquidus of

the platinum-rhodium 6 percent element, about

1820 °C [4]. The actual upper useful limits are below the temperatures mentioned, especially for

the thermocouple of the two alloys where the uncertainty in the liquidus of the 6 percent alloy may

be as great as 20 °C. It probably is safe to say,

however, that the upper usable limit is extended

by at least 30 °C through the use of the all-alloy

thermocouple. As an indication of the upper

limit of the rhodium 40 percent versus rhodium

20 percent alloy, Acken [4] shows the liquidus

of platinum-rhodium 20 percent to be about

1850 °C±20 °C.

Another decided advantage of the 30-6 thermocouple that may be more important than those

mentioned above is the very small emf in the nor- mal reference temperature range, from 0 °C to 50

or 100 °C. The emf of this thermocouple is only

about 7 microvolts (/xv) at 25 °C [5] when the

reference junction is at 0 °C, and increases to 56

^l^r at 100 °C. Errors arising from uncertainties

in or ignoring the temperature of the reference

junction are therefore relatively small for measurements at high temperatures. The errors for

several reference and measuring junction temperatures are given in table 1. The negative sign of the table points out that

values taken from the calibration are too low, and

that corrections to temperatures corresponding to

the observed emf must be positive. The error is seen to be about 5 °C at a reference temperature

of 100 °C and measured temperatures of 1200 to

1800 °C. The errors are greater at the lower

measured temperatures ; but when it is remembered

that this thermocouple is intended for use in the

upper range and the reference temperature of 100

°C probably is unrealistic for most cases, the temperatiire of the reference junction may be ignored

Table 1. Errors due to assuming the temperature of the reference junction of a Pt-Rh 30 percent versus Pt-Rh

6 percent thermocouple to be at 0 °C when it is at some

higher temperature

Actual tein- Errors, degrees C at measuring junction temperatures of

reference junction 600 700 800 1,000 1,200 1,400 1, 600 1,800

°C

20 -0.8 -0.7 -0.7 -0.6 -0.5 -0.4 -0.4 -0.4

40 -2.4 -2.0 -1.9 -1.5 -1.3 -1.2 -1.2 -1.2

60 -4.4 -3.7 -3.4 -2.8 -2.5 -2.3 -2.3 -2.3

80 -6.7 -5.6 -5.3 -4.3 -3.8 -3.6 -3.5 -3.5

100 -9.4 -8.0 -7.4 -6.

1

-5.3 -5.0

' -5.0 -5.0

in general use. This idea is substantiated by the

smaller errors at lower reference and higher measured temperatures.

If the temperature of the reference junction is measured, the corrections can be appUed to the

temperatures corresponding to the measured

thermal emf. Linear interpolation between refer- ence temperatures of table 1 causes no further

errors, and when appHed to measured temperatures

leads to an added error of only 0.3° at 750 °C

when the reference temperature is 100 °C. The

errors introduced by linear interpolation become

smaller at higher measured temperatures, and thus

are insignificant. If the highest accuracy is desired, the use of an ice bath [6] to keep the reference

jimction at 0 °C is indicated.

Errors for a type S thermocouple imder similar

conditions are presented for comparison in table 2.

Table 2. Errors due to assuming the temperature of the reference junction of a Pt-Rh 10 percent versus Pt thermocouple to be at 0 °C when it is at some higher temperature

Actual temperature of

Errors, degrees C at measuring junction

temperatures of

junction

800 1,200 1,600

°C

20 -10.4 -9.4 -9.5

60 -33.4 -30.3 -30.7

100 -60.1 -53.6 -54.3

Here the value at a reference temperature of 20 °C

and a measured 1600 °C is —9.5 °C or about 24

times that for the 30-6. At temperatures of 100"

and 800 °C the error is —60.0 °C, and the ratio

of errors therefore is about 8. The advantage,

consequently, from the standpoint of the small

effect of the temperature of the reference junction

of the all-alloy thermocouple is obvious. Errors

for the type R thermocouple are smaller than

those presented in table 2 by about 12K to 14

percent because of the greater thermal emf of the

type R thermocouple in the upper temperature

range.

Reference tables for the platinum —10 and — 13

percent rhodium versus platinum thermocouples

are given in the National Bureau of. Standards

Reference Tables [7]. No NBS reference table

is yet available for the platinum —30 percent

rhodium versus platinimi —6 percent rhodium

thermocouple, but a table published for this com3
biBation is presented in table 1 of the appendix.

Of the thiree thermocouples under discussion,

type R provides the greatest thermoelectric output and thermoelectric " power, type S the next,

and the 30-6 the smallest. A comparison of the

three, from the references just cited, is given for

600 °C and above in table 3.

Table 3. Thermal emf and thermoelectric power of type R,

type S, and Pt-Rh SO percent versus Pt-Rh 6 percent {30-6)

thermocouples

Emf and thermoelectric power, reference junction at 0 °C, for temperatures, °C, of

600 800 1,200 1.600

mv mv mv mv R

S

30-6

5. 563 5.224 1.796

11.35

10.20

5.95

7.924

7. 329 3.162

12.30

10. 90

7. 55

13.93

11.935

6. 811

13.85

12. 00

10. 50

18. 727

16. 716

11.260

13.70

11.85

11.30

The thermal emf of the 30-6 combination

ranges from 34 percent of the type S and 32 percent of the type R, at 600 °C, to 67 and 60 percent

at 1,600 °C. The ranges of thermoelectric power

for the same temperatures are from 58 and 52

percent to 95 and 82 percent. Thus it is seen that over the range considered here no great loss

of precision will be introduced through the use

of the 30-6 thermocouple, and that any such loss

may be outweighed in ordinary use by the advantages of this thermocouple.

A disadvantage of the smaller emf of this

thermocouple, especially in the lower range, is that the noise of a control system containing

electronic components becomes a larger fraction

of the total signal as the thermocouple emf decreases. The accuracy of a measuring system

that includes a low-emf thermocouple will be

lowered unless some way is employed to eliminate

or compensate for the noise. Another feature

that is related directly to the low emf in the

reference temperature region is the small thermoelectric power at 500 °C and below. The change

in emf per degree C is about 5mv at 500 °C and

drops to less than 0.5 ixy per degree at 25 °C.

This thermocouple cannot be used even with an ice bath for high accuracy in this region, and

if high accuracy is required in both the lower and

upper ranges, the type R or S thermocouple

should be used with an ice bath. None of these

thermocouples is intended primarily for use in

the lower range, though, and unless some experimental factor requires the use of one thermocouple from the lowest to the upper range, a more

suitable thermocouple should be used in the lower

range.

The limits of error for types R and S thermocouples are given by suppliers normally as 3 °C

(5 °F) in the lower range, and to % percent or K

percent of the measured temperature in the upper

range. In one case, and possibly more, the limits are given as 1.4 °C (2.5 °F) and ±0.25 percent

for the lower and upper ranges. Limits as stated

in catalogs generally express the degree of agreement with the tables of NBS Circular 561. The

ranges are not imiform among supphers ; the lower

range as quoted varies from —18 °C (0 °F) or 0 °C (32 °F) to from 538 °C (1,000 °F) to 721 °C

(1,330 °F). The upper limit cited extends from

1,316 _°C (2,400 °F) to 1,482 °C (2,700 °F).

The limits of error given in reference 1 for platinum-rhodium versus platinum thermocouples are ±5 °F (2.8 °C) from 32 to 1,000 °F (0 to 538 °C)

and ±0.5 percent from 1,000 to 2,700 °F (538

to 1,482 °C). These thermocouples can be used

at temperatures above the maximum limits given,

with possible shortening of their useful hfe.

Accuracy of calibrations made at the National

Bureau of Standards of types R and S thermocouples as described in reference [8] follows. The

first statement is for calibrations against an NBS

standard platinum-rhodium 10 percent versus platinum thermocouple.

"The certified accuracy of cahbration of platinum versus platinum-rhodium thermocouples is 0.5 degrees C (0.9 °F) in the range 0 to 1,100 °C

(32 to 2,012 °F), and ranges from 0.5 degree at

1,100 °C to 2 (3.6 °F) degrees at 1,450 °C (2,642

°F). Results above 1,100 °C are obtained by

extrapolation."

For the primary cahbration at four fixed points

the accxu-acy is given as : . . the certified accu- racy of cahbration is 0.3° (0.5 °F) from 0 to 1,100

°C and ranges from 0.3° at 1,100° to 2° (3.6 °F)

at 1,450 °C (2,642 °F). Accuracy of certification

at the fixed points is 2 /iv. If the submitted

thermocouple meets the International Temperature

Scale requirements for standard thermocouples

(see the International Temperature Scale of 1948

by H. F. Stimson, J. Research NBS 43, 209, 1949), a quadratic equation fitted, at the freezing points

of antimony, silver, and gold wiU also be furnished." Specifications for thermocouples submitted for either type of cahbration will be found

in reference [8]. In the laboratory a thermocouple may conveniently be connected to a measuring instrument

using copper leads which join the thermocouple

wires in an ice bath. This is normally not practicable in industrial apphcations where the instrument is remote from the sensing part of the system,

since it would require either considerable amounts

of prohibitively expensive wire or locating an ice

bath near the thermocouple. Lead, or extension,

wires of base metals and alloys are available which

have the same temperature-emf relationship

aroimd ambient temperature as the rare-metal

thermocouple for which they were designed as

leads. For base metals, smaller wires of the same

nominal composition as the thermocouple elements

very often are used as extension leads.

Proper use of such leads makes it possible and

practical, in effect, to extend the thermocouple to

the measuring instrument which customarily in

industrial applications contains automatic compensation for the cold junction. Leads for the

4
types R and S thermocouples have copper as the

positive element and a copper-nickel alloy as the

negative wire. Generally the range for these leads

is stated as 24 to 204 °C (75 to 400 °F) and the

limit of error resulting from the use of the leads

for regular-grade wires is given as ±7 °C (13 °F).

In one instance and possibly others, limits of error

for the regular-grade leads are given as ±6 percent

or ± 5 °C (9 °F) for both types R and S, and the

limits for premium-grade extension wires are given

as ±2}i percent for type R and Iji percent for

type S. Limits given as percentages apply to the

difference between the temperature at the connection to the thermocouple and the temperature

at the measuring instrument. Where the error derived from the percent value is smaller than that

given in degrees, it is considered to be the Umit,

rather than the quantity given in degrees.

A word of caution may be worthwhile here on

the use of extension leads. Although the lead

wires have, within limits, the same temperatureemf relationships as the rare-metal thermocouples,

the positive copper and platinum-rhodium are not

thermoelectrically neutral, nor are the negative

copper-nickel alloy and platinum. These facts

lead to the requirement that the temperature of

the two leads be the same at any junction with

any other material, as for example, at the connection to the thermocouple or to switches. If this

condition is not fulfilled, large errors may result.

The possibility of large errors normally is not

present in thermocouples mounted by suppliers or manufacturers in tubes with heads in which connectors are mounted. The chief danger lies in the

use of unmounted wires where the junctions may

be separated and subjected to different conditions

of drafts, radiation^ etc. Physical and chemical properties of thermocouple elements of the platinum metals have been

assembled from several sources, one of the most

fertile of which is the excellent work of Vines [9]. Many of the references cited here have been taken

•directly from his book, and are so credited. Another excellent source of information on metals in

general is the Metals Handbook, American Society for Metals, 1948 edition [10]. The properties given for the platinum metals in this section

and for platinum and other metals later are those

that may influence the utility of a thermocouple

and the accuracy of results obtainable with it. One of the more important properties of good

thermocouple wires is the uniformity of composition and physical condition throughout. Errors

will arise when a region of chemical or physical

inhomogeneity of a thermocouple is in a temperature gradient. This is so, not only for the rare- metal thermocouples discussed in this section, but

also for any thermocouple used for measurement

of temperature. Among those who have presented

discussions on the subject of homogeneity are Roeser and Lonberger [6], McElroy [11], and

Potts and McElroy [77]. In addition, references

[11] and [77] give quantitative information on

effects of inhomogencities and equipment for

testing for homogeneity. These reports involve

chiefly the nickel-base elements exclusive of those

with copper, but the treatment is applicable to

any thermocouple wires. The chief point that

is made, however, is the real importance of

homogeneity.

Platinum and the alloys considered here are

available in a wide range of wire diameters from,

within reasonable limits, as large as desired down

to a fraction of a mil (0.001 in.). The most

popular diameter, from the standpoint of economy

and convenience of handling, seems to be about

20 mils or 0.5 mm. The larger wires are used

where greater strength and rigidity are wanted,

and where fast response to temperature changes

and effects of conduction and radiation are relatively unimportant. Smaller wires are used where

one or more of these items becomes important.

As are the other platinum metals, platinum is resistant to corrosion by the common acids and

chemicals. It is unaffected by any single acid,

but is dissolved b}^ aqua regia. The platinumrhodium alloys are equally resistant and are said

to be unaffected by aqua regia when they contain

more than about 20 percent rhodium. These

thermocouples should be used under oxidizing or neutral conditions. Platinum and its alloys with

rhodium deteriorate under reducing conditions at

high temperatures by absorbing gases and metals

reduced from the oxides of insulating refractories

in contact with them. Commonly used refractories are sources of silicon contamination under

adverse conditions. Among the more common

reducing atmospheres encountered in practice are those containing carbon and sulfur. Exposure of

the platinum versus platinum-rhodium thermocouples to these conditions at high temperature

must therefore be avoided. The effect of silicon

in a reducing atmosphere was pointed out by

McQuillan [12], who found that platinum in the

presence of silicon failed in less than an hour.

No effect was observed though, in the presence of

alumina, berylha, or carbon in Qji hr.

A more complete statement on the resistance

of platinum to specific agents is found in reference

[10], page 1,122, the material of which is credited

to the Corrosion Handbook [13]. F. E. Carter

[10] gives the resistance to corrosion of the platinum-rhodium alloys as ". . . as good as or better

than that of pure platinum at ordinary temperatures, and at high temperatures is usually better

than the resistance of pure platinum, except under

sulfidizing conditions."

In unpublished work by Olsen at the National

Bureau of Standards, platinum has been found

to be an active catalyst in dilute mixtures of

hydrogen in air, and carbon monoxide in air, to

the extent that temperature measurements in such

mixtures are subject to large errors. Products of- incomplete combustion in, for example, burners

618629—62 2 5
using hydrocarbon fuels thus may be responsible

for the same effects which appear at temperatures

as low as about 120 °C.. Although no quantitative corrections can be given for thermocouples

exposed to residual combustibles and oxidant,

caution should be exercised in interpretation of

data obtained under such conditions with platinum

versus platinum-rhodium thermocouples.

Annealed platinum is so soft that is cannot be

used bare in high-velocity streams of hot gases or

as a long cantilever in quiescent hot gas or vacuum

without bending. Some means of protection must

be used in such cases, but this often imposes

other disadvantages, such as slower response to

changes, increased losses due to conduction, etc. The all-alloy thermocouple described earher has

some advantage in this respect because of the

increased stiffness of the alloy wires over that of

the platinum.

Complete annealing of very pure platinum is said by Vines [9] to occur at 400 °C (752 °F).

The time required for this treatment is not specified, but Wise and Vines [14] have stated the

temperatures required for complete anneahng of

99.9+ percent pure platinum after exposure for

15 min. Their statement is: "The temperatures

required to produce complete softening of this

platinum in a 15-minute anneal are approximately

765, 700, 635, 600, 545, and 425 °C respectively

for reductions in thickness of 19, 39.5, 50.8, 66,

80.5, and 89.5%." Here it is seen that as the

percentage reduction of area, or cold working, is increased, the temperature for complete softening

is decreased; and that after severe reduction by

drawing of extremely pure platinum of thermocouple grade, anneahng at 500 °C (932 °F) for 15

min probably would be more than sufficient. At

the National Bureau of Standards, however, the

regular procedure is to anneal electrically for one

hour at 1,450 °C (2,642 °F). This does not produce appreciable adverse effects in the strength of

the wire, and may oxidize or otherwise minimize

effects of impurities in the wires.

Although the user of thermocouples of platinum

and its alloys with rhodium normally is not con- cerned with numerical data on their mechanical

properties, certain properties are related to mechanical behavior and wUl be given. The an- nealed condition is the only one of interest for

thermocouples, but some information on the coldworked state is presented for comparison. Grades

of platinum discussed are according to the designations of table 1 of Vines [9], which follow in

table 4. Table 4.

Grades of platinum

Grade Minimum

platinum

content

Refiners' designations

%

1 99. 99 Thermo-element, physically pure or chemically pure.

2 99.9 Chemically pure, special pure,

or specially refined.

These grades are equivalent to types A and B of Carter [15].

The tensile strength of grade 1 annealed platinum has been given by Sivil [16] as about 18,000

psi. Wise and Eash [17] found that the tensile

strength of grade 2 platinum increased from 22,000

psi in the annealed condition to 36,000 psi after a reduction in area of 50 percent by cold-drawing.

In later work an increase was found from 19,350

psi to 29,500 psi after cold-swaging 50 percent.

The above values obtained at room temperature.

Wise and Eash [17] reported a tensile strength

at room temperature, after annealing at 1,200 °C

(2,192 °F), for an alloy of 90 percent platinum

and 10 percent rhodium, of 47,000 psi. After 50

percent reduction by cold working the tensile

strength was 84,300 psi.

The reduction in tensile strength on heating of

platinum and platinum-rhodium 10 percent, an- nealed at 1,100 °C (2,012 °F), is seen in the following values taken from Wise and Eash [18].

Table 5. Tensile strengths of platinum and platinumrhodium 10 percent at room temperature and at 1,000 °C

{1,832 °F)

Composition

Tensile strength

Room

temperature

1,000 °C

psi psi Grade 2 platinum. 20, 700 4, 080 Pt-Rh 10 percent- 47,900 13, 600

The variation of tensile strength with temperature for the platinum-rhodium 10 percent alloy is

given in table 6 from Carter and Stauss [19].

Table 6. Tensile strength of platinum-rhodium 10 percent

at elevated temperatures

Temperature

Tensile strength,

psi. Wire diameter, in.

0.050

0.010 0.003

"C °F

20 68 45, 000 45, 000

500 932 29,700 31,950

700 1,292 24, 300 25, 650 900 1,652 17, 100 19, 350

1, 100 2,012 10, 350 13, 050

1,300 2,372 6, 750

1,500 2,732 3, 600

The above values have been derived from the

ratio of tensile strength to that at 20 °C given by

Vines from Carter and Stauss.

Vines also gives the values of tensile strength

as determined by Carter and Stauss for several

alloys of platinum and rhodium which cover the

thermocouple compositions up to 20 percent rhodium. This information, in table 7, is for a temperature of 20 °C.

Small amounts of alloying impurities have an

appreciable effect on the tensile strength of

platinum, according to Vines [9J, and their effect

is even more pronounced on the proportional

limit. This is shown along with the proportional

limit for platinum-rhodium 10 percent in table 8

taken from table 14 of reference [9].

6
Table 7. Tensile strengths of several platinum-rhodium

alloys at20°C {68 °F)

Ultimate tensUe Bhodium strength

Hard Annealed

% psi psi

0 34, 000 18, 000

3.5 60, 000 25, 000

6 70,000 30. 000

10 90,000 45, 000 20 130, 000 70, 000

Table 8. Proportional limits of platinum and platinumrhodium 10 percent

Alloy

Proportional

limit, cold worked, 50 percent reductionAnnealing

temperature

Proportional

limit annealed

Grade 1 platinum

psi °C

900

1,000

1,200

psi

<2,000

2, 000-5, 500 17,000 Grade 2 platinum

Platinum-rtiodium 10 percent27,000

55,600

Most of the more commonly considered physical properties of the platinum and platinum-rhodium thermocouple elements have direct bearings

upon their performance in temperature measurements; some, in addition, are of value in selecting

wires of high purity and for special uses. The purity of platinum often is inferred from

two electrical quantities, the thermal emf and

a= (i?ioo— i?o)/(100i?o), where B, is resistance and

the subscripts refer to degrees C. Because all known impurities likely to be found in platinum,

with the exception of gold in detectable quantity,

make the thermal emf positive, the more negative

a sample of wire is in this respect, the purer it is considered to be. A sample of extremely pure

platinum made at NBS in 1922 and known as Pt 27 has been kept as a reference, and has served

as a standard against which much of the platinum

used in thermocouples has been compared. In

the early days of Pt 27 practically all platinum was

.thermoelectrically positive to it, but recently high

grade platinum neutral to Pt 27 has become

fairly common, and some samples as much as S or

10 p.Y negative have appeared.

The a mentioned above probably is a more

reliable indication of purity of platinum than is the thermal emf; and can, for very pure platinum,

be correlated with thermal emf. Since any

irnpurities reduce the value of a, the correlation is between ascending a and more negative thermal

emf. It appears at present that the ultimate

value of a for platinum with no impurities present

is in the neighborhood of 0.003928, a value which

has been obtained for a few samples. Platinum

with an a of 0.003923 is available commercially

and values up to 0.003927 are attainable if sufficient care in refining and handling is exercised. It

thus appears that a material standard such as Pt 27 has outlived its usefulness, and that refer- ence samples kept for comparison might be

described in terms of the relationships between

thermal emf and a of the samples.

Data on the resistivity of 0 °C and relative

resistivity of platinum and the conventional

platinum-rhodium thermocouples taken from

Roeser and Wensel's [20] table are presented in

table 9. Resistivity is given here in ohms per

circular mil foot (ohms/cmf), and the ratios in

some cases have been adjusted slightly to convert

to degrees C 1948 Int.

Table 9. Relative electrical resistivities at temperatures to 1,500 °C of platinum, Pt-Rh 10 percent, and Pt-Rh 13

percent

Platinum Pt-Rh 10 Pt-Rh 13

0 °C, ohms/cmf 59.1 110.7 114.3

Temperature ° C RiIRo RtIRo RilRa

0 LOOO 1.000 1.000 20 » 1. 056 b 1. 030 <>1.03 100 1.392 1. 166 1.156 200 1.773 1.330 1.308 300 2. 142 1.490 1. 456

400 2. 499 1.646 L601

500 2.844 1.798 1.744 600 3. 178 1.947 1.885 700 3.500 2.093 2. 023

800 3. 809 2. 233 2. 156 900 4. 108 2. 369 2.286

1,000 4. 395 2. 503 2. 414

1,100 4.672 2.633 2. 538 1,200 4.933 2. 760 2.659

1,300 5. 184 2. 8h3 2. 779 1,400 5.423 3. 009 2.896

1, 500 5. 650 3. 130 3.011

» Calculated from Rn and equation of reference [9], page 20.

i> From reference [9], table 42.

0 From figure 3 of reference [21]. An equation relating the resistivity of platinum

at temperatures up to 1,500 °C, and giving values near those of Roeser and Wensel is:

Rt=Ro{l + 3.9788 X 10-^ t - 5.88 X 10"^ t^)

The electrical resistivities at 20 °C of four

platinum-rhodium alloys are given in table 10 [22].

Table 10. Electrical resistivity at 20 °C of four Pt-Rh

alloys

Percent rhodium

Resistivity

at 20 °C

ohmslcmf

3.5 99.9

5 105.3

10 115.5 20 125.1

The density of annealed platinum at 20 °C

appears to be generally accepted as 21.45 g/cm^.

The densities of some platinum-rhodium alloys as given by Carter [22] are presented in table 11.

Table 11. Density at 20 °C of some platinum-rhodium

alloys

Weight percent rhodium

0 3.5 5 10 20 30 « 100 1>

Density... -.g/cms..

Density lb/in. 21.45

0. 7749

20.9

0. 755

20. 65

0. 746

19. 97

0. 721

18.74 0.677

17.6 0.636

12. 414 0.448

• From reference [4], figure 3, density at 25 "C, sample annealed at l,500°O.

b As given by Vines [9], attributed to Holzmann (1931).

7
Table 12. Thermal expansion of platinum, Vt-Kh 20 percent, and rhodium relative to length at 0 °C (It/h)

1 emperature

"Plif'intim X IdtlllLllii X L-xvJJ ZU percent percent

'C

U 1.000000 1.00000 1. 00000 20 J.. UUUJ.OU 1 nnni

s

i. UUUio 1 nnm fi 1. UUUID

25 1. 000225 1. 00022 1. 00020

ou 1. 000452 1. 00044 1. 00040 75 1. 000680 1. 00067 1. 00062

inn J.UU 1. 000909 1. 00089 1. 00085 onn 1 001841 1J.. nmUUi.Oi R9

^nn ouu 1. 002796 1. 00278 1. 00280

400 1.003773 1. 00377 1. 00385

500 1.004776 » 1. 00480 1. 00490

DUU 1. 005803 1. 00586 1. 00600

vnn /uu 1.006853 1. 00695 1. 00710

800 1. 007931 1. 00807 1. 00825 900 1. 009039 1.00922 1. 00949

1, 000 1.010176 « 1. 0104 1. 01080

1,100 1.0116 1. 0121 1,200 1. 0128 1. 0135

1,300 1.0141 1,400

1,500

a 1.0154 1.0167

» These points are given in reference [24].

The thermal expansions of platinum [23], platinum-rhodium 20 percent [24], and rhodium [9] are presented in table 12.

Very few data seem to be available on the alloys,

and rhodium is included in table 12 as a guide for

approximate estimation. Only three values were

available for the alloy of 20 percent rhodium, and

so the others were obtained by interpolation with

a cubic equation. Slight adjustments were made

in a few cases to place the data on the 1948 temperature scale. The equation given by Esser and

Eusterbrock in reference [23] for the thermal ex- pansion of platinum is

:

Z,=/o(i+8.9877X 10-6^ +0.0010652X 10~H^

+0.0000001256X IQ-H^)

Jaeger and Rosenbohm [25] have given equations for the heat content of platinum at temperatures from 0 _to 1,600. °C and values of mean

specific heat (Cp)_from 20 to 400 °C. An equation is given for Cp from 20° to temperatures in

the range 400 to 1,600 °C. The data are said by

the authors to be good to ±0.1 percent. The

equations for the heat content are:

for temperatures between 0 and 500 °C

Q(,o_oO) =0.031357t-F0.04507XlO-*i2_o.0161

X 10-^^^ I (A)

and for temperatures between 400 and 1,600°

Q(,o_oo, =0.031622t-f0.03172XlO-''fl I (B)

Values of specific heat for platinum in table 14

have been obtained by differentiating and substitution of temperatures in the above equations.

In the region of overlap, 400 to 500 °C, agreement

between data obtained is not so good, and a slight

discontinuity is seen in the region where the two

equations join. Values at 400 °C and below were

obtained from equation 1(A) and those above are from 1(B).

Mean specific heats above 20 °C are tabulated

up to 400 °C and an equation is given for higher

temperatures. The tabulated values are given

in table 13, and the equation for the mean specific

heat, (7p(<o_2oo,, follows. From 400 to 1,600 °C

(7,,(,o_2oo,=0.031701-f0.03164XlO-*i.

Table 13. Mean specific heat above 20 °C of platinum

Temperature Cp ((°-20°) cal/g °C

°C

100 0. 03187

200 . 03228

300 . 03264 400 . 03296

Other values of specific heat of platinum are found in Vines [9] and the 1948 edition of the

Metals Handbook.

No determinations of the specific heats of

platinum-rhodium alloys seem to be available.

The values of Jaeger and Rosenbohm [25] for

rhodium up to 1,300 °C are presented in table 14.

Table 14. True specific heats of platinum and rhodium

Temperature

Specific heat, cal/g °C

Temperature

Specific heat, cal/g °C

Platinum Rhodium Platinum Rhodium

°C "C

0 0. 03136 0. 05893 800 0. 30670 0. 07618

25 . 03158 . 05922 900 . 03733 . 07814

50 . 03180 . 05953 1,000 . 03797 . 07969 100 . 03221 . 06026 1,100 . 03860 . 08074

200 . 03297 . 06203 1,200 . 03924 . 08119

300 . 03363 .06415 1,300 . 03987 . 08092

400 . 03419 . 06650 1,400 . 04050

600 . 03466 . 06899 1,500 .04114

600 . 03543 . 07150 1,600 . 04177

700 . 03606 . 07393

The equation from which these values were calculated is that given by Jaeger and Rosenbohm

:

(7^=0.05893 + 1.066X 10-* i+2.7744X lO"* f

-1.7642X10-" f.

Further values of the specific heat of rhodium

may be found in reference [9], and numerous

values of thermal properties of platinum and some

other metals are included in reference [26], which,

together with reference [27], is a prolific source of

thermal properties of some metals.

Table 15. Thermal conductivity of platinum

Temperature Thermal

conduptivity

Temperature Thermal

conductivity

"C callsec,cm''icmj°C °C callseclcm^iCmrC

0 0.1660 600 0.1943

20 .1669 700 .1991 100 .1707 800 .2038

200 .1754 900 .2085 300 .1802 1,000 .2132

400 .1849 1,100 .2180

500 .1896

8
Almost all of the information on thermal

conductivity of platinum is for the low range,

from room temperature to. 100 °C, and no data

seem to be available for the platinum-rhodium

alloys in the higher range. Values calculated

from the equation of Holm and Stormer [28] are given in table 15, and their equation follows.

The equation used to calculate the values of

table 15 is

yt=0.699[l +2.83X 10-* (i-19.5)].

In this case the thermal conductivity, k, is in

watts/cm7cm/°C. Values obtained from the

equation were multiplied by 0.2388 to convert to

the units given in the table.

Only one value of thermal conductivity for

platinum-rhodium alloys has been found. This,

for platinum-rhodium 10 percent (reference [3], p. 138), is 0.072 cal/sec/cm7cm/°C at a temperature of 18 °C. Alpha (a) in the equation

ki=ko{l-\-oL{t—to)] is given as +0.0002, where

is the conductivity at 18 °C and i refers to some

other temperature. No reference to the original

paper is given in reference [3], so that the limitations of this equation are unknown.

A considerable amount of information is found

on the emissivity (e) of platinum. Some disagreements are seen between the various investigators, and the tables that follow are given either

because they provide the most recent data, or

are the only sources found. No information on

the platinum-rhodium alloys seems to be available.

The total emissivity of platinum as given by

Geiss [29] follows in table 16. Note that temperature in this table and some of the following are in

°K and not °C.

Table 16. Total emissivity, e, of platinum

Temperature e Temperature e

"K percevt "K percent

.500 7.31 1,100 13.4

600 8. 41 1,200 14.3

700 9. 48 1,300 15.2 800 10.5 1,400 16.1 900 11.5 1,500 17.0

1,000 12.75 1,600 17.8

Emissivity, in this paper, refers to the ratio of

energy radiated hemispherically per unit area per

unit time by a surface to that radiated by a blackbody at the same temperature.

Spectral emissivities of platinvmi of Worthing

[30] are given in table 17.

Table 17. Spectral emissivities of platinum at wavelengths (microns) of

Temperature 0.665 n 0.535 M 0.460 iJi

1,200

1,600

1,850

0.295 0. 325

0. 335

0. 375

0. 310 0. 390

Values of table 18 by Stephens [31] are for the

emissivity of platinmn at 0.660m, in the wavelength region most commonly used in optical

pjrrometry. Brightness temperatures corresponding to true temperatures are included.

Table 18. Emissivity and brightness temperature of

platinum at 0.660 n

True temperature ^ 0.660 JLi

Bright- ness temperatureTrue temperature * 0.660 MBright- ness temperature

"K "K °K °K

1,200 0.283 1,121.6 1,600 0. 291 1, 466. 5 1,25C .284 1,165.4 1,650 .292 1,508.8

1,300 .285 1.209.0 1,700 .293 1,550.8 1,360 .286 1, 252. 5 1,750 .294 1, 592. 7

1,400 .287 1.295.6 1,800 .295 1, 634. 4 1,450 .288 1,338.6 1,850 .296 1,676.0

1,500 .289 1.381.4 1.900 .297 1,717.4

1,550 .290 1, 424.

1

Emissivities of platinum in the infrared as determined at 1,125 °C by Price [32] are presented

in table 19.

Table 19. Spectral emissivity of platinum in the infrared

at 1,125 °C

e

percent percent

0.65 22.0 2.5 21.8 1.0 29.3 2.75 20.6

1.1 28.7 3.0 19.6 1.2 28.4 3. 25 18.8 1.3 28. 4 3.5 18.0

1.4 27.6 3. 75 17.2 1.5 27.0 4.0 16.5 1.75 25.5 4. 25 15.7 2.0 24.0 4.5 15.0

2.25 22.8 4. 75 14.5

The first value of table 19, 22.0 percent at

0.65 n, was suspect, and on referring to the original paper was foimd to be as published. On

further examination, however, it was found that

this value apparently should be 33.0, which still seems to be too great a change in the opposite

direction.

Neither platinum nor its alloys is magnetic

at room temperature.

The volatility of platinum is indicated, according to the work of Jones, Langmuir and MacKay

[33], by the data given in table 20, taken from

reference [9]. Table 20. Volatilization of platinum in vacuo

Temperature, °C 527 727 1227 1727

Loss, g/cmV-'jeo. -1. 39 X 10-28 6. 7 X 10-2" 5.23 X 10-11 1. 24 X 10-9

According to Crookes [34] platinum is twice as

volatile as rhodium in air at 1,300 °C, one-third

as volatile as palladium, and one-thirtieth as

volatile as iridium. According to Vines, "It should

be emphasized . . . that this order of volatility

applies to tests in air where the oxidation of some

of these metals and volatilization of the oxides is a factor . . ."

9
b. Iridium Versus Iridium-Rhodium Alloys

Feussner [35] proposed in 1933 the use of

thermocouples of iridium-rhodivun alloys versus iridium "for very high temperatures." Several

alloys have been proposed for the positive wire of

the thermocouple, chief among these being aUoys

of 40, 50, and 60 percent iridium with rhodium.

Various reasons have been given for the choice of a particular one of these compositions. The

thermal emfs of the 40 and 60 percent wires are not widely different, and are not too far below the

maximum, which occurs at about the 50-50 conposition. One point made by some is that loss

of iridiimi from the 60 percent wire will alter the

thermal emf toward its maximmn and not toward

the steeper slope in the other direction, and that

the temperature range of the alloy containing

more iridium will be somewhat greater. The

latter point is well taken, and because of this the

60 percent alloy is enjoying greater popularity as time passes. The aUoy of 40 percent iridium60 percent rhodium has been used in the past,

however, and probably wiU continue to be used

for some time.

A considerable amount of work has been done

on the properties of iridium, but information on

the alloys of iridium and rhodium is very scarce, almost to the point of nonexistence. The little

data that are available are given, however, and

some calculated data that should be used with

caution are added.

The temperature hmit of use of the iridiumrhodium versus iridium thermocouples is set by

the hquidus-sohdus temperature of the aUoy

wire, and not, as in the case of the platinum-rhodium versus platinum thermocouples, by the freezing point of the pm-e wire. The freezing point of

iridium is given by Henning and Wensel [36] as 2,454 ±3 °C on the International Temperature

Scale of 1927. This becomes 2,443° on the 1948

scale. No Uquidus-solidus temperatures are found

for the iridium-rhodium system, but approximate

values may be inferred from the freezing points

of iridium and rhodium [37]. If the assumption

is made that the Uquidus-solidus curve for the iridiiun-rhodium system is similar to Acken's [4

J

for the platiniun-rhodium system, then values

obtained by linear interpolation from iridium to

rhodium woiild be on the safe side. Justification

for this is tenuous, but some values are given in

table 21.

One approximate check has been made in Blackbum and CaldweU's work [38] on the vahdity of a value for the 60 percent rhodium-40 percent

iridium aUoy. On heating a thermocouple having this aUoy as one wire until it broke, the Hmiting temperature was found to be below 3,900 °F.

The calculated temperature from table 21,

2,157 °C, is 3,915 °F. The upper safe Umit for

cahbration of this thermocouple accordingly was

set at 3,800 °F (2,093 °C). Judging from this case, it will be necessary, in order to provide a safe

margin, to keep the temperatures of the other two

Table 21. Calculated Uquidus-solidus temperatures for

three iridium-rhodium alloys

Liquidussolidvis Composition temperature

70 °C

100 Rh • 1960

60 Bh 40 It 2,153

50 Rh 50 It 2,202 40 Rh 60 It 2,250

100 It 2,443

< The value of the freezing

point of rhodium has been

adjusted to put it on the 1948 Temperature Scale.

alloys at least 60 °C (108 °F) below the values of

table 21. These limits were arrived at solely on

the basis of temperatures at which the wires

soften or melt; other properties that hmit the use

or hfe of such thermocouples are discussed later.

Several determinations have been made of the

temperature-emf relationships of iridium 40 percent-rhodium 60 percent versus iridium thermocouples. Among these are the work of Droms

and Dahl [39] of the General Engineering Laboratories of the General Electric Company, and of

Blackburn and CaldweU at the National Bureau

of Standards. A report on the latter work, containing reference tables, is included in this symposium [38]. From results of the investigation

of this alloy, and from prehminary observations

on aUoys containing 60 percent iridiiun and 50

percent iridium, it appears that the 50-50 alloy

wiU be the optimum, even at the expense of a shght lowering of the upper range of use under

that of the alloy containing 60 percent iridiimi.

A condensed version of Blackhm-n's reference

table for the iridium 40 percent-rhodium 60 percent versus iridium thermpcouple is given in table

2A of the appendix.

Table 3A of the appendix gives the emfs of the

elements of this thermocouple against copper for use in making corrections for temperatures of

reference jxmctions where connections to the two

wires are not at the same temperature.

The thermoelectric power of the iridium-rhodium 40 percent versus iridium thermocouple is seen

to be a maximum of about 6 hy/°C (3 mv/°F) in

the upper part of the range. In the region of

room temperature and somewhat above, the

thermoelectric power is about one-half the maximum. The emf is not so low in the lower range

though, that, as in the case of the 30-6 thermocouple, the temperature of the reference junction can be ignored without the danger of introducing

an appreciable error. Thus junctions of leads of

this thermocouple with copper should be in an ice

bath, or temperatures of the jimctions should be

measured and corrected for. Normally the latter

coiu'se is taken because of the high cost of the alloy

and iridium wires.

An estimate of accm-acy of the reference table

for the thermocouple with the alloy of 40 percent

iridium is given by Blackbimi, This is for this

table only, and does not contemplate errors that

10
may be involved in the use of this thermocouple.

Accuracy is given by one of the suppliers of the

iridium 60 percent-rhodium 40 percent versus

iridium thermocouple as ±40 °F [40] in the upper

part of the range. This is for a thermocouple in

packed ceramic insulation, but there seems to be

no reason to beheve that this type of moimtitig

should affect the accuracy obtainable to any

appreciable extent. The accuracy of the thermocouple imder discussion may be expected to be

about the same. A statement is made in refer- ence [40] also that "Corrections for radiation and

conduction cannot be given due to the multiplicity of factors affecting these corrections." This

should be borne in mind for this and other thermocouples; such corrections may amount to many

times the accuracy stated above.

Cahbration data for the iridium 60 percentrhodium 40 percent versus iridium from reference

[122] are included in table 7 of the appendix.

No extension leads are known to be available

for the iridium-rhodium versus iridium thermocouples.

No detailed information is available on homogeneity of wires of iridium-rhodium versus iridium

thermocouples, but from experience at the National Bureau of Standards it appears that thermal

emfs of wires of the same size from the same lot do

not vary appreciably. Some small differences are seen though between wires of diameters of 0.035

in. and 0.020 in. that presumably are from the

same lot. No work has been done at NBS on

wires smaller than 0.020 in. nor larger than 0.035 in.

These thermocouple wires are available in diameters from 0.002 in. up to any reasonable size.

The most popular diameter here also seems to be

20 mil, but as in the case of the platinum-rhodium

versus platinum thermocouples, experimental conditions may dictate that other sizes be used.

Iridium, according to Vines [9] "is the most

corrosion resistant element known ; it is unaffected

by common acids and is even resistant to aqua

regia and fuming sulphuric acid. Small percentages of iridium, up to about 20 percent, also

greatly increase the corrosion resistance of platinum and palladium."

The iridium-rhodium versus iridium thermocouples have been used at the National Bureau of

Standards in exhausts of afterburners and ramjet

burners up to about 50 hrs at temperatures of

3,000 °F. (1,649 °C) and above before failure.

The gases were somewhat oxidizing because of

incomplete combustion. The thermocouples were

mounted for this use in water-cooled mountings,

so that only a small portion of the thermocouple

w^as exposed to the hot gases. Failure was caused

by loss of the pure wire in the exposed part.

Apparently the iridium wire oxidized; and the

oxide having, according to Crookes [34], a high

vapor pressure, volatihzed. No quantative measurements have been made on loss by volatilization

from the iridium wire, but on seeing the cloud of

black smoke from a length of wire heated electrically above 3,000 °F it is not hard to believe the

statement of Crookes [34] that "the loss of iridium

in air at 1,300 °C is ... 30 times that of platinum

and 60 times that of rhodium." The alloy wire,

from casual examination, did not appear to have

lost metal.

A coating ranging from gray to black was seen on the wires on returning from high to room temperatures in the work at the National Biireau of

Standards. This coating was thought to be oxide,

and subsequent observations were made in an

atmosphere of helium that had been passed

through a Uquid nitrogen trap to remove water

vapor and oU. The coating persisted, however,

even in hehum, and no determination has been

made as to the source. It has been established,

though, that formation of this coating has no

appreciable effect on the thermal emf.

Thus these thermocouples should not be used in

strongly oxidizing atmospheres if long life is wanted. They are, however, usable in neutral

atmospheres and in vacuo. No statement has been

found of effects of reducing atmospheres on these

wires or of effects of various refractories. Until

information does appear, it seems reasonable to

assume for the sake of accuracy and safety that

iridium and its alloys with rhodium are affected in a manner similar to platinum.

Iridivmi is used as a catalyst in some processes,

but no definitive work seems to have been done on

catalytic effects under conditions of thermocouple

use in burners. It seems reasonable to expect that

such effects will be present in cases of incomplete

combustion, and so care should be taken in interpreting data obtained with iridium-containing

thermocouples under such conditions, especially in

the lower temperature range.

No procedures for annealing iridium and its

rhodium alloys are stated in the literature, but one

suggested by Brenner [41] has been adopted at the

National Bureau of Standards. In this, a small

sample of wire is heated electrically imtil it melts;

the apparent temperature of melting is observed

with an optical pyrometer, and the thermocouple

wire is annealed for a few minutes at an apparent

temperature about 200 °C lower than that observed at melting. Care must be taken in this procedure, especially with the pure wire, because of

the effect on the reading of the optical pyrometer

of the black smoke generated. Immersion of the

hot wires in an atmosphere of inert gas is helpful

in preventing oxidation. Helium has been used at

the National Bureau of Standards during calibrations.

Care is needed in handling these wires. They

are flexible in the annealed state, but cannot stand

much cold working such as the bending that occurs

in normal use. Wire of 0.035 in. diam has

been bent at the National Bureau of Standards

into an eye of about 0.25 in. diam. without breaking, but it cannot be straightened a^ain without

heating. Several cases have occurred in which the

repeated mild bending incidental to changing the

reference junction ice bath has broken the wires.

11
Very little information is available on the

mechanical properties of iridium, and none is found on its alloys with rhodium. Perhaps a clue

might be had to the tensile strength of iridium

from values given in reference [42] for alloys of

5 to 30 percent iridium in platinum. Here the

ultimate tensile strength for the hard alloys is seen to increase from 70,000 psi for 5 percent

iridium to 200,000 psi for 30 percent iridium. The

increase over the same range of compositions for

annealed wire is from 40.000 to 160,000 psi.

Young's modulus for hard iridium is given as 7.47X10^ psi.

The thermal emf of iridium versus Pt 27, up

to 2,500 °F, from the work of Blackburn [38] is given in condensed form in table 22. The thermal

emf of the iridium 40 percent-rhodium 60 percent

wire against platinum can be obtained by adding

the thermal emf of the iridium 40 percent rhodium

60 percent versus iridium thermocouple to that of

iridium versus platinum.

Table 22. Thermal emf of iridium versus Pt 27

Temperature Emf Temperature Emf

°C mv o p °C mv

32 0 0 1,300 704.4 7, 729 100 37.8 0. 227 1,400 760.0 8.596

200 93.3 0. 616 1,500 815.6 9.499 300 148.9 1.066 1,600 871.1 10. 437

400 204.4 1.565 1,700 926. 7 11.410

500 260.0 2.105 1,800 982.2 12. 419

600 315.6 2. 685 1,900 1,037.8 13. 460

700 371.1 3.299 2,000 1,093.3 14.532 800 426.7 3.947 2,100 1,148.9 15, 632

900 482.2 4.630 2,200 1, 204. 4 16. 753

1,000 537.8 5. 349 2, 300 1,260.0 17.893

1,100 593.3 6. 106 2,400 1,315.6 19. 048 1,200 648.9 6,899 2, 500 1,371.1 20. 211

The electrical resistivity of iridium is given in

reference [42] as 6.08X10"^ ohm-cm (36.58 ohms/

emf) at 0 °C. Wilcox and Doring [43] give a value of 31.8 ohms/cmf, which corresponds to the

5.3X10"^ ohm-cm at 20 °C attributed by Vines

to Jaeger and Diesselhorst [44]. The temperat\u-e

coefficient of resistance, as determined by Holborn

[45] is given in table 23. A 1958 value of the

temperature coefficient from 0° to 100 °C a=

(i?ioo— i?o)/(100i?o) given in reference [43] as 0.0040

per °C indicates that the older value, 0.003925 per

°C probably is low.

Values of resistivity of iridium based on resistivities of references [42], [45] are given in table

24. The temperature coefficients of resistance

used are those of table 23.

Table 23. Temperature coefficient of electrical resistance

of iridium

Temperature Temperature

range coefficient

"C per °C

0-100 0. 003925 0-200 .00398 0-300 .00404 o^m .00408 0-500 .00414

Table 24. Electrical resistivity of iridium

Temperature

Resistivity

Reference

[42] Reference

[45]

°C ohmslcmf ohms/cmf

0 36.6 29.6

on 39.5 31.9

100 51.0 41.2 200 65.6 53.2

300 80.6 65.6

400 96.3 77.6

500 112. 90.8

The two columns of table 24 differ widely, and

which is the better is not known. Vines [9] believed the value of reference [45] to be the best,

while Garter and Stauss seemed to prefer the

newer value at 0 °C of reference [42]. The Smithsonian Physical Tables [3] give 6.lX 10~* ohm-cm

on page 384 for the resistivity at 0 °C, and also

the resistivity at lOO °C as 8.3X 10~® ohm-cm, a value that agrees with neither in the table.

The density of iridium from reference [3] is 22.42 g/cm^ (1400 Ib/ft^). Vines cites the value

of 22.41 g/cm^ of Holborn and co-workers [46],

and also points out that the density "calculated

from the space lattice, which . . . may be more

rehable is 22.656 g/cm^ at 0 °C and 22.65o at 20 °C

according to Owen and Yates [47]". The value

given in reference [43] is 22.54 g/cm^. No information is available on the density of the alloys

with rhodium; but the value of 12.4 g/cm^ for

rhodium at 20 °C, as given in reference [47], may

be used to calculate approximate densities of the

aUoys.

Holborn and Valentiner [48] give the equation

Z,=;o(l-f 6.6967X 10-«

'^-f 1.158X lO"* t'),

from which the length of iridium at temperatures

up to 1,000 °C can be calculated. Values calculated from this equation follow in table 25.

Table 25. Thermal expansion of iridium relative to length

at 0 °C Htllo)

Temperature lilU Temperature

"C °C

0 1.000000 600 1.00443 100 1.000681 700 1. 00526 200 1.001386 800 1. 00610 300 1.002113 900 1.00696

400 1.00286 1,000 1.00785

500 1.00364

Jaeger and Rosenbohm [49], among others, have

determined the specific heat of iridium. Their

equation is Cp=0.030725+7.4004X 10"^ ^. Values

of true specific heats calculated fr-om this equation

are presented in table 26.

The heat content above 0 °C is given by the

equation

Q(^o_oo) =0.037025 «-H0.0000037002<2.

12
Table 26. True specific heat of iridium

Temperature Specific heat Temperature Specific heat

°C callg °C "C callg °C

0 0. 03072 900 0. 03742 100 . 03147 1,000 .03816 200 .03221 1, 100 .03891 300 . 03296 1,200 .03965 400 . 03370 1,300 . 04040

aoo .03444 1,400 .04114

600 . 03519 1,500 . 04189

700 . 03593 1,600 . 04263

800 . 03668 1,700 .04337

The thermal conductivity of iridium has been

given by Barratt and Winter [50] as 0.141 cal/

sec/cm7cm/°C at 17 °C and 0.135 at 100 °C.

Information on the emissivity of iridium is scattered. Roeser and Wensel, on page 313 of

reference [20], give a value of 0.30 at 0.65^ for the

unoxidized solid. Goldwater and Danforth [51]

give a value of 0.33 at the same vp^avelength.

Fulk and Reynolds [52] give the total normal

emissivity at 295 °K to be 0.04, and a list of nor- mal spectral emissivities at the same temperature.

These latter values are given in table 27.

Table 27. Normal spectral emissivities of iridium at 295 °K

Wavelength e Wavelength e

% %

1.0 22 7.0 5 2.0 13 9.0 4 3.0 9 10.0 4 4.0 6 12.0 4

5.0 6

Values of the reflectivity of iridium at three

wavelengths, from Wise [53], are given in table 28.

Table 28. Reflectivity of iridium

Percent reflectivity at wavelengths of

0. 450 0. 550 0. 750a»

64 70 78

Except for the data of Fulk and Reynolds, temperatures at which emissivities and reflectivities are given are not specified. Goldwater and Danforth

do say, however, that their value for the emissivity

was computed from the relationship between true

and brightness temperature at a pyrometer wavelength of 0.65m.

Both elements of the iridium-rhodium versus iridium thermocouples are nonmagnetic at room

temperature.

c. Platinum-Iridium 15 Percent Versus Palladium

This thermocouple having as its elements the

positive platinum-iridium 15 percent and the

negative palladium wires was developed by the

General Electric Company [54] under sponsorship

of the Wright Air Development Division of the

Air Force. It was developed to provide a sensor that is suitable for extended use at temperatures

above the usable range of the more common basemetal thermocouples, and that has a thermoelectric power of the same order as that of the

base-metal thermocouples.

The upper range of the platinum-iridium 15

percent versus palladium (PIP) thermocouple is

limited by the freezing point of palladium, given

as 1,552 °C (2,826 °F) on page 72 of reference [3]. The solidus of the platinum-iridium vpire is seen from figure 22 of reference [9] to be about 1,790

°C. This, when corrected to the 1948 temperature scale, is about 1,785 °C, or 3,245 °F. In

most cases the upper useful limit of this thermocouple will be found to be somewhat below the

freezing point of palladium.

Determinations of the temperature-emf relationship of the PIP thermocouple have been made

at the General Electric Company [54] and at the

National Bureau of Standards [55]. Tables of

reference [55] are simUar to those of NBS Circular

561 in which reference tables for the conventional

thermocouples are given. A condensed version of

these tables for the PIP thermocouple is in table

4A of the appendix. Thermal emfs versus copper

of the elements are given in table 5A of the appendix. Thermal emfs of the thermocouple elements versus Pt 27 are presented in table 29.

Table 29. Thermal emf of platinum-iridium 16 percent

and palladium versus Pt £7

Temperature

Platinumiridium

15 percent Palladium

op °C mv mv

32 0 0.000 0.000 200 93 1.309 -0. 521 400 204 3. 124 -1.224

600 316 5. 096 -2. 034 800 427 7. 168 -3.004

1,000 538 9.296 -4. 173 1,200 649 11. 475 -5. 577 1,400 760 13.692 -7. 218

1, 600 871 15. 950 -9. 080

1,800 982 18. 258 -11. 142

2,000 1,093 20. 592 -13. 390

2, 200 1,204 22. 943 -15. 787 2,400 1,316 25. 262 -18. 306

2,550 1,399 26. 989 -20. 266

Extension leads of alloys of base metals have

been developed by the General Electric Company

for the PIP thermocouple that provide a reasonable match with the thermocouple up to about

1,300 °F. The cahbration has not yet been pubhshed, but it is anticipated that it vnll appear in

the near future.

Six thermocouples of platinum-iridium 15 percent versus palladium were used in the determination of the reference tables. Homogenity of

the 0.040-ia. vnres examined is radicated by the

maximum divergence of the six thermocouples at

2,500 °F (1,371 °C) of about 3 °F, and much

smaller differences at lower temperatures.

618629—62 -3

13
Some of the 0.040-in. wires have been drawn

down to 0.010-in. diameter at NBS, and there is

no reason to beheve that they cannot be reduced

to a diameter of a mil of so. Resistance to corrosion of the platinumiridium alloy is excellent, and apparently sur- passes in this respect the platinum-rhodium alloys

discussed earUer. Some loss of iridimn is said to occur in the region of 900 °C (1, 652 °F) and above

because of formation and volatihzation of the

oxide. A gray to black fibn has been seen to form

on this wire at about the same temperature when

the wire is heated in air. This film may disappear

on heating to a temperature two or three hundred

degrees higher. Although no detailed information on the resistance of the platinum-iridium alloys to

the ordinary products of combustion and reducing

conditions is found, as a matter of safety, precautions similar to those described for platinum

should be observed.

Palladium is not so resistant to attack by common chemicals as are the other platinum metals

discussed previously. According to Vines [9] "Palladium is resistant to corrosion at room temperature by hydrofluoric acid, perchloric acid,

phosphoric acid and acetic acid. It is sHghtly

attacked by sulphuric, hydrochloric and- hydrobromic acids, especially in the presence of air,

and readily attacked by nitric acid, ferric chloride,

hypochlorites and moist chlorine, bromine and

iodine. In ordinary atmospheres palladium is resistant to tarnish, but some discoloration may

occur on exposure to moist industrial atmospheres

containing sulphur dioxide." Ihnat [54] points

out the following characteristics of palladium:

"1. Palladium is superficially oxidized if heated to a

temperature of 700 °C (1,292 °F). The oxide (PdO)

formed decomposes above 875 °C (1,607 °F) and a bright

metal remains.

2. "Palladium will absorb as much as 800 to 900

times its own volume of hydrogen over a range of temperature.

"3. Subjected to alternate oxidizing and reducing

atmospheres surface blistering is apt to result."

The first and third of these have been confirmed

approximately in the work on this thermocouple

at the National Bureau of Standards.

Both the platinum-iridium and palladium wires

are catalytic, and accordingly care should be taken

in interpretation of data obtained with them.

Both elements of this thermocouple are readily

handled in the annealed state without danger of

breaking. Vines' comments on annealing of the

wires follow:

"Alloys containing more than 10% iridium require

rather high annealing temperatures, in the neighborhood

of 1,400 °C to completely soften them in a short time,

although in practice, anneals at about 1,100 to 1,200 °C

for 30 to 45 minutes are sometimes employed.

"Palladium is very ductile and can be worked hot or

cold. As with platinum, it is usual to begin working ingots

at about 800 °C to secure more rapid reduction and then

finish by cold working. Palladium withstands drastic cold

working and, like gold, can be beaten into leaf as thin as 1/250,000 of an inch.

"It is possible that pure palladium can be annealed at

a lower temperature than pure platinum but reliable information is not available. Although marked softening occurs at 700 °C, commercial palladium requires an annealing temperature of 800 or 900 °C to obtain maximum

ductility with a short anneal. At temperatures outside

this range, selective grain growth resulting in lowered

ductility may occur in some lots of commercial palladium.

"Palladium is preferably annealed in nitrogen or carbon monoxide as it oxidizes in air if the temperature is below about 800 °C . . ."

The following annealing procedures, adopted at

the National Bureau of Standards, are those suggested by Mr. L. J. Stiles of the General Electric

Company. The elements were separated, and

the platinum-iridium wire was heated electrically

to 1,300 °C (2,372 °F) in air for one minute.

Appropriate corrections to observations, made

with an optical pyrometer, took account of the

emissivity, about 30 percent of the wire. The

palladium wire was heated electrically at 750 °C

(1,382 °F) for 25 min. Emissivity of the palladium was assumed to be about 35 percent.

The ultimate tensile strength of platinum

iridium alloys as given Dy Carter and Stauss in

reference [19] is presented in table 30.

Table '30. Ultimate tensile strength of platinum-iridium

alloys, psi

Percent Ir Hard Annealed

5 70,000 40,000

10 90, 000 55,000

15 120, 000 75,000 20 145, 000 100, 000 25 170, 000 125, 000 30 200, 000 160, 000

The tensile strength at temperatures up to

1,100 °C of commercial palladium annealed at

1,100 °C, according to reference [18], is given in

table 31. The tensile strength of pure paUadimn

that had been annealed at 800 or 900 °C for about

five min, according to reference [17], increased

from 30,000 psi to 47,000 psi when reduced 50

percent in area by cold drawing.

Table 31. Tensile strength of commercial palladium at temperatures up to 1,100 °C

Tensile

strength, Temperature commerical

palladium

°C psi Room 28, 000

200 24, 600

400 18, 100 600 12,700

800 8,300

1,000 3,820

1,100 2,920

Wise, CroweU, and Eash [17] found the proportional limit of commercial palladium to be about

5,000 psi after anneahng at 800 °C and 30,000 psi

after a 50 percent reduction in area by cold

drawing.

14
Values of resistivity at 20 °C of platinumiridium alloys and the temperature coefficient of

resistance from 10 to 160 °C, from Carter and

Stauss [19] are given in table 32. The authors

regard these data "as giving good average values

of the alloys rather than as giving exact physical

constants."

Table 32. Electrical resistivity and temperature coefficient of resistance of some platinum-iridium alloys

Percent

It

Resistivity, 20 °CTemperature

coeflficient of resistance

ohms/cmf 10°-160 °C

5 115 0.0020

10 150 .0013

I5 170 .0010 20 190 .0008

The electrical resistivity of palladium as determined by Grube and Knabe [56] is given in table

33. Along with this is given the relative resistivity at temperatures up to 800 °C as given by

Conybeare [57]. The data presented were taken

from reference [9].

Table 33. Electrical resistivity of palladium at temperatures up to 1,400 °C, and relative resistivity up to 800 °C

Temper- Resis- Ri/Ro Temper- Resis- R</Ro

ature tivity ature tivity

"C ohms/cmf °C ohmslcmf

0 1.000 700 199.7 3. 220

100 84.2 1.372 800 214.2 3.449

200 104.7 1.730 900 228.0

300 126.3 2. 078 1,000 240.6

400 146.2 2.395 1,200 269.5

500 164.8 2.704 1,400 297.8

600 182.9 2. 972

Resistivity at 0 °C is given by Conybeare as 63.95 ohms/cmf (10.63 microhm-cm), and his

value for the a is 0.00372. Vines [9] gives the

best value as being about 0.00377 per °C, but does

point out that it is dependent upon the temperature used to anneal the palladium.

No direct data have been found on the density

of the platinum-iridium alloy, but approximate

values may be calculated from the information for

platinum and iridiimi. Vines [9] gives the density

of palladium as 11.96 g/cm^ at 18 °C.

No information has been found on the linear

coefficient of thermal expansion of the platinumiridium 15 percent alloy, but a comparison of data

in table 34 for the 20 percent iridium alloy with

that for platiaum in table 12 may be informative.

These data are from reference [3], page 151.

Table 34. Coefficient of thermal expansion of the platinumiridium 20 percent alloy

Temperature Coefficient range of ejcpansion

X 10«

"C per °C

0-100 8.3

0-1,000 9.6 0-1,600 10.5

Vines [9] has computed a table of values of the

relative length of palladium assxmiing unit length

at 0 °C from the equation of Holborn and Day [58]:

^,-^o(l + 1.167X10-^ ^+2.187X10-^ f)

Their value of the linear coefficient of expansion

at 0 °C is given as 1 1.67 X 10"" per °C. Values of

Itjlo are presented in table 35.

Table 35. Thermal expansion of palladium

Temperature hlh Temperature UlU

°C °C

0 1.000000 500 1.0064

20 1. 000234 600 1.0078

100 1.00119 700 1.0092

200 1.00242 800 1. 0107

300 1.0037 900 1.0123

400 1. 0050 1,000 1.0139

No data are available on the specific heat of the

platinum-iridium 15% aUoy. The true (Cp) and

mean (Cp) specific heats of palladium as determined

by Jaeger and Veenstra [59] are given in table 36.

The mean specific heat is over the range from 0 °C

to the upper temperature.

Table 36. True and mean specific heats of palladium

Temperature

Specific heat cal/g °C

Temperature

Speoiiic heat cal/g °0

True Mean True Mean

"C °C

0 0. 05838 0. 05838 700 0. 06694 0. 06264

25 . 05868 .05853 800 . 06819 . 06326

50 . 05898 .05868 900 .06944 .06387

100 . 05959 . 05898 1,000 . 07069 .06449 200 . 06080 . 05959 1,100 . 07195 . 065H

300 . 06202 . 06019 1,200 . 07321 . 06574

400 .06324 .06080 1,300 .07448 .0663(5 500 . 06447 . 06141 1,400 . 07576 . 06698

600 . 06570 . 06202 1,500 . 07704 . 06761

The only information found on the thermal

conductivity of a platinum-iridium alloy is from

reference [3]; this gives a value for platinumiridium 10 percent of 0.074 cal/sec/cm7cm/°C at

17 °C.

Very little is available on the thermal conductivity of palladium. The experimental values

given by Hall [60] at 0° and 100 °C are 0.165 and

0.182 cal/sec/cm^/cm/°C respectively.

Information on emissivity of the aUoy leg is not

available, but indications from the spectral emissivities of platinum and iridium at 0.65 n are that

the emissivity of platinum-iridium should be

about 30 percent at this wavelength.

The spectral emissivities of solid palladium at

its melting point are, according to reference [3],

38 percent at 0.55 ^ and 33 percent at 0.65 n. Forsythe [61] gives the values of table 37, attributed to Burgess and Waltenburg (1915), of the

total emissivity of palladium. Tkese values

presumably are for polished, unoxidized palladium.

Neither element of the platinum-iridium 15

percent versus palladium thermocouple is known

to be magnetic at room temperature.

15
Table 37. Total emissivity of palladium

Temperature Emissivity

" K

1,000 0. 36

1,400 .33

1,500 .31

d. Platinel Thermocouples

Two types of Platinel ^ thermocouples have

been developed by Engelhard Industries, Inc. for use up to 1,300 °C (2,372 °F), about the same

range as that for which the platinum-iridium 15

percent versus palladium was designed. These

thermocouples are new, and so have not yet been

in wide use. A considerable interest has been

generated in them, however, to the extent that it seems desirable to include them here.

The negative wire for both thermocouples

Platinel No. 1 and No. 2 is a gold-palladium aUoy

designated by Engelhard as Platinel No. 1503.

The positive wires are gold-palladium-platinum

alloys designated as Platinel No. 1786 for thermocouple No. 1 and as Platinel No. 1813 for thermocouple No. 2. The exact compositions are proprietary and are not available for publication.

Because httle information is available on the

properties of these thermocouples, none at all except a provisional reference table being found

for Platinel No. 2, the properties at hand are summarized in table 38. These are as supphed by

Wilcox and Doring in reference [43]. Temperatures for which these values apply are not given,

but presumably determinations for which temperatures are not given are for 0 °C or room temperature. These wires are said to be obtainable in

diameters from 0.001-in. up.

Table 38. Properties of Platinel

Property

Platinel No.

1503 1786 1813

Density (calculated) (g/cm 3)_ 15.87 13. 05 14. 91 Eesistivity (Hard)

ohms/cmf (20 °C) 141.0 115 188 Resistivity (annealed) ohms/cmf (20 "O 146.0 117 188 Temperature coefficient of resistance (Hard) (O-lOO °C) (per °C).. 0. 0036 0. 00183 0. 000865 Hardness (BHN, 60 percent reduction) 149 Tensile strength (Hard) (psi)_. 88, 360 47, 000 98, 000 Tensile strength (annealed at 1200 °C)„- 33,000 30, 500 47,000 Composition Au, Pd Au, Pd, Pt Au, Pd, Pt

The thermal emf of the Platinel No. 1 thermocouple approximates that given in NBS Circular

561 for Chromel-Alumel * thermocouples. From

work done at the National Bureau of Standards [62]

the two temperature-emf relationships cross, the

emf of the Platinel becoming the greater at about

' According to Information from Engelhard Industries [43] Platinel Is their

registered trade mark, and patents on the thermocouples are pending.

* Chromel and Alumel are registered trade marks of The Hoskins Manufacturiag Co.

850 °F; they cross again at about 1,800 °F. The

maximum divergences measured between the

Platinel and the NBS table were about 400 at

1,400 °F and about 1,050 mV at 2,200 °F. Obviously corrections should be appHed for maximum

accuracy if values from NBS Circular 561 are used

as a reference. A calibration of Platinel No. 1 from reference [43] and one made at the National

Bureau of Standards are given in table 39. Provisional data on Platinel No. 2 from reference [43] are included.

Table 39. Calibration of Platinel thermocouples

Temperature

No. 1 No. 2

Engelhard NBS Engelhard

°C Emf mv EmJ mv Emf mv

0 0 0 0 100 3.6 3. 55 200 7.6 7.57 300 12.1 11.81 400 16.3 16.25 15.7

500 20.8 20.75 600 25.2 25.20 24.7

700 29.4 29.54

800 33.6 33. 68 33.5 900 37.5 37. 61

1,000 41.2 41.26 41.7

1,100 44.6 44.70

1,200 48.0 47.86 49.0

1,300 51.1 62.6

The values for No. 1 from Engelhard are given

as good to ±0.1 milhvolt (mv) up to 600 °C and

to ±0.2 mv above. No estimate is made as to

the accuracy of the NBS results.

Some of the differences between the two calibrations of Platinel No. 1 may have been caused

by different annealing treatments before calibration. At the National Bureau of Standards the

negative gold-palladium wire was heated electrically at about 2,000 °F (1,093 °C) for 15 min.

The positive wire was held at 2,200 °F (1,204 °C)

for 15 min. No record is available of the treatment of the thermocouples from which the Engelhard table was devised.

The provisional cahbration table over the

range 400 to 1,300 °C in reference [43] for Platinel

No. 2 differs somewhat from that for Platinel No.

1. Some advantages are claimed for No. 2 over No. 1, but they are not clearly enough defined at

this time to give a complete discussion of them.

2.2. Base Metals

Thermocouples discussed in this section are limited to those made of metals and alloys whose

usual usable tempeiature range is considered

to be 2,300 °F (1,260 °C) and below. Four

ISA types [1] are given in table 40.

Some hberty has been taken, in indicating that

the second and third thermocouple combinations

of table 40 are type K. This designation in

reference [1] refers to Chromel-Alumel only;

16
Table 40. ISA types of base-metal thermocouples

ISA type

K

K

K.._ -. Y b and J ». Y and J Y

JT

Trade name or elements

Chromel-Alumel »- - - Kanthal P-N » Tophel-Nial « Iron-constantan Iron-Advance » Iron-Copnio <• Iron-Cupron <• Copper-constantan =.

Table 41. Special tolerances for different sizes of ChromelAlumel thermocouple wire

Trade mark of

Hoskins Mfg. Company

The Kanthal Corporation

Wilbur B. Driver Company

Driver-Harris Company

General Electric Company

Wilbur B. Driver Company

» When so marked in the table this indicates that the namfes of the alloys

are registered trade marks of the companies of the third column.

>> ISA type Y refers to the tables of NBS Research Paper 1080 [63].

= ISA type J refers to the tables of NBS Circular 561 [7]. This is said m

reference [1] to be the calibration most widely used today.

<i Owners of the rights to these names are not known, but these designations

are used by the companies listed.

e Copper is used in conjunction vrith Advance, Copnic, Cupron, and

constantan, aU of which are thermoelectricaUy similar.

but because claims for properties and. performance

of the three are similar, it seems reasonable to

include Kanthal P-N and Tophel-Nial thermocouples under this type.

The Geminol ^ P-N thermocouple and thermocouples composed of the positive element of the

type K and constantan have no ISA designations.

Because some of the elements of this section

are paired with more than one other in forming

thermocouples, presentation of information is in a different order from that of the section, on

rare metals. So far as it is practical, data on all thermocouple combinations are given, and properties of the separate elements follow. Also,

information available from the proprietors of

the thermocouple combinations, when they are known, are given when differences are found

between their data and those of suppliers.

a. ISA Type K Thermocouples

Thermocouples of this type that are discussed

are those listed in table 40. Among the excellent

works on these thermocouples are those of Roeser,

Dahl, and Gowens [64]; Dahl [65]; McElroy [11];

Potts and McElroy [66], and Potts and McElroy

[77]. Catalog literature is, of course, another

source of information, and other references are

cited as the need arises.

The usual range of these thermocouples is given

by Roeser and Lonberger [6] to be up to 1,100 °C

(2,000 °F), and the maximum temperature for

them is said to be 1,350 °C (2,450 °F). The

makers clahn [67, 68, 69] the standard tolerances

[1] of ±4 °F (2.2 °C) up to 530 °F (277 °C), and

±% percent from 531 °F to 2,300 °F (1,260 °C)

are applicable. In addition, thermocouples are

offered to meet closer tolerances when needed for

precision work. One statement of these tolerances

that is available follows in table 41 from reference

[67]. In other cases the maximum deviation from

the NBS tables is given as one half the standard

tolerance of reference [1]. The certified accuracy

of calibration at the National Bureau of Standards

of base-metal thermocouples (type K in this case)

Wire size Temperature Tolerance range

24 to 40 gage 1, 000-1,600 "F ±5 °F

18, 20, 22 gage 1,000-2,000 °F ±5 °F

[8] is 1 degree in the range from 0 to 1,100 °C

(32 to 2,000 °F). Base-metal thermocouples are not calibrated above 1,100 °C (2,000 °F). The

tables for Chromel-Alumel thermocouples in NBS

Circular 561 [7] serve as reference tables for ail

of the type K thermocouples, and the tolerances are in terms of the maximum disagreement with

these tables.

The thermoelectric power of the type K

thermocouples is about 40 iiY per degree C at

0 °C (22 Mv/°F at 32 °F) and 36 fiv per degree C

at 1,260 °C (20 Mv/°F at 2,300 °F). Thus the

necessity of correcting or compensating for the

temperature of the reference junction is obvious

in cases where a properly maintained ice bath is not used. Extension wires are furnished for these

thermocouples that may be used effectively to

extend the thermocouple to the measuring instrument. In the vast majority of industrial installations the measuring instrument is equipped with

reference junction compensation so that the only

added error from the use of the extension wires is that resulting from the deviation of the calibration

of the extension wires from that of the measuring

thermocouple. The standard tolerance (limit of

error) of the extension leads is given in reference

[1] as ±4 °F (2 °C) over the range 0 to 400 °F

( — 18 to 204 °C). The limits of error of alternate

extension wires, of copper-constantan and of ironalloy, are listed as ±6 °F (3 °C) over the ranges of

75 to 200 °F and 75 to 400 °F, respectively.

These limits are said to be for a reference junction

temperature of 75 °F only.

Type K thermocouples are available in sizes

ranging from No. 2 B&S gage down to No. 40.

Maximum recommended operating temperatures

from references [67], [68] are given in table 42.

Table 42. Maximum recommended operating temperatures

for type K thermocouples

a. From reference [67] b. From reference [68]

B&S gage Maximum

B&S gage Maximum temperatemperature ture Minimum Usual

2, 6, 8, and 11

14, 16, and 18 20 and 22

24 through 40

"F

2,300

2, 000

1,800

1,600

111822

38

6- 8

8-11 11-16

16-24

°F

2,300

2,010

1, 830

1,650

' Geminol is a registered trade mark of the Driver-Harris Company.

Some suppliers recommend still narrower limits.

In one case [70] the range for bare thermocouples

is to 2,000 °F for 8-gage and to 1,300 °F for 30-gage

wires, and to 2,300 °F and 1,500 °F for the same

sizes when protected.

17
McElroy [11] points out that appreciable inhomogeneities have been found in thermocouples

of this type, and that they vary from wire to wire

and between producers. The as-received alloys are said to contain mechanical inhomogeneities

which are removed to a certain extent by shorttime heatings. Much information on this and

other phases of use and treatment of, and results

to be expected from, the type K thermocouples is found in the comprehensive reports of references

[11] and [77].

Dahl [65] conducted stability tests in air on

thermocouples among which were the type K.

Because of the nature and number of the tests re- ported in this paper, it is not possible to give detailed results here. The general svunmarization by

Dahl is "From the observations reported it is seen

that long-time exposure ... to high temperatures causes the emf corresponding to a given temperature to increase, or the temperature corre- sponding to a given emf to decrease." Oxidation

of the wires when exposed in 1,000-hr tests to temperatures of 1,800 °F (982 °C) and above was

appreciable, to the point that one 8-gage thermocouple oxidized nearly through after 300 hrs ex- posure to 2,200 °F (1,204 °C). In tests at 1,600 °F

(871 °C) and below, oxidation was not enough to

materially decrease the diameter of the wires.

Potts and McElroy [77] give more recently obtained information on this subject.

Changes in calibrations of 8-gage thermocouples

given by Dahl are listed in table 43.

Table 43. Changes in the calibration of ChromeUAlumel

{type K) thermocouples healed in air {electric furnace)

Exposure

temperature

Exposure Maximum

change

Exposure

temperature

Exposure Maximum

change

"F

800 hours1,000

1,000

1,000

1,000

"F <1

<1

+2

3

1,600 hours

1, 000

1,000

1,000

200

°F

58

19

21

1,000 1,800 1,200 2,000 1,400 2,200

In spite of these apparently adverse findings,

type K thermocouples are used successfully in

practically aU types of industrial operations and

control systems suitable to their temperature

range. With this, as with any other measurement

system, the components must be used intelligently

in order to achieve the best performance.

b Geminol Thermocouples

The thermocouple Geminol-P versus Geminol-N

was developed, according to the maker [72], to

meet the need of the pyrometer industry for a thermocouple to withstand reducing atmospheres.

The information presented herein is taken largely

from reference [72], because very little information

on Geminol is available in the general literature.

The range of Geminol, from the temperatureemf relationship given in [72], is from 0 to 2,300 °F

(—18 to 1,260 °C). No NBS reference table has

been determined for this thermocouple, but a condensed calibration is given from reference [72] in

table 6 of the appendix. The maker claims that

the elements are manufactured so that any positive and any negative leg can be combined to be

within tolerances of ±4 °F (2 °C) in the range up

to 500 °F (260 °C) and ±% percent above 500 °F.

Geminol extension lead wires with the standard

tolerance of ±4 °F (2 °C) in the range 32 to 400

°F are furnished. Geminol was included in the

work of Potts and McElroy [77], and a calibration

is given by them.

The approximate thermoelectric power of Geminol at several temperatures is given in table 44.

The smallest size wire listed in reference [72] is B & S gage No. 30, and the largest is No. 6.

Table 44. Thermoelectric power of Geminol

Temperature Thermoelectric Temperature Thermoelectric power power

op °C plvI°F "F °C

20 -7 13.3 23.9 1,000 538 19.1 34.4 100 38 13.6 24. 5 1,200 649 20.1 36.2 200 93 14.0 25.2 1,600 871 21.6 38.9

400 204 15.0 27.0 1,800 982 22.1 39.8

600 316 16.6 29.9 2, 000 1,093 22.4 40.3

800 427 17.9 32.2 2, 200 1,204 22.2 40.0

The thermoelectric power in the table, an aver- age over the 100 °F interval above the listed

temperature, increases from 13.3 /uv per °F- to a

constant value of 22.2 at 2,000 °F or a little below

From 600 or 800 °F up, however, the change of

thermal emf with temperature is comparable to

that of type K. Above 1,200 °F the slope of the

calibration curve is very close to that of the type

K, and the thermoelectric power of Geminol is even

a little greater at the top of its usable range.

The low change with temperature of thermal emf

in the range near 32 °F is not a disadvantage,

because this thermocouple apparently is not

designed for use in the low range. It might even be said to provide a smaU advantage, since the

temperature of the reference junction is a little less

critical than for the case when the thermoelectric

power is constant over the entire range. This

advantage, if any, is slight here though, and the

usual precautions mentioned earlier about the ref- erence junction should be observed. Extension

wires, with the stated limits of error of ± 4 ° up to

400 °F, are available.

Data are presented in reference [72] on the

reproducibility of Geminol thermocouples under

oxidizing and reducing conditions. The resistance

to oxidizing atmospheres from the standpoint of

change of thermal emf was examined by cycling

the thermocouple to 2,200 °F (1,204 °C) and

measuring the resulting changes. No mention of

the time of exposure is given. The change at

2,200 °F was comparable with that found for the

type K thermocouple by Dahl [65] after exposure

to 2,200 °F in air for about 50 hr.

Potts and McElroy [77] concluded that Geminol

has superior qualities with respect to drift after

18
long service at elevated temperatures, and to

mechanical strength, or resistance to oxidation.

The thermocouple was subjected to an atmosphere containing 10 percent CO, 5 percent CO2,

1 percent CH4, 16 percent H2, O2—nil, and the

balance N2, at 1,750 °F (954 °C), conditions chosen

to simulate those said to promote "green rot"

attack on nickel-chromium alloys in a reducing

atmosphere. After 212 hrs the emf at 1,750 °F of

two Geminol thermocouples decreased by 0.13 and

0.01 mv, very small changes as compared with

those of the "comparison couples" used.

Maximum operating temperatm-es for the different sizes of Geminol [72] are given in table 45.

Table 45. Maximum recommended temperatures for

Geminol thermocouples

B & S gage Maximum

temperature

B & S

gage Maximumtemperature

30

0 p

1, 300

1,500

1,700

18

op

1,900

2, 100

2, 300 28 14 24 8

c. Iron-Constantan Thermocouples (ISA Types Y and J)

The iron-constantan thermocouple appears to

have been the first of the base-metal thermocouples to be accepted on a large scale, and it continues to enjoy wide popularity. Information

on this type of sensor is found in references [63,

65, 72], and in catalog literature. In addition, the

work of Corruccini and Shenker [73] presents the

reference table for iron-constantan that is included

in NBS Circular 561 [7], and helps eliminate some

of the confusion that had existed regarding the

two existing calibrations (ISA types Y and J [1])

for this thermocouple. As mentioned in the footnote to table 40, the type Y thermocouple is that

to which the calibration of RP 1080 [63] applies,

and the tables of NBS Circular 561 apply to type J., According to Corruccini and Shenker, the type J

is the most widely used, but type Y is used exclusively, of the iron-constautan thermocouples, in

the field of military aircraft.

The usual range of the iron-constantan thermocouples is given in reference [6] to be —310 °F

(-190 °C) to 1,400 °F (760 °C) and the maximum

temperature is given as 1,800 °F (982 °C). Suppliers' estimates of the usual range are not consistent, and vary from 1,200 °F (649 °C) for bare

B & S 8-gage and 1,400 °F (760 °C) for protected

8-gage wu-es, to 1,600 °F (871 °C) with no specifications as to size or protection. The maximum

service temperature is said in reference [72] to

be 1,500 °F in air and 1,750 °F in a reducing

atmosphere.

Suppliers generally claim the standard tolerances, or limits of error (deviation from tables of

reference [7]), of reference [1], and in this case

limits of error for special wires are given in refer- ence [1] as one half the standard. This is so for

both thermocouple and extension lead wires.

The certified accuracy of calibrations of ironconstantan thermocouples at the National Bureau

of Standards [8] is 1 °C.

The thermoelectric power of the iron-constantan

thermocouple is about 50 nv per degree C at 0 °C

(27 iiy/°F at 32 °F), increases to about 54 nv per

degree C at 100 °C (30 at 212 and 64 hy

per degree C at 760 °C (36 mv/°F at 1,400 °F).

It is always greater than that of the type K

thermocouple, and the type J or Y sometimes is selected because of the supposed increase of

accuracy resulting from the greater thermoelectric

power. This increase of accuracy is not, however,

great enough to be a deciding factor in most cases, and one of the most potent factors in selecting

iron-constantan is said to be its lower cost. Because of the high thermal emf in the normal

range of temperature of reference junctions,

proper precautions should be taken if extension

leads to a compensated measuring instrimient are not used.

The iron-constantan thermocouples are available in wire diameters from 0.001 in. up. The

maximum size mentioned generally is B & S 8-

gage, but apparently larger sizes may be had.

Pipe therniocouples also are made of this combination. The outer sleeve, of iron, may be as large

as %-in. pipe (about ^Ke-in. diameter), vpith the

constantan inside in the form of a wire insulated

from the pipe and welded to the closed end to form

the measuring juntion. This type of thermocouple is described in reference [74].

Recommended upper temperature limits for

several wire sizes are given, from three sources,

in table 46.

Table 46. Recommended upper temperatures for different wire sizes, iron-coritantan

Upper temperature limits

gage

Refer- ence [1] Reference [70] Refer- ence [72]

°F 0 p "F °F

8 »1,400 1,200 "1,400 1,500 12 1,300 14 1,100 900 1,100 1,100 16 900 1,100 900

20 900 800 900 700

24 700 650 700 30 700 600 700 500

» Tbese columns are given for conditions under which the thermocouples

are protected.

Dahl [65] commented:

"The exposure tests on 8-gage iron-constantan thermocouples showed failure of the materials within the 1000-

hour heating time for the tests at 1600 °F and above.

Failure occurred after 12- hours at 2000 °F, after 28 hours

at 1800 °F, and after 300 hours at 1600 °F. The 18-gage

iron-constantan thermocouple failed after 500 hours at 1400° F, whereas the 8-gage thermocouple remained

serviceable throughout the 1000-hour test at 1400 °F.

However, at the conclusion of the test, the diameter of the

8-gage materials had been reduced to about Ho of their

original value."

19
Changes in calibrations of 8-gage thermocouples

also given by Dahl are presented in table 47.

Table 47. Changes in the calibration of iron-constantan

thermocouples heated dn air (electric furnace)

Exposure Exposure A'Ta.Yimnm temperature change

°F ftr op

800 1,000 <1

1,000 1,000 <1

1,200 1,000 -4

1,400 800 -7

1,600 100 -10

1,800 28 -18

2,000 8 -19

On comparing these changes with the standard

tolerances for new wires, they are seen to be well

within the standard limits up to 1,600 °F (871 °C),

and at 1,200 °F (649 °C) and below, are within even the special limits.

d. Copper-Constantan Thermocouples (ISA Type T)

The copper-constantan thermocouple has been

used for a long time, and it still finds favor in the

sub-zero and relatively low ranges above 0 °C.

Some information on this thermocouple is to be

found in references [63, 65, 75, 76] and in the trade

literature. The reference tables of NBS Circular

561 [7] are said to be based on the work of .Roeser

and Dahl [63] which resulted in the so-called 1938

table of Adams.

The usual temperature range, from reference [6], is given as -190 to +300 °C (-310 to +570 °F),

and the maximum temperature is cited as 600 °C

(1,100 °F). The NBS tables are given up to

400 °C (752 °F) [7]. Generally, 750 °F is the maximum temperature mentioned in catalogs; in

some cases this temperature is recommended for

intermittent service, and 600 °F (316 °C) is mentioned for continuous use of 20-gage B & S wires.

The highest temperature mentioned in connection

with copper-constantan thermocouples in reference

[1] is 700 °F.

The standard limits of error, from reference [1], are ±1^ °F (0.8 °C) for the range -75 to 200 °F

( —59 to 93 °C) and ±% percent from 200 to

700 °F (371 °C). Special limits over the same

temperature ranges are one half the standard. Extension leads are listed for use over the range —75 to 200 °F, and the limits of error for them are the same as the standard limits for the thermocouple in this range. These same limits of error are given by some suppliers of type T thermocouples.

The thermoelectric power of the copper-constantan thermocouple, from reference [7], is about

21.3 per degree F at 32 °F (38.4 mv/°C at 0 °C),

26.0 MV per degree F at 212 °F (46.8 mv/°C at

100 °C), and 33.9 juv per degree F at 700 °F (61.0

Mv/°Cat371 °C). This high thermoelectric power

is one of the reasons for selection of this thermocouple for accurate work in the lower temperature

range above 0 °C, and below 0 down to —200 °C

or so. Other reasons will become apparent as properties of the individual elements are given.

These thermocouples are available in diameters

from 0.001 in. up. It is probable that, of the basemetal thermocouples, the copper-constantan is used with a reference junction ice bath proportionately more than the others, because it is used

in the laboratory and in industrial applications

where relatively high accuracies are required.

Recommended upper temperature limits for

different wire sizes are given, from two sources, in

table 48.

Table 48. Recommended upper temperatures for different wire sizes, copper-constantan

B & S

gage

Upper temperature limits

Reference

[1] Reference [70]

0 p "F

14 »700 600 "700

16 500 600 20 500 400 500 24 400 400 400

28 400

30 400 400

a These columns are given for conditions under which the thermocouples

are protected. No recent detailed information is available on

the homogeneity of copper-constantan thermocouples, but from their wide use in the low and

sub-zero ranges it seems certain that materials of

adequate homogeneity must be available. Dike

[76] says that "Copper-constantan thermocouples

made of wires taken at random from stock will

match the 1938 N.B.S. temperature-emf table for

copper against Adams constantan within limits of error of ±0.5 percent or 1.5 deg. F, whichever is larger, between —75 and +200 F. Between 200

and 700 F the limit is ±0.75 percent of the measured temperature. Those limits can be cut in

half, or even less, by selecting constantan which

matches tabular values more closely."

Because of the high degree of reproducibility of

the thermal emf of good copper, the thermal emf

of costantan versus platinimi is a good indication

of the homogeneity of the copper-constantan

thermocouple. Roeser and Dahl [63] gave the

outputs of thermocouples of three sizes of con- stantan from the same coil versus Pt 27. Table

49 is the lower half of their table 2 in which this

information is presented.

Table 49. Thermal electromotive force of different sizes of constantan wire drawn from the same coil [reference material NBS Pt 27)

Temperature

Electromotive force (reference junctions at

0 °C)

No. 8 No. 14 No. 20

"C mv mv mv

0 0 0 0 100 3. 51 3. 51 3.50 200 7. 43 7. 44 7. 43 300 11.69 11.70 11.68 400 16.17 16. 19 16.16

20
I This table does not necessarily, of course, show

the homogeneity of the wire because of the possibility of effects of the rather drastic reductions

I by drawing of 75 and 96 percent. It does, though,

I show a maximum difference due to combined

I causes of 0.03 mv at 400 °C (752 °F). 0.02 mv

[ at 300 °C (572 °F) and only 0.01 mv at 200 °C

(392 °F) and below. In each case the difference

is, in terms of temperature, well within the preI scribed limit of error.

I On the assumption that the greater part of the

I change in service of a copper-constantan thermo1 couple is due to changes in the constan tan wire,

Dahl's results [65] on the stability of constantan

I after periods of heating at 800 °F (427 °C) up to

! 1,000 hr are of interest. The thermal emf inI creased 10 on exposure for 1,000 hr and 6 hy

after 100 hr. These changes, resulting from rather

drastic treatment of constantan wires to be used

with copper, are not great, and surely should not

I be so great in normal use for this type of

i thermocouple.

e. Chromel-Constantan Thermocouples

' According to Dike [76] the Chromel-constantan thermocouple is an excellent thermocouple

"with excellent properties, both elements being

resistant to corrosion and capable of operating

j at temperatures up to 2,000 °F. It has been used

in thermopiles for radiation pyrometers where its

high thermoelectric power is of advantage. It is sometimes used in place of Chromel-Alumel in

industrial thermocouples."

The tables of NBS Circular 561 [7] for this

combination were based on tables for Chromelj platinum in NBS RP 767 [64] and tables for constantan-platinum given in NBS IIP 1080 [63].

The thermal emf at 750 °C(1, 382 °F) is given as 57.12 mv as compared with 42.32 and 31.23 mv for

iron-constantan and Chromel-Alumel. The approximate thermoelectric power of this thermocouple

is given iti table 60.

Table 50. Thermoelectric power of the Chromel-constantan

thermocouple

Temperature Thermoelectric power

°C MV/^C

0 58.5 200 74.5

400 81.0

600 81.0

800 78.5

1,000 75.0

No limits of error for this thermocouple are given in reference [1].

f. Properties of Base-Metal Thermocouple Materials

The type K wires considered here are

the positive Chromel, Kanthai P, and Tophel;

the negative wires are Alumel, Kanthal, and Nial.

McElroy and Potts, and McElroy [11,77], as

mentioned earlier, carried out a very comprehensive investigation of properties including

inhomogeneity and drift of Chromel-Alumel and

Kanthal P-Kanthal N thermocouples, and included, in addition, "A. C. Scott Chromal and

Alumel." Potts and McElroy [66] carried this

work further, and described their so-called unilateral temperature gradient furnace and its use

in determinations of homogeneity. Dahl's work

[65] included determinations of drifts of Chromel

and Alumel on exposure to temperatures up

to 2,200 °F. The findings are too numerous to

give here, and reference to the original papers is

suggested. Some inhomogeneities are found in

the individual wires and, as may be expected,

their outputs drift, depending upon the time and

temperature of exposure. The prospective user must therefore decide from the literature the

importance in his work of the inhomogeneities

and drifts which generally are of such magnitudes

as to be relatively unimportant for normal

applications.

Compositions of the type K wires, as given

by the makers of the first three thermocouples of

table 40 are presented in table 51.

Table 51. Compositions of type K thermocouples

AUoy

Percent of constituents

Ni Cr Mn Si Al Fe Co C

Chromel P" [67] Kanthal P [68] 90

90

98-97

93

10 Tophel [69] 9.3 0.4

2-3 1.05

0. 55 0. 05 0.03 Alumel [67] Kanthal N [68] Nial [69] 2. 5 2.0 0. 25 0.50 0.02

» "A . . . nickel-chromium alloy containing nearly ten times as much

nickel as chromium, but also incorporating nine additional minor constituents ..."

^ "It contains nickel, manganese, aluminum and silicon with appreciable percentages of the last three elements . . . plus eight other minor constituents . . . ." Hoskins alloy No. 196 is said by Potts and McElroy [66] to be a nickel-silicon alloy containing between 2 and 3 percent silicon. This, termed Special Alumel, is stated in reference [66] to have given "superior

performance." This alloy appears to be similar to the Kanthal N.

Analyses, from several sources, of the type K

wires are given in references [11] and [77]. Here

the positive elements are seen to contain from

about 89 to 90 percent nickel, 9 to nearly 9.5

percent chromium, silicon in amounts up to

nearly 0.5 percent and iron from 0.02 to 0.65

percent. Manganese, found in most of the

analyses, was present in amounts ranging from

0.01 to 0.8 percent.

Analyses showed the negative elements to be

composed of from somewhat less than 94 percent

to 96 percent nickel, 1 to 1.5 percent sUicon except

for Kanthal N which has 2.40 percent, aluminum

1.3 to 2.5 percent, manganese 1.8 to 3.25 percent,

and iron and other constituents in smaller quantities. Materials other than nickel and silicon are present in Kanthal in only small amounts. According to Sibley [71], the Special Alimiel "meets

the guaranteed emf curve for regular Alumel at

all temperatures except those within the range

from about 100 to 300 °F." On the assumption

21
that this is alloy No. 196 (table 51, footnote b) or one whose composition is close to it, the negative

element of Kanthal is similar, and might be

expected to exhibit the- same departure from the

reference tables. The standard tolerances are as mentioned earlier, listed for Kanthal, and also for

Tophel-Nial.

Type K thermocouples find their applications

in oxidizing or neutral atmospheres over the higher

temperature range up to 2,300 °F (1,260 °C).

Even though both elements are subject to oxidation in air above about 1,600 °F (871 °C), they are the most oxidation-resistant of the commonly

used conventional base-metal thermocouples. Reducing conditions have adverse effects, and the

"green rot," which is associated more with the

positive leg of the type K thermocouple than with

the negative, is said to be a result of a marginally

reducing or oxidizing atmosphere. Chromiimi is oxidized preferentially and the emf of the wire

and consequently of the thermocouple in which it

is used is lowered. Environments containing

hydrogen, carbon monoxide, and other strongly

reducing gases should be avoided.

The negative legs of the type K thermocouples

are, as indicated in table 51, of substantially different compositions. None of these alloys seems

to be affected by the "green rot" as are the positive legs, but the wires of low silicon content are

less resistant to oxidation than are those with

more. The increase of silicon seems to increase

resistance to oxidation to about the level of that

of the positive wire. The low-silicon negative

wires are subject to embrittlement by sulfur to

the point of breakage; and Sibley [71] said that,

from a limited amount of data, the modified

(Special) Alumel has provided no improvement in

resistance to sulfur.

Potts and McElroy [77] exerted a considerable

effort on determination of the effects of coldworking in these alloys, and quotations from their

summary follow:

"Generally, the effects realizable from cold work

produce calibration shifts of less than 10 °C in nickelbase thermocouples."

"As-received thermocouple wire is generally coldworked less than 5%."

"Depending on the composition of the alloy, the per- centage of cold-working, and the time at temperature, the

recrystallization temperature of the nickel-base thermocouple alloys is between 500 and 750 °C. The recovery

temperature range is between 250 and 450 °C."

"Heat treatment designed to produce the recovered

state in originally cold-worked material causes a recovery

of nearly 90% of the error induced by cold-working in

Chromel-P and somewhat less in Alumel, principally

because the initial change in Alumel is less." Some physical properties of the type K wires, as given by the makers [67,68,69] are presented in

table 52.

Tables have been given by the makers from

which the resistance of the positive and negative

wires at elevated temperatures may be obtained.

Because they are in different forms, tables 53 a, b,

and c are given as presented in references [67],

[68], and [69].

Table 52. Some physical properties of type K thermocouple wires

Chro- mel

P

Alumel Kanthal P

Kanthal N

Tophel Nial

Melting point °0.. 1430 1400 1430 1410 1425 1400 Melting point °F_. 2605 2550 2605 2570 2600 2550

Resistivity, 20 "C (68 °F) ohms/cmf.. 425 177 415 133 410 191 Coefficient of resistance X10<

per °C, 20-100 °C.. 3.2 18.8 4.4 30 3.9 24.0

Coefficient of expansion XIO'

per °C, 20-100 "C. 13.1 12.0 14 13.5 13.37 12.25

Specific heat

eal/g °C, 20 °C.. 0. 107 0. 125 0.11 0. 12 0. 11 0. 12 Thermal conductivity,

100 "C

caI/seo/cmVem/°C._ 0. 046 0. 071 0.05 0. 07 0. 042 0. 064 Density— g/cm'.. 8.73 8. 60 8.68 8.69 8. 63 8.47 Density ..lb/in.'.. 0. 315 0. 311 0. 313 0. 314 0.312 0.306 TensUe strength annealed

1,000 psi— 95 85 92 72 88 83 Tensile strength, work

hardened 1,000 psi— 165 170 Yield point annealed

1,000 psi 40

22 No

30

35 Yes

30 28 Elongation aimealed

percent Magnetic No Strong No Yes

Table 53. Resistance of type K thermocouple wires at elevated temperatures

a. Average increase (percent) in resistance when heated

Degrees F 68 100 200 400 600 800 1,000 1,200 1,400 1,600 1,800 2,000 Degrees C 20 38 93 204 315 427 538 649 760 871 982 1,093 Chromel P.-- 0 .67 2.8 7.1 11 14 19 21 24 28 31 34 Alumel 0 4 17 39 51 60 69 78 87 97 107 116

b. To obtain resistivity at working temperature, multiply by factors below

Degrees

C 20 100 200 300 400 500 600 700 800 900 1, 000 1, 100 Degrees F 68 212 392 572 752 932 1, 112 1,292 1,472 1,652 1,832 2.012 Kanthal P 1.000 1.035 1.068 1. 103 \. 144 1. 182 L202 1.226 1.249 1. 268 L305 L328

Kanthal N 1.000 1.28 L50 1.64 1.76 1.87 1.98 2. 10 2.22 2.35 2.50 2.64

c. Percent change In resistance

Temperature

68 100 300 500 700 900 1,000 1,200 1,400 1,600 1,800 2, 000 Tophel 0 1 6.5 11 14 17.5 19 22.5 25.5 28.6 31.6 34.5 Nial- 0 5.5 37.5 47 55 62.6 66 73.5 80.6 88 95.7 105

Information on the resistance and feet per

pound for several sizes of Chromel-Alumel and

Kanthal wires is given from references [67] and

[68] in table 54.

Table 54. Resistance and length per pound of several sizes of Chromel-Alumel and Kanthal wires

B & S gage Wire

diameter

Ohms per ft at 68 °F

Approximate Chromel ftAb P

Alumel Kanthal P

Kanthal N

in. 8 0. 1285 0.0257 0.0107 0. 0252 0. 00806 21

14 .0641 .104 .0432 .101 .0324 83

16 .0508 . 164 .0681 .161 .0516 131 20.-- .0320 .415 .173 ."407 .130 332

24 .0201 1.05 .438 1.03 .329 839

28 .0126 2.68 1.12 2.61 .836 2, 130 30 .0101 4. 25 1.77 4. 07 1.30 3,320 36 .0050 17.1 7. 12 16.6 6.32 13, 600 40 .0031 45.4 18.5 43.2 13.8 35, 300

22
The thermal coefficients of expansion at various

temperatures of Tophel and Nial [69] are given in

table 55.

Table 55. Thermal coefficient of expansion of Tophel and

Nial

Thermal coeflBcient of expansion, in./in. °CX10», 20 °C to temperature

Temperature, °C20 100 200 300 400 500 600 700 750

Tophel

Nial

13. 37

12. 25 13.9413.06

14. 36

13. 79

14.74

14. 21

15, 15 14.6715. 59

15. 05

16. 05

15.50 16.2615.69

The specific heat at elevated temperatures of

the nickel 80 percent-chromium 20 percent alloy,

from reference [92] is given in table 56.

Table 66. Specific heat of 80 percent Ni-20 percent Cr

Temperature Cal/g °C Temperature Cal/g °C

"C "C

0 0. 1033 400 0. 1264 25 .1052 500 .1310

50 .1071 600 .14

100 .1109 700 .1470 200 . 1171 800 . 1515 300 . 1217 900 .1563

Information from reference [69] in table 57

shows the change of thermal conductivity of

Tophel and Nial with temperature.

Table 57. Thermal conductivity of Tophel and Nial

Thermal conductivity, cal/sec/cmVcm/°C at temperature

Temperature °C 20 100 200 300 400 500 600 700 750

Tophel 0.042 0.047 0. 052 0.058 0. 063 0.068 0.073 0. 078 0. 080 Nial 0. 064 0. 003 0.070 0.077 0. 084 0. 090 0. 096 0. 102 0. 104

The only type K wires for which the thermal

emfs against platinum are foxmd are Chromel and

Alumel. Tables of thermal emf for these alloys

from reference [77] are given in table 58.

Table 58. Calibrations for ChromeUplatinum" and

Alumel-platinum^ thermocouples

Temperature Chromel Alumel Temperature Chromel Alumel

°C emf, mv emf, mv °C emf, mv emf, mv

0 0 0 700 22. 9610 6. 1835

ICQ 2. 8110 1. 2885 800 26. 2240 7.0820

200 5. 9695 2. 1640 900 29. 4115 7. 9530 300 9. 3260 2. 8860 1,000 32. 5320 8. 7850 400 12. 7655 3. 6335 1,100 35. 5830 9. 5785

500 16. 2150 4. 4310 1,200 38. 5475 10. 3420

600 19. 6245 5. 2875 1,300 41. 4005 11.0660

» Based on Hoskins Mfg. Co. Table E-271-CC.

' Based on Hoskins Mfg. Co. Table E-271-AA.

This table is condensed from the original in

which the intervals are 10 °C. According to the

authors, values of their table for the ChromelAlumel thermocouple may depart from those of

NBS Circular 561 by as much as 10 /xv. No

statement is found about the identity of the ref- erence platinmn used, but presmnably the data of

table 58 are given against NBS Pt 27 or against a specimen of low thermal emf versus Pt 27.

Little if any definitive information is found on

emissivity of the type K wires. Koeser and

Wensel give on page 1313 of reference [20] the

emissivity of unoxidized Chromel P at a wavelength of 0.65m to be 0.35, and that for Alumel to

be 0.37. The probable value at the same wavelength for oxide formed on smooth Chromel P and

Alumel is given as 0.87. Later, on page 1315, the

total emissivity of oxidized 80 percent nickel-20

percent chromium is given as 87 percent at 100

and 600 °C and 89 percent at 1,300 °C. The

warning is given here that "Most values are uncertain by 10% to 30%. In many cases value

depends upon particle size." Emissivity at room

temperature (presumably total) is given in ref- erence 70 as 0.64 to 0.76 for Chromel P and 0.60

to 0.75 for Alumel.

The Geminol wires are the positive Geminol P,

said in reference [72] to comprise 20 percent chromium, 1 percent colimabium (niobium), and the

balance nickel, and the negative leg containing 3

percent silicon in nickel. Analyses presented by

McElroy [11] show Geminol P, 20-gage and 24-

gage wires, to contain nickel 78.67 and 79.03 percent, chromium 17.92 and 18.07 percent, silicon

0.74 and 0.85 percent, iron 0.73 and 0.76 percent,

columbium 0.99 and 0.94 percent, and copper 0.02

and 0.03 percent. Geminol N of the same sizes is reported to comprise nickel 96.43 and 96.78 percent, and silicon 2.75 percent, columbivun 0.05

percent, carbon 0.01 percent, and chromium 0.005

percent in each.

Reference [77] points out the superiority of both

Geminol wires imder adverse conditions over the

type K. The higher chromiima content of the positive leg and the greater percentage of silicon in the

negative leg are indicated as the responsible factors. This comparison does not include ChromelSpecial Alumel, but it does include Kanthal N

with 2.40 percent silicon as opposed to 2.75 percent

sUicon in Geminol N.

Among features claimed by the makers for Geminol P are its "unexcelled oxidation resistance,"

and resistance to "green rot." The negative leg

has a resistance to oxidation that is comparable

with that of the positive wire of the type K thermocouples. According to the makers, the Geminol

thermocouple is resistant to change of calibration

when used under at least some reducing conditions.

The separate wires are said to be made to close

enough tolerances that any Geminol P, connected

to any Geminol N, will provide a thermocouple

having a calibration within the standard Hmits of

error. Some physical properties of Gemiflol P and N

from reference [72] are given in table 59.

Except for conversion of the value for thermal

conductivity from watts to calories per second, the

values of the table are as given. No temperatures

23
Table 59. Some physical properties of Gemmol P and

Geminol N

Property

'

Melting point °C__

Melting point °F..

Resistivity ohms/cmt (20 °C)--

Coefflcient of resistance X 10< per °C--

Coefflcient of expansion X 10' per °C.. Thermal conductivity cal/sec/cm2/cm/°C (100 "C).. Weight lb/in.3—

Specific gravity --- Magnetic strength

Geminol

1,400 2,562

655

1117

0. 0267

0. 034

8. 41 None

N

1,420 2,588

16224

14.5

0. 0907

0.311

8. 60 Strong

are given in the reference, but from another table

in the same reference it seems reasonable that the

temperatures inserted in the "property" column

apply. Other temperatures that seem to apply to

the values in the table are the range 20 to 500 °C

for the temperature coefl&cient of resistance of

Geminol P, 20 to 100 °C for that of Geminol N,

10 to 1,000 °C for the coefficient of expansion of

the P wire, and 20 to 500 °C for the Geminol N.

The tensile strength at 20 °C is given as maximum

and minimum of 200,000 and 100,000 psi for an

alloy close in composition to Geminol P, an-d

150,000 and 75,000 for an alloy having a composition near that of Geminol N. The specific heat of

these same two alloys is given as 0.104 and 0.128

cal/g °C. No temperature for which these values

apply is given.

Information on the resistance and feet of wire

per pound of Geminol P and N in table 60 is taken from reference [72].

Table 60. Resistance and length per pound of several sizes of Geminol P and Geminol N wires

B&S gage Wire

diameter

Ohms per foot Approxi- mate

ftAb Gem. P Gem. N

In. 8 0. 1285 0. 0400 0. 00989 21

14 .0641 .160 .0395 85

16 .0508 .252 .0623 134

20 .0320 .e-io .158 341 24 .0201 1. 62 .401 864 28 .0126 4. 12 1.02 2,200

30 .0101 6.55 1.62 3,490

The thermal emf of the Geminol wires versus Pt 27 is tabulated in reference [72]. This

reproduced in table 61.

IS

Table 61. Thermal emf of Geminol P and Geminol N

versus Pt S7

Tempera- Geminol Geminol Tempera- Geminol Geminol

ture P versus N versus ture P versus N versus Pt 27 Pt 27 Pt 27 Pt 27

°F mv mv "F mv mv

32 0. 00 0. 00 1,200 +12. 70 -6. 35 200 +1.22 -1.04 1,400 -f15. 66 -7. 44

400 +3. 11 -1.96 1,600 4-18. 74 -8. 56 600 -f5. 24 -2.94 1,800 +22. 00 -9.65

800 -1-7. 27 -4.09 2,000 +25. 43 -10. 65 1,000 +9. 96 -5.24 2,200 +28. 80 -11.75

No information is available on the emissivity

of these wires, but it seems reasonable that the

values given for the type K wires should at least

approximate those for Geminol.

Thermocouple iron, unlike materials discussed

previously, is not normally made expressly for

thermocouples, but is obtained by selection from

lots of pure iron made for other purposes. Differ- ences between lots of iron appear to have become

less since the introduction of the iron-constantan

thermocouple, though, and by selection of both

the iron and constantan wires it is possible to

provide thermocouples to match the ISA type

J or type Y tables. This is done to the extent

that, according to Dike [76], "Well over two

himdred tons of these thermocouples are supplied

to industry annually in the U.S."

Originally, according to Dike, the use of iron

in thermocouples was opposed strongly by authorities in the field of thermoelectric pyrometry

because of the inhomogeneity of the iron wire

obtainable. This objection has been overcome,

and its popularity has survived because of its

large thermoelectric power with constantan, its

adaptability to use under both oxidizing and

reducing conditions, and its low cost. Much of the information that follows on properties of iron has been taken from reference [26]

and the monograph of Cleaves and Thompson on

iron [78]. Although no assurance exists that the

iron for which properties are given matches that

of thermocouple wire, the information presented

here is for commercially pure or electrolytic iron

which should be expected to be fairly representative of thermocouple iron.

Analyses of the iron wires used in determination

of the modified 1913 reference tables of NBS

Circular 561 [7] by Corruccini and Shenker [73] are given in table 62.

Table 62. Chemical and spectrographic analyses of iron

samples

Values are given in percrent.

Sample C Mn P S Cu Si» Ni» Cr» Sn »

A-1 0. 07 0. 25 0. 015 0. 031 0. 12 <0.01 0.044 0.015 <0. 01 C-1 .11 .38 .005 .031 .054 <.01 .030 .016 <.01 D-1 .04 .29 .009 .024 . 14 <.01 .042 .016 <.01

F-1 .02 .03 .004 .029 .15 <.01 .039 .025 <.01 F-3 .08 .43 .080 .028 .018 .02 .011 <.01 .01 F-4 .02 .23 .011 .016 .023 <.01 .014 <.01 <.01 F-5 .06 .21 .006 .020 .022 .01 .013 <.01 <.01 F-6 .05 .22 .006 .018 .022 <.01 .012 <.01 .01

» Spectrographic determination. From reference [78], exposure of a fresh surface

of iron to air, in the absence of moisture, causes a rapid development of a superficial coating of

iron ioxide. At temperatures below 150 °C (302

°F), the coating is said to be so thin as to be invisible, but suffices to prevent further oxidation.

At temperatures above 200 °C (392 °F) the rate

and extent of reaction between iron and air

increase with temperature. Liquid water, even though only an adsorbed film, is said to be neces24
sary for corrosion in the atmosphere, and rusting

is speeded up by temperature fluctuations causing

condensation from the surroimdings. Rusting

may result from wet steam, and according to

Cleaves and Thompson, iron ma}' be attacked

appreciably by atmospheres of hydrogen, nitrogen, or ammonia.

A statement from reference [78] summarizing

the information on corrosion of iron in aqueous

solutions follows:

"Even the purest iron dissolves to a slight extent in pure

water in the absence of oxygen and rusts in pure water in

the presence of oxygen. In general iron is subject to corrosion in aqueous solutions of acids, alkalies, and salts. The rate and extent of corrosion vary with a number of

factors including hydrogen-ion concentration, presence of

oxygen, agitation, temperature, and nature and concentration of the solution. Some degree of protection may

result from passivity or from the formation of protective

films but these factors, and the effect of purity and homogeneity in the metal, are usually of slight importance in

submerged corrosion."

In connection with the popularly held conception

that pure iron is markedly corrosion resistant,

Cleaves and Thompson say that.

"The effect of purity, in comparison of very pure irons

with commercial products or in comparison of different

grades of commercial products one with another, usually

is less important than the effect of external conditions.

Real resistance to corrosion is obtained only by the incorporation of considerable amounts of added elements, such

as silicon in cast irons or chromium and nickel in stainless

steels, but even these products are not universally resistant

to corrosion. The corrosion resistance of iron may be

summarized in two words—iron rusts." The melting point of iron is, from reference

[3], 1,533 °C (2,791 °F).

Information on the tensile properties of iron, even in its pur^r states, is widely divergent.

Much information is avilable, but selection of

data that apply strictly to thermocouple iron is practically impossible. The summary of data on

tensUe properties from reference [78] foUows:

"Available information indicates that electrolytic iron

in the as-deposited condition may have tensile strengths

and yield strengths of more than 100,000 lb. per sq. in. with elongations as small as '3 per cent. However, the

properties of the same material in the annealed condition

are roughly comparable to those of Armco iron. The

tensile strength of annealed electrolytic iron is reported to be between 40,000 and 55,000 lb. per sq. in., yield strength

between 20,000 and 25,000 lb. per sq. in., with elongations

between 25 and 45 per cent. Fusion in vacuum with

subsequent annealing treatment further softens electrolytic iron. The reported properties of such material are:

tensile strength between 35,000 and 40,000 lb. per sq. in., yield strength between 10,000 and 20,000 lb. per sq. in. elongation between 30 and 60 per cent, and reduction of area between 70 and 90 per cent."

The Armco iron mentioned above is described as one of the commercially pure types. The temperature of annealing that caused the reduction

in tensile strength of the electrolytic iron is not

specified definitely, but annealing for 1 hr up to

300 °C (572 °F) had no effect. Several annealing

temperatures in the range from 900 °C (1,652 °F)

to 960 °C (1,760 °F) are listed.

The variations of tensile strength and modulus

of elasticity of Armco iron with temperatiire are given in table 63.

Table 63. Variation of tensile strength and modulus of

elasticity of Armco iron with temperature

Temperature JL Clio lie Rlastin J-^ IdrO 1/1^ oti cii^i'ii TTinflnlns

°C "F psi psi X /03 0 32 44, 400 29, 000- 100 212 30, 300 28, 100 200 392 30, 700 25,700

300 572 45, 700 22, 200

400 752 34, 300

500 932 21, 500

600 1,112 11,800

700 1,292 7,800

800 1,472 3, 700 900 1,652 3, 900

1,000 1,832 5,700

The above values from reference [78] and attributed to Lea [79] are taken from a small-scale curve and so must be considered to be approximations.

Hardness of the specimens used in the work of

reference [73] is given in table 64.

Table 64> Results of hardness measurements on samples

of iron wire

SatQple

Viekers number " Equivalent

Rockwell B number

Sample

Viekers number » Equivalent Rockwell B number

A-1 99.8 55 F-3 210 94 C-1 106 59.5 F-4 83.6 39 D-1 138 76 F-5 138 76 F-1 127 71 F-6 131 73

a Determinations were made with a 10-kg load applied through a square- based diamond pyramid-indenter.

The resistivity of iron relative to that at 0 °C

(32 °F), from reference [20], is given in table 65.

Some values of the table have been adjusted

slightly to bring them more nearly into agreement

with the International Temperature Scale of 1948.

Information on resistivity is presented in some

detail in reference [78].

Table 65. Electrical resistivity of iron relative to the

resistivity at 0 °C (Rt/Ro).

[The actual resistivity (ohms/cmf) is given for 0 °C].

Temperature Temperature Ri/Ro

"C op °F "F

0 32 1.000 500 932 6.162

0 32 (51. 56) 600 1,112 7.839 100 212 1.650 700 1,292 9.785 200 392 2. 454 800 1,472 12.000 300 572 3. 485 900 1,652 12. 787

400 752 4. 716 1,000 1,832 13. 069

The suggested value for the density of pure iron

at "room temperature" in reference [78] is 7.87

g/cm^ (0.2843 Ib/cu in.).

The thermal coefficient of expansion of electrolytic iron is given in reference [80]. Both the

relative length at elevated temperatures and the

25
true thermal coefficient of expansion are given in

table 66.

Additional information on different high-purity

irons is found in references [26] and [78].

True and mean specific beats of electrolytic iron

from reference [80] are given in table 67.

A large quantity of information on thermal conductivity of iron is found in reference [26], and

table 68 gives values obtained at BatteUe [81] for

Armco iron of 99.906 percent purity.

Averages of the values of the thermal emfs for

the eight samples of iron versus Pt 27, from refer- ence [73], are given in table 69.

Although a considerable amount of information

is found on the emissivity of iron, large differences

exist between different observers for conditions

that apparently are similar. Wilkes [82] gives the

total normal emissivity of iron to be 0.05 and 0.07

at 100 and 500 °F. This is in fair agreement with

values from McAdams [83], of 0.052 to 0.064 over

the temperature range from 350 to 440 °F. For

polished iron the total normal emissivity ranges

from 0.144 to 0.377 over the range 800 to 1,800 °F.

"Wrought iron, highly polished, is reported as having an emissivity of 0.28 over the range from

100 to 480 °F.

Smooth oxidized electrolytic iron, according to

McAdams, has a total normal emissivity in the

range 260 to 980 °F varying from 0.78 to 0.82.

Oxidized iron at 212 °F has an emissivity of 0.736.

Emissivities of 0.85 to 0.89 are given for iron

oxide over the range 930 to 2,190 °F.

The emissivity of electrolytic iron was determined by Wahlin [84] to be approximately 0.345

from about 1,000 to near 1,178 °K, then decreased to 0.325 at 1,178 °K (A3 point) with a dip

to approximately 0.318 at about 1,360 °K. The

emissivity increased to about 0.325 near 1,677 °K

and 0.338 at 1,677 °K (A4 point). This work was

done with an optical pyrometer and so the spectral

region presumably was near 0.65 or 0.66 fj.. The

total emissivity of iron (black) at 200 °F is re- ported by McDermott [85] to be 0.56, and

McAdams [83] gives 0.657 for the same temperature. Roeser and Wensel [20] give 0.35 at 0.65 n

for imoxidized iron, and 0.87 for the probable

emissivity at 0.65 of the oxide formed on smooth

cast iron.

Copper, from the standpoint of utility in thermocouples, has some unusually desirable featm-es.

Thermoelectric properties of the commercial metal

are exceptionally uniform, and mishandling re- sulting in severe cold working does not produce the

large thermoelectric changes that it does in some

other materials. Roeser and Dahl [63] found a

total spread of 29 iiv, with junctions at 0 and

350 °C, in the emf of 31 samples of copper ranging

from 8-gage B & S to 40-gage. Differences

amounting to only 24 juv between cold-roUed and

annealed wires, with junctions at 0 and 300 °C

have been encountered by others. Results on

copper drawn from a rod of one-half inch diameter

to sizes from 8-gage to 22-gage show the maximum

difference between the cold-worked and annealed

Table 66. Thermal expansion of electrolytic iron

t

Coefficient Coefficient

of Ulh t of Ulh

expansion expansion

°c X10«per°C "C X10« per°C

nu 1. 000000 325 15.39 1.004341 100 12.62 1.001158 350 15. 54 1. 004725 110 12.67 1.001284 375 15.74 1.005114

120 12.76 1.001411 400 15.88 1. 005507 130 13.32 1. 001539 425 16.01 1.005904 140 14. 25 1.001672 450 16.17 1. 006304

13. 32 1. 001743 500 16. 37 1.007115

150 13. 12 1.001810 550 16.47 1.007935 160 13. 56 1. 001941 600 16. 61 1.008762

170 13. 82 1.002076 650 16.64 1.009592 180 14.02 1.002146 700 16.68 1.010424 190 14.14 1.002355 732 1. 010884

200 14.32 1. 002496 733 1. 010827 •Shrink225 14. 54 1. 002853 age

250 14.78 1. 003217 736 1. 010669

275 14. 97 1. 003587 750 20. 97 1. 010972 300 15.20 1. 003961 770 20.95 1. 011390

800 20. 97 1. 012018

Table 67. True mean specific heats of eletrolytic iron

a. True specific heat b. Mean specific heat

Cp bet Cal/g °C t Cal/g "C t t' tween t and t'

°C °C °C "C cal/g °C

105 0. 112 850 0.190 100.3 21. 267 0. 1127

150 0. 132 900 0. 182 200.2 21.861 0. 1161 200 0.123 975 0. 142 301.2 22. 996 0. 1202 250 0. 134 1, 000 0. 144 402.4 22. 673 0. 1266 300 0.136 1,050 0.148 500.4 23. 453 0. 1312

350 0.143 1,100 0.151 605.0 23. 172 0. 1386 400 0.146 1,150 0.156 707.9 24. 458 0. 1467 450 0. 151 1,200 0.162 770.3 25.442 0. 1557

500 0.160 1,250 0.169 825.1 24. 846 0. 1597

555 0.169 1,300 0.176 910.1 25. 260 0.1623

600 0.175 1, 350 0.178 1, 067.

5

24. 144 0.1644 650 0. 186 1,400 0.180 1,197.2 24. 212 0. 1635

700 0.213 1,450 0.200 1,391.1 25. 375 0. 1651 750 0.281 1,500 0. 200 1, 403.

9

25. 032 0. 1662 800 0.213 1,491.1 25. 598 0. 1686

Table 68. Thermal conductivity of Armco iron

Temperature cal/seo/cm2/cm/°C Temperature cal/sec/cmVcm/°0

"C °C

0 0.180 600 .095 100 .162 700 .086 200 .146 800 .079 300 .132 900 .083 400 .118 1,000 .086

600 .106

Table 69. Thermal emf of iron verse Pt 27

Temp. Emf Temp. Emf

op mv °F mv

32 0.00 1,000 7.03 100 0.67 1,100 7.63 200 1.64 1,200 8.31 300 2. 56 1,300 9. 11 400 3. 41 1,400 10.04 600 4.16 1,500 11.04 600 4.82 1,600 12.02 700 6. 47 1,700 12.86 800 6. 96 1,800 13. 67 900 6. 48

26
copper, with the junctions at 0 and 350 °C, to be

9 nv. According to Dike [76], copper need not

be specially selected for thermocouple use if it conforms to ASTM Spec. B3-45 for soft or annealed

bare copper wire.

A disadvantage which restricts the use of copper

in thermocouples generally to temperatures of

400 °C (752 °F) and below is its oxidation at

relatively low temperatures. Its high thermal

conductivity may be advantageous in some instances, but additional care is required when using

copper to insure that both the measuring and

reference junctions assmne the desired temperatures. Copper wire is available in diameters from

0.001 in. up, and because of its higher thermal

conductivity (about 20 times that of Chromel P)

and electrical conductivity (about 40 times that

of Chromel P) often is used in smaller sizes than

are other wires.

Copper is extremely resistant to corrosion under

normal atmospheric conditions; and according to

Gilbert, page 380 of reference [86], its good behavior depends to a considerable extent on the

maintenance of an inhibitive film of oxide or other

insoluble corrosion product. Corrosion resistance

of copper, imlike that of some other metals, is not

influenced by the grade or impurities present.

Copper is said to be "usually virtually unattacked

in most non-oxidizing solutions", and under fairly

mild conditions copper is successfully used for handling solutions of hydrofluoric, hydrochloric, sulfmic, phosphoric, and acetic and other fatty acids.

Increased concentration, temperature, and flow

rate cause increased corrosion. Copper is said to

be attacked to some extent by aerated alkaline

solutions, and its use should be avoided where

ammonia is present. It is unsuitable for use in

hydrogen peroxide' and in molten sulfur, and

corrosion is accelerated by hydrogen sulfide. Sulfur dioxide is corrosive at high relative hmnidity

(about 65 percent), and becomes more so with an

increase of concentration.

The mechanical properties of copper are affected

by a wide variety of conditions, and so it is not

possible to give a complete picture of them here.

Some of these properties presented by Smart in

reference [86], page 374, are listed in table 70.

Table 70. Some comparative stress-strain relations of

tough-pitch and oxygen-free copper

Tough-pitch Cu Oxygen-free Cu

Property

Hard Annealed Hard Annealed

Modiilus of elasticity psiXlO-S- 16.95 15. 82 17.54 16.20 Yield strength (0.5 percent strain)psi__ 46, 000 5, 500 47,000 5, 000 Tensile strength psi - 46, 900 31, 500 47, 500 '30, 900 Elongation in 2 in %- 14.8 53.0 20.5 60.0 Reduction of area - -%- 58.5 71.4 86.4 92.1

According to Smart, "High puritj^ copper is so

ductile that it is difficult to determine the existence

of a finite limit beyond which it cannot be cold

worked. No evidence of such a limit has been

observed in wires drawn 99.4 percent . . . ." According to Jennison and Smith [87], perfectly

pure copper, under extended periods of annealing,

will recrystaUize at about 212 °r. Smart, however, gives on page 413 of reference [86] a table of

softening temperatures of pure copper with individual impurity additions. Some of his data are presented in table 71.

Table 71. Softening temperature (degrees' C) of pure

copper containing individual impurity additions

Samples annealed 14 hr at 600 °C (1,110 °F) prior to final cold reduction of

75 percent.

Softening temperature of pure copper=140 °C (284 °F). \

\ Quan-

\tity

Im-X

purity \^

0.002 percent 0.005 percent

T.P." Oa-free T.P. Oj-free

O2 140 140 140 140 Fe 140 146 140 153 Sb^ 180 192 258 282 As 168 168 189 189 Ni 140 140 140 140 Pb« 146 250 146 270 Bi^ 210 260 247 300 Ag 148 148 172 172 Sn 140 198 140 27/

Si> 181 181 183 183 Seb 222 222 234 234 Teb 212 212 228 228 P 140 258 140 284

Si 161 181 Co

CrZn Cd>

145 152

148 183

152

190

151 248

» Copper containing oxygen in the tough-pitch range.

•> Tough-pitch values reflect partial oxide formation. Values equivalent

to those given for oxygen-free copper can be obtained by heat treatment at temperatures in excess of 700 °C (1,290 °F).

« Much higher values obtainable by heat treatment to increase solid solubility.

Some physical properties of copper are summarized in table 72, taken from a table by Smart,

page 363, reference [86].

Table 72. Some physical properties of copper

Melting point, 1083°±0.1 °C

1981.4° ±0.2 °F

CoefBcient of thermal expansion at 20 °C, 0.0000165 per

°C; for0to300°C, L, = Lo[l-t-(16.23t +0.00483«2j XlO-s]

Specific heat at 20 °C, 0.092 cal/g °C

Specific heat, solid, 0 to t °C, 0.092+ 0.0000250t cal/g °G

Thermal conductivity at 20 °C, 0.934 cal/sec/cm2/cm/°C

(adjusted to electrical conductivity of 101 percent

Resistivity at 20 °C,» 1.7241 X10-« ohm cm, 10.371

ohms/cmf

Temperature coefficient of electrical resistivity at 20 °C,

0.00394 per °C

Emissivity,

At wave length fi.. 0.70 0.65 0.60 0.55 0.50 Percent e at 1,000 °C (1,932 °F) . . 8. 7 11. 2 18. 5 31. 3 40.

4

Percent < at 1,125 °C (2,057 °F).. 11.8 14.8 20.5 29.3 38.7

• lACS copper, 100 percent, 0.15328 ohm at 20 "C (meter gram).

A table of factors to reduce the resistivity of

96 to 100 percent conductivity and at temperatures from 0 to 75 °C is given on page 407 of

reference [3].

27
Table 73. Electrical resistivity of copper relative to the

resistivity at 0 °C (Ri/Ro)

The actual resistivity (ohms/cmf) is given for 0 °C.

Temperature RilRn Temperature

°C °F °C "F

0 32 1.000 500 932 3. 210 0 32 (9. 38^ 600 1, 112 3.695

100 212 1.431 700 1,292 4. 208

200 392 1.862 800 1,472 4. 752 300 572 2. 299 900 1,652 5. 334 400 752 2. 747 1, 000 1,832 5. 960

Table 74. True and mean thermal coefficients of expansion

of copper

Temperature

Expansion coefficient X108 TemperatureExpansion coefficient X106

True

at t° Average,

0° to t° True

at t" Average,

0° to t°

"C "C

0 16. 73 600 20. 63 18.63

100 17.27 17. 00 700 21.46 19. 01 200 17.83 17.29 800 22. 33 19. 41 300 18. 45 17. 60 900 23. 25 19.83 400 19. 13 17.92 1, 000 24. 16 20.26

500 19. 85 18. 27

The resistivity of copper relative to that at 0 °C

(32 °F) from reference [20] is given in table 73.

A standard density at 20 °C of 8.89 g/cm^

(0.321 Ib/cu in.) was adopted for copper of 100

percent conductivity. Smith [88], however, ac- cepts 8.94 g/cm* (0.323 Ib/cu in.) as the density

of annealed pure copper.

The true and average thermal expansion of

copper, from reference 23, follow in table 74. The

length of a heated piece of copper, Z<, is given as:

Z,=Zo(l + 16,733X10-«i+0.002626X10-¥

+0.00000091 X10-¥).

True and mean specific heats of electrolytic

copper as given by Jaeger, Rosenbohm, and

Table 75. True and mean specific heats of copper

Temperature

Specific heat, cal/g °C

True, at t° Mean, 0° to t°

°C

300 0. 0988 0. 0957 400 .1009 .0968

600 .1030 .0978 600 .1051 .0988 700 .1072 .0999 800 .1092 .1009 900 .1113 .1020

Bottema are presented in table 75. The equation

for the total heat content Qo at temperature t° C

is Qo=0.092597i+0.10416X10-*i^

for the true specific heat Cp,

Cp=0.092597+0.20832X10-%

and for the mean specific heat above 0 °C, c'p is

Cp=0.092597 +0.10416X10-^t

The thermal conductivity of copper, from Smith

[88], is given in table 76.

Table 76. Thermal conductivity of copper

Tempera- Thermal con- ture ductivity

"C cal/sec/cm^/cml'' C

0 0.912 20 .910 100 .901 200 .890 300 .879 400 .867 500 .856 600 .845

The thermal emf of copper relative to platinum,

from Roeser and Wensel [20], is given in table 77.

Table 77. Thermal emf of copper relative to platinum

Temperature Emf Temperature Emf

°C o p mv °C op ma

0 32 0 600 1,112 8. 34

100 212 0.76 700 1,292 10. 49 200 392 1.83 800 1,472 12.84 300 572 3.15 900 1,652 15. 41 400 752 4. 68 1,000 1,832 18. 20

500 932 6. 41

Constantan is a name applied to a family of

copper-nickel alloys most of which were designed

originally to provide wires having a low temperature coefficient of resistance. According to Dike

[76] the composition ranges from copper 50 percentnickel 50 percent to copper 65 percent-nickel 35

percent, with small percentages of manganese

and iron as well as some trace impurities, such as carbon, magnesium, silicon, cobalt, etc. The

precise composition is not specified ; but it depends

upon the use to which the &,lloy is to be put, such

as to match thermoelectrically a particular iron

for an iron-constantan thermocouple, or to be

used with copper or a positive type K element to

match particular thermocouple tables. According to Roeser and Dahl [63] "Constantan . . . includes the alloys made in this country under

such trade names as Advance (Ideal), Copel,

Copnic, Cupron, etc., most of which contain

approximately 55 percent of copper and 45 percent

of nickel."

The approximate melting point of constantan,

although not a limiting factor in its use in thermocouples, is given as 1,210 °C for Advance [72], an

alloy of copper 57 percent-nickel 43 percent.

The stability of constantan was pointed out

by Dahl [65]. After 1000 hrs of heating in air at

800 °F (427 °C), which is above the recommended

temperature for the copper-constantan thermocouple, the change in 8-gage constantan at

800 °F corresponded to only about 0.5 °F. This

is well within the tolerances for the copperconstantan thermocouple, and since copper is highly reproducible, this is the drift that might

be expected from prolonged use of this thermocouple in its extreme range.
On heating 8-gage constantan in air to 1,800

°F, it was found to deteriorate to the point of

imminent failure after 28 hrs. The Ufe of the

iron wires used was about" the same as that of

the constantan. The maximum change in the

constantan wire corresponded to about an in- crease of 13° at 1,200 to 1,400 °F and the change

decreased to about 8° at 1,800 °F. The maximum

change is greater than the allowable limit of

error for new thermocouples; and when the con- stantan was coupled with the iron used, the

change in the iron, corresponding to a decrease of

about 9° at 1,800 °F, gave a net decrease of emf

at 1,800° that corresponded to about 17 °F:

After exposure for 200 hrs to 1,600 °F, the in- crease of the emf of the constantan corresponded

to about 10° at 1,600 °F and the decrease of the

iron to about 10 "^F; the net decrease of emf at

1,600° thus was about 20 °F.

As mentioned above, constantan oxidizes at

high temperatures, and this fact must be taken

into account when it is considered for use. Its

ability to withstand some reducing conditions

is in its favor and is taken advantage of in some

industrial applications when it is used as one

leg of an iron-constantan thermocouple.

Some properties of constantan from references

[43] and [72] are given in table 78.

Table 78. Some properties of constantan

Nominal specific resistance at 20 °C, 68 °F: 49 ohm

cm X 10-«, 294 ohms/cmf

Nominal temperature coefficient of resistance:

0.00001 (68-400 °F) per °F

±0.00002 (20-100 °C) per °C

Specific heat: 0.094 g cal/°C

Thermal conductivity at 100 °C: 0.0506 cal/sec/cm2/cm/°C

Nominal coefficient of. linear expansion, (20-100 °C):

14.9 X 10-e per °C

Tensile strength at 20 °C: maximum 100,000; minimum

60,000 psi

Density: 8.90 glare?; 0.322 Ib/cu in.

Wise [90] gives curves of properties of commercial annealed alloys. Approximate values of some

of the mechanical properties of constantan at

presumably room temperature are : tensile strength

72,000 psi, elongation 48 percent, and elastic

limit about 23,000 psi. Stauffer [91] points out

that "the 55% copper-45% nickel has about the

highest electrical resistivity, the lowest temperature coefficient of resistance, and the highest

thermal emf against platinum of any alloys of these

metals." The values of properties given by

Stauffer agree very closely with those of table 78;

and he gives, in addition, temperature coefficients

of resistance over the ranges from 0 to 250 °C

and 20 to 500 °C as 0 and ±0.000025 per degree- C.

He cites 500 °C as the maximum temperature of use of constantan in resistors and 900 °C as the

upper limit for thermocouples.

The thermal emf of constantan is tailored to

meet the use to which it is put, and so, a table of

its thermal emf versus Ft 27 must be regarded as

approximate. Table 79 gives the averages of the

values obtained for seven samples by Corruccini

and Shenker [73] and those obtained by Roeser

and Dahl [63].

Table 79. Thermal emf of constantan versus Pt 27

JL U 'li ptrl a,- T? ofd"i3n Oil Xvclcl cllLc xVclcI cULc R o Td roTi Oct

I' IDOJ ture f7^1 LDoJ

° F emf, mv emf, mv o p emf, mv emf, mv

32 0. 00 n. 00 1,000 -22. 46 -22. 55 100 -1.26 -1.27 1,100 -25.07 -25. 15 200 -3. 24 -3.26 1,200 -27. 67 -27. 77 300 -5. 35 -5.39 1,300 -30. 29 -30. 39 400 -7.58 -7. 64 1,400 -32. 89 -32. 99

500 -9.92 -9.98 1,500 -35. 48 -35. 58 -12. 32 -12. 40 1,600 -38. 04 -38. 14 700 -14. 80 -14. 88 1,700 -40. 57 -40. 66

800 -17. 33 -17. 41 1,800 -43. 06 -43. 13 900 -19.88 -19.97

The variation with temperature of the resistivity of constantan (55 copper-45 nickel) as given by Roeser and Wensel [20] is presented in

table 80.

Table 80. Electrical resistivity of constantan (55 Cu-45

Ni) relative to the resistivity at 0 °C (Rt/Ro)

The actual resistivity (ohms/cmf) is given for 0 °C.

Temperature Rt/Ro Temperature

°C °F "C "F

0 32 1.000 600 1, 112 1.024

0 32 (294. 2) 700 1.292 1.040

100 212 0. 999 800 1,472 1.056 200 392 0. 996 900 1,652 1.074 300 572 0.994 1,000 1, 832 1.092 400 752 0.994 1,100 2,012 1.110

500 932 1.007

2.3. Refractory Metals

The materials discussed in this section are generally the non-noble metals that can be used,

alloyed or alone, to form thermocouples usable in

the ranges above those covered by the current

conventional thermocouples. The large amount

of effort that has been and continues to be spent

on the refractory metals is a direct result of the

higher temperatures encountered in the jet, missile,

and reactor technologies. The principal materials

under investigation are the so-called refractory

metals—^tungsten, molybdenum, rhenium, tantalum, niobium, and their alloys. Iridium also

has been considered for use in one leg of a hightemperature thermocouple, either alone or alloyed

with one of the other metals mentioned.

Descriptions and properties of thermocouples of

the refractory metals are found in the works of

Sanders [93], Kuether [94], Lachman and Kuether

[95], Kuether and Lachman [96], Lachman and

McGurty [97], and of others as cited. The legs of

the various thermocouples mentioned will be

identified by the chemical symbols (tungsten W,

molybdenum, Mo, etc.) in the discussion that

follows, and temperatures will be reported as given; no corrections have been made to convert

to the 1948 scale. Information on the refractory

thermocouples will be presented along with

properties of the wires, and the separation

29
Table 81. Physical properties of tungsten

Property Unit Condition Values

Mechanical properties

DensityHardness.

Tensile strength and elongation.

Yield point

Hot tensUe strength and corresponding elongatlo J.

Young's modu'usg/cm'..

V.P.N.

1,000 psl and percent.

1,000 psi.

1,000 psi and percentpsi X 10-5.

Presintered at about 1,500 °C (2,700 °F)_

Sintered at up to 3,000 °C (5,400 °F) Swaged

Drawn

Sintered bar _-_ Swaged bar Cold rolled sheet. Recrystallized sheet Sintered bar Swaged bar Wire 0.04 in. diam

.02 in. diam__

.008 in. diam

.004 in. diam.._-

.0008 in. diam

Wire 0.004 in. diam, annealed (recryst.). Single crystal (containing Th)

Wire 0.02-0.04 in. diam

Annealed Unannealed

Wire 0.025 in. diam

400 °C (750 °F)

800 "C (1,470 °F)_...

1,200 °C (2,200 °F)..

1,800 °C (3,270 °F)..

20 °C (70 °F)

1,000 °C (1,830 °F)..

10. 0-13.

0

16. 5-17. 5

18. 0-19.

0

19 0-19.

3

200-250

35(1-500 450-500 260-380

1850-213

256 1 to 4 284 1 to 4 355 1 to 4 427 1 to 4 582

156

166 20

99-113

213

170-227 2 to 3 113-142 4 to 5 57-85 5 to 6 14-42

59

47

Thermal properties

Melting point Rate of evaporation.

Specific heat

Thermal conductivity

Linear coefficient of expansion .

°C... ., g/sq cm/seccal/g °C

cal/sec/cm2/cm/°C .

deg-i C

3,410±20 (6,170 °F3 _ 2,500 °C (4,530 "F).

3,000 °C (5,430 °F)

.

20 °C (70 °F)

1,000 °C (1,830 °F)_

2,000 °C (3,630 °F)

.

20 °C (70 °F)

1,000 °C (1,830 °F).

1,600 °C (2,910 °F)_

20 "O (70 °F)

1,000 °C (1,830 °F).

2,000 °C (3,630 °F).

7X10-S

2.4X10-5

0.033

0.041 0.047

0.31 0.27 0.25 4.43X10-'

5.17X10-8

7.24X10-6

Electrical properties

Specific resistance microhm-cm.,

ohms/cmf

20 °C (70 °F)

1,000 °C (1,830 °F).

2,000 °C (3,630 °F).

3,000 °C (5,430 °F).

5.5 33

33

198

66 397

103 620

Optical properties

Total intensity of radiation watts/sq cm 1,000 °C (1,830 °F). 2.5 2,000 °C (3,630 °F) 47

3,000 °C (5,430 °F).. 240

made previously between facts on thermocouple

combinations and the separate elements will not

be made.

The majority of the thermocouples proposed

for use at the extreme upper temperatures for

immersed sensors have tungsten for one leg. Because of its importance in this area, properties of

tungsten are given in tables 81 and 82 in some

detail from reference [101].

Similar information on molybdenum is given

from reference [102] in tables 83 and 84. Information similar to that of tables 81 to 84 is found

also in references [103], [104], and [105]. Other

sources are available, but these cited, together

with [101] and [102], were supplied in answer to

a request circulated when this paper was contemplated, and supply most of the information

required on tungsten and molybdenum.

The smallest diameter found in the trade literature for tungsten is 0.4 mil, and that for molybdenum is 1 mil. Larger sizes of wires and rods

to any desired diameter are available.

From the chemical properties of tables 81 and

82, it is seen that at temperatures above 500 °C

(932 °F) tungsten and molybdenum cannot be

used in an oxidizing atmosphere and must therefore be protected if measurements are to be made

under oxidizing conditions. No reaction between

tungsten and nitrogen or hydrogen is said to occur

at even the "highest temperatures," but molyb30
Table 82. Chemical properties of tungsten

Corrosive

Hydrochloric acid, sulphuric

acid, or nitric acid.

Aquaregia

Hydrofluoric acid... _-- Hydrofluoric and nitric acid mixed

Alkalies—

Ammonia..

Sodium nitriteCarbon Oampblack, graphite,

charcoal) and hydrocarbonsSulphur

Mercury and mercury vaporBehavior

or

Cold, dilute insoluble Warm, dilute attack Cold: practically insoluble. attack Cold or warm: insoluble Rapid attackor concentrated: practically

concentrated: slight Warm: rapid

Cold aqueous caustic soda or potash:

practically insoluble. Molten caustic soda or potash or alkaline carbonates:

a) in air: slow oxidation

b) in the presence of oxidizing agents,

e.g., KNO3, KNO2, KCIO3,

PbOj; rapid solution

Practically insoluble to attack by aqueous

solution, slight attack in the presence of

Molten: rapid solution above 300 °C (570

Formation of carbides from 1,400 °C

(2,550 °F), complete carburization to WC

at 1,400 to 1,600 "C (2,550 to 2,900 °F) Molten or boiling: slow attack

Practically insoluble

Corrosive

Aluminum oxide, magnesium

oxide. Thorium oxide

Air and oxygen

Water

Water vapor Hydrogen - Nitrogen

Ammonia

Carbon monoxide -- Carbon dioxide Halogens

Hydrogen chloride gas

Nitric oxide --- Hydrogen sulphide

Sulphur dioxide Carbon disulphide

Behavior

Reduction to metal in contact with tungsten above 1,900 °C (3,450 °F) Reduction to metal in contact with tungsten above 2,200 °C (4,000 °F)

Insoluble at room temperature; oxidation

starts from 400 to 500 °C (750 to 930 °F);

at temperatures above this rapid oxida- tion to WO3 and vaporization

Insoluble at all temperatures At red heat rapid oxidation to WO3

No reaction up to melting point

No reaction up to highest temperatures

Slight formation of nitrides with powder

from 700 °C (1,290 °F). Nitrides dis- sociate above this temperattu'e Carburization begins at 800 °C (1,470 °F) Oxidation above 1,200 °C (2,190 °F)

Fluorine: attack at normal temperatures

Chlorine: attack at 250 °C (480 °F) Bromine and iodine: attack at light red

heat Up to 600 °C (1,110 °F) no attack if free of oxygen

Oxidation to WO3 at high temperatures

Surface attack at red heat Oxidation at high temperatures Attack at red heat

Table 83. Physical properties of molybdenum

Property Unit Condition Values

]Mechanical data

Density

Hardness - ---

Tensile strength and elongation

Yield point--.

Hot tensile strength

Modulus of elasticity

g/cm^

Vickers (kg/sq mm).-

1,000 psi and percent .

1,000 psi

1,000 psi and percent1,000 psi.

Pressed, unsintered

Sintered Swaged . Sintered bar Swaged bar Annealed sheet Drawn wire:

1 mm (0.04 in.) diam

0.05 mm (0.002 in.) diam

Annealed wire:

1 mm (0.04 in.) diam

0.03 mm (0.0012 in.) diam..- - --. Single crystal Unannealed wire: 0.1 to 0.5 mm (0.004 to 0.02 in.) diam.

Wire 0.6 mm (0.024 in.) diam:

200 °C (390 °F)

400 °G (750 °F)

800 °C (1470 °F)

1200 °C (2,190 °F/

6.1 to 6.3 9.2 to 9.4

10.0 to 10.28 150 to 160

200 to 230

250 to 300

142 to 200 1 to 3 312 to 355 1 to 3

114 to 142 10 to 15

114 to 170 10 to 20 ~50 ~30

80 to 90 percent of the tensile strength.

114 to 142

85 to 100

71 to 85

28 to 43

47,800

4 to 5

4 to 5 4 to 5

5 to 6

Thermal properties

Melting point

Vapor pressure

Specific heat

Thermal conductivity

Linear coefl. of expansion

°C (°F) mm Hg

cal/g "C

cal/sec/cmVcm/°C

deg-'C -

2,630±40 (4,740±72 °F)

1,500 °C (2,730 °F)

1,800 °C (3,270 °F)

2,000 °C (3,630 °F)

20 °C (68 °F)

1,000 °C (1,830 °F)

1,400 °C (2,550 °F)

20 °C (68 °F)

1,000 °C (1,830 °F)

1,600 °C (2,910 °F)

20 °C (68 °F)

6.4X10-«

8.0X10-'

4.15X10-5

0.065

0.075

0.080

0.37

0.25

0.16 5.3X10-'

Electrical properties

Resistivity microhm-cm 20°C(68°F) 5 ohms/cmf. 30

1,000 °C (1,830 °F) 27

162

1,500 °C (2,730 °F) 43259

2,000 °C (3,630 °F) 60

361

31
Table 84. Chemical properties of molybdenur

Reagent

Hydrochloric acid, sulphuric

acid.

Nitric acid and aqua regia

Hydrofluoric acid Hydrofluoric acid-nitric acid mixtures.

Alkalies

Fused oxidizing salts (KNO3

KNO2 NajOa KCIO3).

Ammonia__

Carbon (lampblack, graphite

or charcoal).

Sulphur. - ,

Mercury liquid and vapor Phosphorus Boron

Silicon ZrOz, MgO, AI2O3, magnesite, chrome magnesite. Air or oxygen

Steam Hydrogen

Nitrogen

Nitrogen-hydrogen (cracked ammonia).

Nitrous oxide and nitric oxide.. Carbon dioxide Carbon monoxide

Hydrocarbons

Halogens

Hydrogen sulphide ..- Sulphur dioxide- --.

Behavior

Cold, dilute or concentrated: practically

resistant Warm, dilute or concentrated: slight attack

Cold, concentrated: slow attack Cold, dilute: more rapid attack Warm, dilute or concentrated: severe attack and complete solution Cold or warm: no reaction Severe attack, rapid solution

Cold aqueous caustic soda or potash: practically resistant. Fused caustic soda or potash or alkaline carbonates:

a) in air: slow oxidation

b) in the presence of oxidizing agents such as KNO3, KNO2, KCIO3,

Pb02: violent reaction, solution Violent reaction, solution

Aqueous solution: slow attack Gas: resistant up to red heat

Surface carburized above 1,100 °C (2,010 °F), completely carburi7ed above 1,300 to

1,400 °C (2,370 to 2,550 °F) No reaction up to 440 °C (825 °F), sulphides form at higher temperatures

Completely resistant No reaction even at high temperatures

Borides form at high temperatures

Silicides form at high temperatures The metal reacts with the pure oxides above 1,600 °C (2,900 °P)

Practically no reaction at room temperature. Oxidation above 400 °C (750 °F). Rapid

oxidation to M0O3 above 600 °C (1,110 °F) and sublimation Rapid oxidation at red heat No reaction below melting point

No reaction up to 2,400 °C (4,350 °F), for- mation of nitrides at higher temperatures

No reaction

Oxidation to M0O3 at red heat Oxidation above 1,000 °C (1,830 °F) Carburized above 1,000 °C (1,830 °F) Carburized above 1,000 °C (1,830 °F)

Fluorine: reaction at room temperature.

Chlorine and bromine: attack at red heat. Iodine: no reaction even when hot Formation of sulphide at 1,200 °C (2,190 "Fl Oxidation at red heat

denum forms a nitride at temperatures above

2,400 °C (4,352 °F). There is no reaction with

hydrogen. The choice of atmosphere in a protection tube containing these materials thus must

be made with care from the standpoints of both

the effect of the atmosphere on the thermocouple

materials and the effect on the insulating refractory.

In connection with the refractories usable in

the temperature regions where these materials

find application; aluminum oxide and magnesium

oxide are said to suffer a reduction to metal in

contact with tungsten above 1,900 °C (3,452 °F),

and thorium oxide is similarly affected at temperatures above 2,200 °C (3.992 °F). Molybdenum

is said to react with pure zirconium, magnesium,

and aluminum oxides at temperatures above 1,600

°C (2,912 °F). Here again caution must be exercised to select materials that are compatible.

The W-Mo thermocouple appears to have been

used in early attempts to make practical sensors

of the refractory metals. It has been used successfully in molten steel and still appears to find

favor in some areas. One reason for its acceptance

seems to be its ability to withstand reducing atmospheres, in spite of its sensitivity to some of

the gases listed in tables 82 and 84. Its disadvantages have been its brittleness, instability in

use, lack of thermoelectric reproducibility, low

thermal emf, and a polarity reversal at about

1,250 °C _ (2,282 °F). Improvement has been

achieved in some of these properties but the undesirable thermoelectric features remain. The

temperature-emf relationship for the W versus Mo thermocouple from 0 °C (32 °F) to 2,200 °C

(3,992 °F), as calculated from the equations of

reference [106], is given in table 85, and approximate values have been taken from the curve of

reference [97].

Table 85. Thermal emf of the tungsten versus molybdenum

thermocouple

a. Reference! 106]

Temperature Emf Temperature Emf

OQ op mv "C °F mv

0 32 0 1, 200 2, 192 -0.98

200 392 -0. 85 1, 400 2, 552 +0.06

400 752 -1. 14 1, 600 2, 912 1.42 600 1,112 -1.36 1,800 3,272 2.88 800 1.472 -1.54 2, 000 3,632 4. 10

1, 000 1, 832 -1.46 2,200 3,992 4.94

b. Reference [97]

Temper- Emf Temper- Emf

ature ature

"F mv °F mv

32 0 2,400 -0. 22 400 -1.00 2,800 +1.30

800 -1.70 3, 200 +2.95

1,200 -1.99 3, 600 +4. 60 1,600 -1.81 4, 000 +6.38

2, 000 -1.23

The emf of the W versus Mo thermocouple

derived from the Battelle equations [106] for W

versus Re and Mo versus Re is lower than that

found generally, even though the Battelle data

for these two thermocouples are in reasonable

agreement with those of most other investigators.

On the whole, the data of Lachman and McGurty

[97] seem to be more nearly representative of most

of the calibrations found of the W versus Mo

thermocouple.

The approximate upper temperature limit for

the W versus Mo thermocouple is given in refer- ence [107] as 2,400 °C (4,352 °F), which is more

than 200 °C below the melting point of molybdenum (see table 83).

Rhenium until recently appears to have been

largely a scarce curiosity with little practical use. According to reference [109], however, "Extensive

work has recently been reported on the metal

rhenium, both in this country and abroad [110,

111]. Rhenium is a strong, ductile, refractory

metal with a melting point of 3,180 °C (5,756 °F).

It has a high electrical resistivity, hexagonal

close-packed crystal structure, and some chemical

and metallurgical properties characteristic of the

precious metals. In England, extremely interesting exploratory work on tungsten- and molybdenum-base alloys containing about 35 atomic

percent rhenium has shown these materials to

32
have exceptional ductility [112]." Rhenium thus

appears to have desirable properties itself, and

also to have the capability of imparting these

properties to its alloys with tungsten and with

molybdenum. Thermocouples of tungsten and

rhenium, and of alloys of tungsten and rhenium

appear to be gaining favor, and it seems probable

that one of these thermocouples will be selected

for most general use at the upper temperatures

as an immersed sensor. Some properties of unalloyed rhenium follow.

Indications of the purity of available metal are seen in table 86 of analyses of rhenium used in

the work of reference [106]. Analyses of tungsten

and molybdenum are included.

Table 86. Analyses of rhenium, molybdenum, and tungsten

[Impurity content,' weight percent]

Element

AlC

Ca

CrCu

FeK

MgMn

Mo

Na

Ni

O

SiW

Rhenium

Powder Strip

<0.01

<0. 01ND

<0.01

0. 0008 ND

<0. 010 ND

ND

ND

<0.01

0. 010

0. 005

"o.'ooB"

0.005

0. 007

Molybdenum

Powder

0. 012

.005 .002

0. 002

'o.'ooi"

0. 005

.010 <.01 <.01

Tungsten

Powder

<0. 001 .002 <.001

<.001

<.001

<.001

.001 <.001

<.001

.004

<.001

<.001

.031 <.001

» ND designates not detected in spectrographio analysis.

Impurities in the rhenium are seen to total

about 0.05 percent or less. No statement of

homogeneity of rhenium wire is available, but

according to reference [106] "good reproducibility

from run to run (of W versus Re and Mo versus Re combinations) was obtained, and the average

curves (thermal emf) obtained were in excellent

agreement with the data obtained from other

laboratories."

Some mechanical properties of rhenium sheet

are given in table 87 [108].

Table 87. Tensile and elongation data for annealed and

cold-rolled rhenium sheet

Annealed Reduced

12.9% Reduced24.7% Reduced30.7%

Rolled thickness _--mils-_ Proportional limit psi-.

0.1 percent offset yield strength__psi..

0.2 percent oflset yield strength- .psi,

.

Ultimate tensile strength psi.. Elongation in 1 in percent Reduction of area ... percent

10.1

31, 700

131,000

135,000

168, 00028

30

8.8

25, 200

233, 000

245, 000

2.50, 000

8

24

7.6

42, 200

279, 000

298. 000

307, 000

2

1

7.0

159, 000

282, 000 311,000

322, 000

2

1

Rhenium wire containing 0.5 percent thoria

prepared for electronic studies [108] was drawn

down to a diameter of 2.9 mils. Below a diameter

of 15 mils, two 10 percent reductions could be

taken between anneals, and the authors believe

that finer sizes could be prepared. No annealing

temperature is given for this particular operation,

but elsewhere annealing at temperatures of 1,700

°C (3,092 °F) and 1,800 °C (3,272 °F) for periods

up to 2 hrs is mentioned.

The modulus of elasticity of rhenium varies

almost linearly from about 67.5X10^ psi at room

temperature to 54X10« psi at 880 °C (1,616 °F).

The electrical resistivitj^ of rhenium containing

copper, tin, aluminum, silicon, magnesium, calcium, and molybdenum to a total of 0.0585 percent and gold given as O.OX percent is given in

table 88.

Table 88. Electrical resistivity of rhenium from 20 to 2,400 °C

Temperature Resistivity Temperature Resistivity

°C "F ohmslcmf °C °F ohmslcmf

20 68 116 1,400 2, 552 541 200 392 197 1,600 2, 912 578 400 752 280 1,800 3, 272 608

600 1, 112 349 2, 000 3,632 632

800 1, 472 403 2, 200 3, 992 650 1.000 1,832 460 2,400 4, 352 662

1,200 2, 192 505

Approximate values of the specific heat of

rhenium, taken from a curve of reference [108]> are given in table 89. The discontinuity at

1,500 °K is a result of extension of the determinations upward in the work of reference [108] from

the earlier determinations of Jaeger and Rosenbohm [110] that covered the temperature range

up to only 1,473 °K.

Table 89. Specific heat of rhenium

Temperature

Specific heat Temperature

Specific heat

°K

300

600

900

1, 200

1, 500

callg "K

0. 0325

0. 0348

0. 0367

0. 0383

0. 0400

°K

1, 800

2, 100 2,400

2,700

callg °K

0. 0432

0. 0457

0. 0483

0. 0503

The density of rhenium, from reference [110],

is 21.04 g/cm^

The linear thermal expansion of rhenium containing about 0.2 percent impurities, from refer- ence [110], is given in table 90.

Table 90. Linear thermal expansion of rhenium

Temperature liiha 'c Temperature Itllm °c

°C "F °C °F

20 68 1. 00000 600 1, 112 1.00396

100 212 1. 00066 700 1,292 1.00462 200 392 1. 00132 800 1,472 1.00528 300 572 1. 00198 900 1,652 1. 00594' 400 752 1. 00264 1,000 1,832 1. 00660

500 932 1. 00330

No information on the thermal conductivity of

rhenium seems to be available for temperatiu-es

above about 175 °K.

The spectral emissivity of rhenium, attributed

in reference [113] to D. T. F. Marple, is given for

two temperatures in table 91.

33
Table 91. Spectral emissivity of rhenium

Tempera- ture

Emissivity, percent at wavelength (microns) of

0.3 0.4 0.6 0.8 1.2 1.6 2.0 2.4 2.8

"R

3,560.__

5,480.._ 42.542.5 42.542.5

41.0

41.0 39.039.0 32.534.0

27.5 30.5

25.0

28.5

22.5

27.0 22.025.5

Table 93. Thermal emfs of W versus Re and Mo versus Re thermocouples [122]

Table 92. Thermal emfs of W versus Tie. and Mo versus Re thermocouples [106]

Temperature

Emf

Temperature

Emf

W-Re Mo-Re W-Re Mo-Re

"C °F mv mv "C op mi) mv

0 32 0 0 1, 200 2, 192 18. 94 19. 92

100 212 0. 55 1. 10 1,300 2, 372 20. 65 21.15 200 392 1.59 2. 44 1,400 2, 552 22.22 22. 16 300 572 2. 96 3. 98 1,500 2,732 23. 66 22. 95 400 752 4. 55 5. 69 1,600 2, 912 24. 96 23. 54

500 932 6. 27 7. 52 1,700 3,092 26. 09 23.93

600 1, 112 8. 06 9. 43 1,800 3, 272 27. 05 24.18

700 1,292 9.90 11.37 1,900 3, 452 27.86 24. 33

800 1,472 11.75 13. 29 2, 000 3, 632 28.56 24. 46

900 1,652 13. 60 15. 14 2, 100 3, 812 29. 15 24. 66

1,000 1,832 15. 43 16. 89 2,200 3,992 30. 00 25. 05

1, 100 2,012 17.23 18.49

Ttie variation of emissivity at 0.665/n with

temperature, credited in reference [113] to Sims,

Craighead, Jaffe, et al., shows in one case that the

emissivity is constant at 40 percent over the range

from 500 to 5,500 °K,. In another set of determinations the emissivity is about 43.5 percent at

500 °R, remains practically constant up to about

1,700°, and drops off to about 42 percent at 3,000

°R. From here it decreases almost linearly to

about 37 percent at 5,000 °R and somewhat more

slowly to a little' over 36 percent at 5,500 °R.

The temperature-emf relationships for the W

versus Re and Mo versus Re thermocouples, as calculated from the equations of reference [106], are given in table 92. Data for the same thermocouples, from reference [122], are given in table 93.

The thermoelectric powers of both thermocouples are seen to decrease in the upper part of

the range. (The apparent increase between 2,100

and 2,200 °C in the table is as calculated, but is not confirmed experimentally.) Various combinations of timgsten and tungsten-rhenium alloys

have been investigated in attempts to find sensors that do not exhibit the flattening of the thermal

Temperature

Emf

Temperature

Emf

W-Re Mo-Re W-Re

"if" mv mv "F mv mv

32 0 0 2,200 18. 99 18. 59 200 0.70 1. 00 2, 400 20. 72 19! 70 400 1. 91 2.60 2,600 22.38 20.67

600 3. 47 4. 45 2,800 23. 96 21.89

800 5.24 6.44 3,000 25.41 22.23

1, 000 7.15 8. 42 3 200 26.64

1, 200 9. 19 10. 41 3! 400 27. 58

1, 400 11.22 12. 31 3,600 28. 32

1, 6C0 13.26 14. 12 3,800 28.92

1,800 15. 24 15. 76 4,000 29. 21

2, 000 17. 17 17.26

Table 94. Calibration data for tungsten versus rheniumalloy thermocouples, and for thermocouples of two tungstenrhenium alloys

W/W-20% W/W-26% W-3% W-5%

Temperature Ke Re Ke/ W-2Uyo Ke/ vV-26% Re Re

"C °F emf, mv emf, mv emf, mv emf, mv

93 200 0.143 0.310 0.763 1.139

204 400 0. 634 1. 110 2. 040 2.864 316 600 1.460 2. 152 3.546 4.596

427 800 2.625 3.660 5. 215 6.682

538 1,000 3.989 5. 482 6.830 8. 760

649 1,200 5. 530 7. 450 8.565 10. 888

760 1,400 7. 248 9.395 10. 255 12. 988

871 1,600 9. 009 11.601 11.980 14.992

982 1,800 10. 875 14. 030 13. 710 16. 939

1,093 2, 000 12. 690 16. 535 15. 420 18. 860

1.204 2, 200 14. .155 19. 052 17. 075 20. 716

1, 316 2, 400 16. 260 21. 395 18. 660 22. 484

1,427 2,610 18. 010 23. 705 20. 290 24. 235

1, 538 2,800 19. 795 25. 675 21.810 25. 898

1,649 3, 000 21.50 27. 87 23. 29 27.63

1,760 3, 200 23.23 30. 14 24.68 29. 13 1,871 3, 400 24.97 32. 28 26. 16 30. 58

1,982 3, 690 26.60 34.20 27. 28 32. 13 2,093 3,800 28. 12 36.12 28. 58 33. 21 2,204 4,000 29. 75 37. 83 29. 71 34.40

2,316 4, 200 31.05 39.64 30. 77 35.49

2,427 4,400 32. 20 41.25 31.69 36.42

2,538 4, 600 33. 45 42. 56 32. 61 37. 31 2,649 4, 800 34. 65 43. 99 33. 55 38. 00 2,760 5,000 35. 60 44. 79 34.38 38. 45

2,871 5,200 36.50 35. 15

emf versus temperature relationship that limits

the usefulness of the W versus Re and Mo versus Re thermocouples. Results of work on these

materials, taken directly from reference [97], are presented in table 94.

Compositions of the thermocouple materials

used in the work of Lachman and McGurty,

taken from reference [97], are given in table 95.

Table 95. Chemical analyses of tungsten-rhenium alloys and tungsten, from reference [97]

Composition

wt%

W-1% Re W-3% Re W-5% Re W-10% Re.... W-20% Re.... W-26% Re.... Tungsten

Chemical analysis

W

wt%

98.12 96.26

93.98

89.68

82. 22 75.32

Re

wt%

1.22 3.24

5. 35 9.98

17.21

23. 85

Mo

wt7o

0. 005

.005

.004 .005

.005

.04 <.001

Fe

wt%

0.003

.002

.004

.005

.005

.01 .001

Cu

wt %

0. 001

.001 .002

.001

.002

<. 001

Al

wt%

0. 01 .006

.001

.002

.004

.001

Si

wt%

0. 001

.003 <.001

Mg

wt%

0.001

"<r6o5"

Ni

wt%

0.004

.005

wt%

0.123

.048

.079

.044

.133

.010

ppm

-186 138

211

173 390

75

ppm

<10

<10

<10

<10

<10

34
From the data of table 94 it appears that the

W versus W-Re 26 percent thermocouple is the

optimum from the standpoints of thermoelectric

power and near-linearity up to about 5,000° F.

Some anomalies are seen in each of the calibrations that could cause complications in the use of

any of these thermocouples. The reasons for

these irregularities is not known; they may be

definite properties of the thermocouples, or they

may be a consequence of the difficulties inherent

in calibrating at the high temperatiu-es covered.

The conclusion of Lachman and McGurty [97] on

their work on rhenium alloys in thermocouples is

. . the tungsten/tungsten-26 percent rhenium

thermocouple offers a vast improvement over the

pure tungsten/rhenium type so far as high temperatme capabilities (in the 5,000 °F range) and

accuracy are concerned."

Tantalum has been considered for use in thermocouples, either unalloyed or alloyed. It does,

however, in the unalloyed form exhibit some irregularities in its behavior above 1,200 °C when

coupled with rhenium, or with any other material

that gives a smooth temperature-emf relationship

with rhenium. According to Steven, page 363 of

reference, [107] "The metals tantalum and niobium fell by the wayside because of their great

affinity for residual gas molecules at elevated

temperatures and the shift in electromotive calibration resulting from this 'getter' action." Some

properties of tantalum along with those of other

refractory metals are given by Steven in the same

reference.

The tungsten versus iridium thermocouple [99]

gives a smooth and probably the most nearly linear

temperature-emf relationship of any of the hightemperature thermocouples. One of its obvious

advantages is its high thermoelectric output, but

because of the iridium leg, its use is limited to a maximum temperature of about 2,200 °C (3,992

°F). Although iridium is more resistant to oxidizing conditions than is tungsten, it does not

survive long exposure near the upper useful limit.

Unlike tungsten, it is not at its best under some

reducing conditions, and so caution should be

exercised that the uses to which the tungsten

versus iridium thermocouple is put are compatible

with good service.

Troy and Steven [99] give the thermoelectric

power of this thermocouple as 22.9 fiv per degree

at 1,000 °C, 25.0 at 1,600 °C, and 26.3 nv per

degree at 2,000 °C. Their calibration is given in

table 96.

•Table 96. Thermal emf of the tungsten versus iridium

thermocouple

Temperature Emf Temperature Emf

"C op mv °C mv

1,000 1,832 14. 25 1,600 2,912 28.62

1, 100 2,012 16.56 1, 700 3, 092 31.12

1,200 2,192 18. 91 1, 800 3,272 33.69

1,300 2,372 21.3 1,900 3, 452 36.28

1,400 2, 552 23.70 2,000 3,632 38.88

1,500 2,732 26.15

2.4. Carbon and Carbides

Carbon and carbides for many years have been

subjects of investigations to avoid problems

inherent in the use at high temperatures of thermocouples of metals and alloys. Thermocouples of

this kind have not yet, from the number in use, assumed an important position in the temperature

measuring field; but some are used now, and the

possibility exists that more will be used in the near

future. Only a brief mention of the several types

of these thermocouples is made here ; the information has been taken chiefly from reference [114].

According to Thielke and Shepard [114] they

have had little success with graphite versus carbon

thermocouples. Their sensor of this type started

off at room temperatiu-e with a thermoelectric

power of 8 nv per degree C (4.4 nv/°F) , and reached

a maximum thermal emf of 7 mv at 1,600 °C.

They do, however, mention the work of Ubbelhode, Blackman and Dundas [115], who have

reported a thermocouple based on the difference

in thermoelectric power between graphite of different crystal orientation. The following quotation is from reference [114].

"They report work on single crystals of graphite where

the limiting value of thermoelectric power in the a-axis

direction is about —3 a'v/°C and the limiting value in the

c-axis direction is +1.86 mv/°C at 20 °C. They (Ubblehode, et al.) formed a couple from materials having the

greatest practical difference in orientation, one element

being a pyrolytic filament and the other (evidently) a pressed natural graphite. The couple has an output of

about 35 mv at 2,400 °C, with a sensitivity at room temperature of about 25 /tv/°C. This is too great to be

accounted for by the algebraic sum of the a-axis and c- axis thermoelectric powers (about 5 /iv/°C), but as they

note, 'the thermoelectric power of various specimens . . . depends on various defects in the carbon networks in graphite, as well as on the crystal orientations'. Even

though the interpretation of the nature of the couple may

be difficult, its output is substantial and its value as an

instrument is very appealing."

Another approach to the effect of crystalline

perfection was, according to Thielke and Shepard

based on the work of Bidwell [116]. The effect of

baking temperature on the thermoelectric power

of graphite was determined, and it was found that

one rod baked to 2,000 °C (3,632 °F) and an identical rod baked to 3,000 °C (5,432 °F) produced an

output of 25 mv at 1,000 °C (1,832 °F) that was

linear from about 200 °C (392 °F) to 1000 °C.

They say that this thermocouple might be useful

up to 2000 °C, but that use above this temperature

would alter the characteristics of the rod baked at

2,000 °C and thus the calibration of the thermocouple.

The most successful combination of carbon

with metals is said in reference [114} to be that of

tungsten and graphite, described by Watson and

Abrams [117], who gave a calibration of this

thermocouple to 1,600 °C (2,912 °F) and reported

the favorable use of it to 2,400 °C (4,352 °F) in a reducing atmosphere and repeated use in vacuum

at 1,850 °C (3,352 °F). Stevens points out in

35
reference [20], pp. 357-376, that this thermocouple is sensitive to moisture content in the

graphite leg, but that the calibration can be re- stored by baking. The maximvmi emf is reported

as 45 mv at 2,100 °C (3,812 °F), although the

upper temperature is said to be 2,400 °C (4,352 °F).

The carbon vs silicon carbide thermocouple

announced by Fitterer in 1933 [118, 119] had an

output of 508 mv at 1,700 °C (3,092 °F) with,

according to reference [114], possible use up to

2,700_ °C (4,892 °F). The maximum thermoelectric power was about 300 nv per degree C

(167 /iv/°F), a colossal figure when compared

with outputs of conventional thermocouples.

Fitterer also reported on thermocouples of C and

MoC giving an output of 16 mv at 1,050 °C

(1,922 °F), and on one of C versus WC that gave

an output of 10.5 mv at 860 °C (1,580 °F). Ridgway [120] received a patent in 1939 on a thermocouple of B4C and C having an output of 600 mv

at 2,400 °C, which is comparable to that of the C

versus SiC thermocouple of Fitterer. Its thermoelectric power is 330 hy per degree C (184 )uv/°F) over the range 500° to 1,700 °C.

Graphite versus doped graphite thermocouples,

have, according to Thielke and Shepard [114], been

used successfully and appear to have possibilities of

continued and expanded applications. The doped

element which they describe contains about 1 percent boron in graphite [120]. This thermocouple

was calibrated to 3,100 °C (5,612 °F) and showed

"a continuously increasing output to that temperature with an output of about 90 millivolts at

2000 °C (3,632 °F) and a sensitivity of 50 microvolts per °C (28mv/°F) between 500° and 1,200 °C

(932° and 2,192 °F) . . . The same authors

also say that "the choice of a heavily doped borongraphite element is based on two considerations:

first, small losses of boron from a heavily doped

element have little effect on the room temperature

thermoelectric power of the graphite; second, the

thermoelectric power of the heavily doped graphite

maintains a high value at high temperatures,

whereas the value for low concentrations of boron

drops off with increasing temperature."

The behavior of the graphite versus boron-doped

graphite thermocouple from field tests has shown,

according to reference [114]:

"(a) Stable operation of the boron/graphite couples

in a vacuum crystal growing furnace for at least 300 hr at 1,400 °C, compared to a maximum of 36 hr using Pt/Pt-Rh.

"(b) Continuous operation of the couples in a refrac- tory-producing furnace operating at 1,800° to 1,900 °C.

"(c) Use of the couples in a high pressure crystal

growing furnace with no failure after 60 heating cycles of 6 hr each to temperatures between 1,500° and 2,000 °C.

"(d) Successful use of the couples as immersion pyrometers for steel melts, providing accuracy to within 10 °C

and response times as low as four seconds."

The usefulness of all of the thermocouples of

carbon and carbides has been limited by the

bulkiness, fragility, and questionable reproducibility. The possibility that fabrication of carbides

may be developed to the point where reproducibility is improved is visioned in reference [114],

and small thermocouples of graphite and borondoped graphite are mentioned. In addition,

pyrolytic filaments of 0.020-in. and 0.0008-in.

diameters have been treated to form graphite vs. doped graphite thermocouples. Further, the

development of graphite cloth and yarn at the

National Carbon Company is said to provide

material from which these thermocouples can be

made in flexible form. It appears that by these

latter two means flexible thermocouples can be

made that will give fast response and freedom

from the limitations imposed by the use of the

bulky, fragile, larger thermocouples of this kind

that have been considered.

Properties of various carbon and carbide

thermocouples are summarized in table 97,

taken from reference [114].

Table 97. Thermocouples using carbon and its modifications

Couple

Maximum

temperatui-e Maximum emf Maximum sensitivity Notes—accuracy, interchangeability, resistance

W/Qr

C/Ni

C/Qr

Gr/Or (BT)

Gr/Gr (a. versus c.) BGr/Gr

B4C/C

Gr/SiC

Gr/MoC

Gr/WC

2, 400

1, 623

1,250 2,000

1.400

2, 400

1,900

2.500

2.600

2, 400

2, 700

1,050 850

mv

45(2,100 °C)

36.22(1,623 "C)-.

0.049(1,900 "C)--

0.04(1,400 °C)

7(1,600 "C)

36(1,900 "C)

34(2,300 °C)

112(2,700 °C)

650(2,400 °C)

508(1,700 °C).._.

16(1,050 °C)

10 J^(850 °C)

29(800 "C)

22(1,500-1,700 °C)_

0.02

8(700-1,000 °C)

27 3^(300-1,000 °C)

20(400 °C)

62(900 °C)

330(500-1,700 °C)_.

300(1,200-1,700 °C)

16(300-1,100 °C)

16(400-900 "C)

'A ohm; ±10 °C below 1,600°; 5 percent inter- changeability.

±50 °C

2,000° baked stock versus 3,000° baked; ±10 °C

accuracy

c-axis graphite versus a-axis graphite; ±20 °C

±10° accuracy; ±35° int?rchangeability; H

ohm resistaiice

5 ohm; ±10 °C accuracy; 1 percent interchangeability

36
3. Ceramic-Packed Thermocouple Stock ^

The ceramic-packed thermocouples discussed

here comprise thermocouple wires packed tightly

in ceramic insulating material enclosed in a

j metal tube. This means of insulation for use at

I high temperatures has been used for a long time

in resistance heating elements, but it is only

' since the arrival on the scene of jet engines and

gas turbines that it has been applied so widely

to thermocouples. Although initial applications were in the turbine-type engines, the versatility

and other desirable properties of the thermocouples

are so attractive that they now are accepted

throughout industry and technology. The purpose here is, because of the widespread interest in

this type of thermocouple stock, to point out

briefly some of its properties that are of interest

in applications.

The general versatility of the ceramic-packed

wire is described in catalogs of some suppliers

of this material. When quotations are given

here from such sources the phrase "ceramicpacked" is substituted for brand or company

names. One statement [121] on the versatility of

this material follows.

"The characteristics of ceramic-packed wire make it ideal for a wide variety of applications. Flexibility is one

of these highly desirable qualities; ceramic-packed wire

can be formed to a radius equal to the sheath diameter

without loss of insulation. It will fit any contour, yet

remain rigid after bending; it can be cut to short lengths

and connectors attached. It may be imbedded in concrete,

plaster, or similar materials or buried underground."

No mention is made in this statement about the

rigorous conditions under which the packed

stock must and does perform when it is subjected

to the temperatures,' vibrations, and atmospheres

encountered in aircraft engine use.

3.1. Thermocouple Wires

All of the conventional thermocouple wires

are available in the ceramic-packed stock, and

some makers indicate that they will supply any

suitable wires in such material. The most

popular thermocouples so mounted today are, however, the type K and iron versus constantan.

The range of wire sizes found in catalogs is. from

a diameter of about 0.001 in. to B & S 12-gage

(0.081 in.). Stock containing one or two wires

is generally available, and some with multiple

wires are in the catalogs.

Several types of measuring junctions are

available. The most common types of bare

measuring junctions are the V, the U, the loop,

and variations of these to meet special requirements. Totally enclosed junctions that protect

the wires from physical or chemical damage fall

' Most of the information available on the ceramic-packed stock is from

catalog or advertising literature, and consequently practically all given here

is from such sources. No implication is intended that materials from industrial organizations whose literature is cited are superior to those of others, and

no endorsement of any product over any other is implied herein.

generally into two styles, one in which the

thermocouple is grounded to the closed end of

the sheath, and the other in which it is not

grounded. Another enclosed-j unction type [123,

124] that uses single-wire stock provides a buttwelded measuring junction in a continuous

unwelded sheath. This type assembly can be

used as a straight-through probe or can be formed

into a loop.

Another type of junction that is used more in

special applications is one in which a thin metallic

film is used to form a surface junction across the square-cut end of coaxial stock. [125] The

sheath serves in this t5'pe as one leg of the thermocouple, and the single, coaxial wire serves as the other. The metallic film is vacuum deposited

to a thickness of about one micron. One of the

advantages claimed for this type of junction is fast response to a temperature change.

Additional types, mentioned in reference, [124] are the differential and the pencil or tapered in

which the closed end at the measuring junction

•is tapered like a pencil point to give less mass

and consequent faster response to a temperature

change.

Various types of connections and connecting

heads are available, but these are outside of the

scope of this paper and are not discussed here.

Makers of the sheathed stock normally claim

that the limits of error for the thermocouple

types listed in reference [1] are within the ISA

tolerances, and in most cases give their own

estimates of limits of error for thermocouples not

listed in reference [1]. The types listed by ISA

are often said to be carried in stock, and the

less common thermocouples are made up on order.

The ceramic-packed stock is available in outer

diameters ranging from 0.010 in. to about in.

The recommended minimum radius of bending

difl^ers among the suppliers, but the range found

is from one to four diameters. Some makers

suggest that the stock be heated to make the

sharper bends.

Information from three suppliers [124,126,121]

on dimensions and wire sizes of available stock is given in table 98. The first two tables are presumably for two-wire stock and the third is for

stock with up to four wires.

Inconel and stainless steels seem to be the most

popular sheath materials, but a large assortment

is offered by the various makers. A composite

list of the various materials said to be available

from several makers is given in table 99 together

with some properties. Some of the information

is from reference [126], and advantage has been

taken of the thorough survey by Anderson and

IVIacKenzie [127] for the most of the remainder

of the data given. Values from tlie latter work

are chiefly scaled from charts, and so must be

considered approximate.

37
Table 98. Dimensions and wire sizes of available ceramicpacked stock

(a)

Sheath Standard Maximum Maxlum

diameter wire wire length

,in. B & S gage B & b gage ft

0. 010 50 46 25 .020 38 38 150

. 025 36 34 150

. 032 35 34 150

. 040 34 32 200

.062 29 28 200

. 091 26 26 100

.114 25 25 100

.125 24 22 100

.187 20 18 60

.250 18 16 60

.312 16 14 60

.375 14 14 40

.437 14 12 40

.500 12 12 25

(b)

Sheath outside diameterOutside diameter

tolerance

Nominal

wall thick- ness

Approximate

wire Maximumlength

in. ±in. in. B & S gage ft 0.020 0.001 0.003 38 250

.032 .001 .005 36 250

.040 .001 .007 34 250 .062 .002 .010 29 150 .090 .002 .014 26 125

.125 .002 .018 24 100

.188 .003 .025 18 60

.2.50 .003 .032 17 40 .313 .003 .040 16 40 .375 .003 .049 14 30

.430 .003 .065 13 30

.500 .003 .065 12 30

(c)

Sheath

diameter

Sheath

wall thickness

Nominal conductor diameters

l-wire 2-wire 3-wire 4-wire

in. in. in. in. in. in. 0.313 0.064 0.051 0. 040 0. 040

.250 .051 .040 .032 .032

.188 .040 .032 .022 .022

.125 o'oii .032 .022 .011 .011 .062 .010 .022 .011 .006 .006 .040 .007 .011 .006 .025 .005 .006 .004

Of the refractories available for use in the

ceramic-packed thermocouples magnesium oxide

appears to have been and to continue to be the

most popular. Aluminum oxide is, however,

gaining favor; and beiyllium, zirconium, and

thorium oxides may be had on special order.

The quality of and some thermal information on

these refractory insulating materials is given in

table 100, taken from reference [121].

The average coefficient of thermal expansion

from room temperature to 700 °C of the refractory

insulating materials, scaled from a chart of

reference, [127] is given in table 101. Values of

copper, stainless steel, and aluminum are included

for comparison.

Resistivities of some ceramic insulating materials, taken from Anderson and MacKenzie's

chart, are given in table 102. These data as well as

Table 99. Sheath materials for ceramic-packed thermocouple stock and some of their properties

Material Melting

point

Rec- ommaxi- miun

in air

Recommended Tensile strength ^

Operating

atmosphere «

Contin- uous

maxi- mum

tempera- ture

at 200 opat 1,600

Stainless steel 304 _ °F

2, 560 "F

1,920 ORNV

ORNV

ORNV

ORNV

ORNV

ORNV

ORNV

ORNV

ORNV

ONVo

ONV«

"F'

1,650

2,000

2, 100 1,700

1,600

1,600

1, 650

1,200 2,000

2, 100 2,200

psi 68,000

psi

2, 560 2,280

2,580

2,600

1,960

1, 760

1,500

1,680

87,000

75,000

70,000

75,000

23,000 23,000

17,000

316

OZi

An'i

446

2, 550

2, 620

2,500 2,650

2, 440

2,420

2,400

2, 240

2, 460

2,200

1,980

1,850

1,220

2, 450

2, 550

2, 550

2,647

2,791 3,090

3, 216

2,000

1,500

1,640

2, 400 1,820

1,820

1,620

1,400 1,640

93,000

150,000

77, 000

106,000

136, 000

147, 000

97, 000

125,000

6,000

11,000 3,000

7,000

64,000

13,000

54, 000

51,000

Hastelloy C

Haynes 25

xiasteiioy r

riasceiioy a

Nionel ...

600

700 800

400 2,000

2, 400

1,500

1,000

1,900

3, 000

ObRNV 600 Brass Aluminum ORNV 700 Beryllium _„ _ _ __ Nichrome

AlumeL- —

Nickel

Iron-,

Zircalloy,. _ _ Platinum ON^^ 3,000

Niobium 4, 379

4, 750

4, 750

4, 750

5,440

3, 300

6, 850

1,600

1, 000

3,680

3, 400

750

600

1,000

VN

VNR

3,800 no 000 Molybdenum 137, 000 30,000 Molybdenum

,

disilicized 4,400 Molybdenum,

chromalized .

Tantalum,.- V

VN

5,000

2 000 96,000 22,000 Titanium.

Graphite,

» This column is from reference [126]; symbols describing atmospheres are: 0—Oxidizing; R—Reducing; N—Neutral; V—Vacuum,

b Scales readily in oxidizing atmosphere,

° Very sensitive to sulfur corrosion.

"i After exposure to temperature of 100 hrs except for stainless steels, Haynes

25, W, Mo, Ta, and Nb.

Table 100. Quality of and some thermal data on insulating

materials used in ceramic-packed thermocouple stock

Mini- Melting Usable Maximum equivaInsulator mum point tempera- lent boron

purity ture content

percent °F 'F ppm

Magnesia (MgO) 99.1 5,050 4,000 39 Alumina (AI2O3) 99.5 3, 650 2, 300 30 Zirconia (Zr02),._ , , 99.4 4,500 1,200 200 ppm of HFj

Beryllia (BeO) 99.8 4, 550 4,000 10 Thoria (Th02) 99.5 5,950 5,000 10

all data on thermal properties of such refractories are affected by the physical and chemical composition and history of the samples of materials

studied; and so the approximate nature of such

information must be kept in mind.

When used in packed stock, the resistance between wires and sheath of magnesia is said in

reference 128 to be "upwards of 2000 megohms

38
Table 101. Comparison of the average coefficients of thermal expansion of refractory insulating materials

with those of three common metals

Coefficient of Material expansion X

106 a

per °C

Copper- 16.5

Stainless steels 13.9 to 16.4 Aluminum 9.6 Magnesium oxide. . 12.9 Beryllium oxide 8.1 Aluminum oxide. 7. 1 to 8. 0 Zirconium oxide 4. 2 to 5. 2

• Average coefficient from room temperature to 700 °C.

I per foot of length at 68F temperature." Reference

I [123] gives "Minimum insulation resistance wire

to wire, or wire to sheath is 1.5 megohms at 500

volts D.C. in sizes .062" diameter and larger for

lengths to 30 feet."

Thermal conductivities of alumina, magnesia,

and beryllia are, among refractories, relatively

high. Actual values would have no significance

here though, because not enough data on the

physical properties such as density, grain size,

degree of sintering, etc. are available.

Pressure tightness and porosity are properties

of the made-up stock for which no generally

acceptable criteria have yet evolved. Some

Table 102. Flectrical resistivities of ceramic insulating

materials

Material

Resistivity, ohm-cm, at temperatures, °C, of

900 1,000 1,100 1,200 1,300 1,400

Alumina,

100%

Alumina,

99%

Beryllia

Magnesia

1.3X108

7. 3X10»

3. 4X10«

1.7X10'

1.8X10'

2. 0X10«

4. 5X10»

3. 4X10'

4.6X10'

6. 7X10*

1.2X10' 8.5X10*

1.5X10'

2.2X10*

4.7X10*

2. 6X10*

4. 7X10'

6.4X103

2.1X10' 6.7X103

2.5X10'

1.4X103

1.0X10* 6.4X10^

suppliers advertise that their assemblies will stand

high pressures; and they will, in the sense that the

stock is not destroyed or damaged under these

conditions. "Gas-tightness" of the packed ceramic

is difficult to attain, however, and this is recognized

by some suppliers who carry special sealing materials to apply to either the hot or cold end to

attain tightness.

Porosity which is related to "gas-tightness" is of importance in uses where substances may permeate the ceramic packing and produce undesirable effects such as partially short-circuiting the

thermocouple wires or by leaving corrosive agents

in contact with the wires or sheath. This too is alleviated to some extent by sealers.

4. Conclusion

The information presented herein has been

obtained from various sources, and is believed to

provide a fairly complete coverage of the field.

Obviously more iijiformation could have been

given, but time and space set limitations that

conld not be ignored. Unfortunately, some sources

of information appeared too late to be used to

advantage here, but they are listed in the bibliography, and probably are available to those

interested.

The search for data to include in this paper has

revealed a paucity of information on some materials, particularly several just now coming into

use. More knowledge concerning the properties

of these materials is needed in order to utilize

them to best advantage in thermocouple

thermometry.

Acknowledgment is made to G. F. Blackburn

and C. Halpern for their assistance in reviewing

this paper, to Mr. Halpern also for his support in

conducting the necessary library research, and to

Mrs. B. De Wane for her patience in typing and

retyping the manuscript and for arranging it in its

proper form. Although the list of those who contributed material for this paper in answer to a

questionnaire cannot be given, their cooperation

and generosity nevertheless are highly appreciated

as contributing to a more nearly complete coverage

of the field.

5. References

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Recommended Practice for Thermocouple and Extension Wires, ISA-RP 1.1-.7 (July, 1959).

[2] W. F. Roeser, F. R. Caldwell, H. T. Wensel; The

freezing point of platinum, BS J. Research, RP.326,

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[3] W. E. Forsythe, Editor, Smithsonian Physical Tables,

Ninth Revised Edition, Smithsonian Institution

(1954).

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[6] W. F. Roeser and S. T. Lonberger, Methods of testing

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Circular 590 (Supersedes RP768), (1958).

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39
[10] T. Lyman, Editor, Metals Handbook, 1948 ed., The

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[39] C. R. Droms and A. I. Dahl, Iridium versus iridiumrhodium thermocouples for gas temperature meas- urements up to 3500°F, Proceedings of the Joint

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40
[541 M. E. Ihnat, The General Electric Company, A jet

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through June 30, 1959, ORNL-2773, UC-37-

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exact measurement of the specific heat and other

physical properties of solid substances at high

temperatures. XI. The specific heat, electrical

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expansion of electrolytic iron, Recueil Trav. Chim.

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five materials, Battelle Memorial Institute, WADC

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[83] W. H. McAdams, Heat Transmission (3d ed.,

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[84] H. B. Wahlin and H. W. Knop, Jr., The spectral

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[85] P. F. McDermott, An apparatus for the measurement

of the total normal thermal emissivity of sheet

materials in the temperature range 140 to 500° F,

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[86] A. Butts, Copper the metal its alloys and compounds

(Reinhold Publishing Corp., New York, 1954).

[87] H. C. Jennison and C. S. Smith, Properties of commercially pure copper, Metals Handbook, 1939 ed.. The American Society for Metals, Cleveland, Ohio,

1389.

[88] C. S. Smith, The physical constants of copper, Metals

Handbook, 1939 ed., The American Society for

Metals, Cleveland, Ohio, 1380.

[89] F. M. Jaeger, E. Rosenbohm, and J. A. Bottema,

The exact measurement of the specific heats of

solid substances at high temperatures. VII- Metals

in stabilized and non-stablized condition: Copper

and gold, Proc. Acad. Sci. Amsterdam, 35, 772

(1932).

[90] E. M. Wise, Copper-nickel alloys, Metals Handbook,

1939 ed.. The American Society for Metals, Cleveland, Ohio, 1413.

[91] G. C. Stauffer, Properties of the alloy 55% copper45% nickel (constantan), Metals Handbook, 1939

ed.. The American Society for Metals, Cleveland,

Ohio, 1417.

[92] T. B. Douglas and J. L. Dever, Enthalpy and specific heat of four corrosion-resistant alloys at high

temperatures, J. Research NBS 54, RP 2560, 15

(1955).

[93] V. D. Sanders, Review of high temperature immersion

thermal sensing devices for in-flight engine control,

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[94] F. W. Kuether, Uniqueness of thermal emfs measured

in molybdenum, rhenium, tungsten, iron and copper, High-Temperature Thermometry Seminar, Oak

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TID 7586 (Pt. 1), 23.

[95] J. C. Lachman and F. W. Kuether, Stability of

rhenium/tungsten thermocouples in hydrogen at- mospheres, ISA Journal 7, 67 (1960).

[96] F. W. Kuether and J. C. LachmaUj How reliable are the two new high-temperature thermocouples

in vacuum?, ISA Journal 7, 66 (1960).

[97] J. C. Lachman and J. A. McGurty, Thermocouples

for 5000° F using rhenium alloys. Presented at the

Electrochemical Society Symposium on Rhenium,

Chicago, May 4, 1960.

41
[98] G. Schneider and A. Boettcher, to Deutsche Goldund Silber-Scheideanstalt normals Roessler; Tungsten vs Rhenium Thermocouples, U.S. Patent No.

2,948,766, August 9, 1960.

[99] W. C. Troy and G. Steven, The tungsten-iridium

thermocouple for very high temperatures. Trans.

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vs iridium thermocouple, U.S. Patent No. 2,588,998,

March 11, 1952.

[100] Technical Bulletin, Engelhard Industries, Inc. 1,

111 (1960).

[101] Tungsten, Metallwerk Plansee Aktiengesellschaft,

Reutte, Tirol, furnished by the Rembar Company,

Inc., Dobbs Ferry, N.Y.

[102] Molybdenum, Metallwerk Plansee Gesellschaft

M.B.H., Reutte, Tirol, furnished by the- Rembar

Company, Inc., Dobbs Ferry, N.Y.

[103] Elmet, tungsten and molybdenum. North American

Philips Company, Inc., Elmet Division, Lewiston,

Maine.

[104] Sylvania Tungsten, Sylvania Electric Products, Inc., Tungsten and Chemical Division, Towanda, Pa.

[105] Molybdenum, Sylvania Electric Products, Inc.,

Chemical and Metallurgical Division, Towanda,

Pa.

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molybdenum or with tungsten and having good

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Dundas, A graphite/graphite thermocoulpe for

high temperatures. Chemistry and Industry, 595

(May 9, 1959).

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graphite, Phys. Rev., (3), 38, 450 (1914).

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measurements of temperatures above 1500° C, Trans. Am. Electrochem. Soc. 54, 19 (1928).

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Trans. AIME 105, 290 (1933).

[119] The Fitterer pyrometer. Iron Age 140, 38 (1937).

[120] R. L. Shepard, R. D. Westbrook, and H. S. Pattin,

A high-temperature boron-graphite-graphite thermocouple. Bull. Am. Phys. Soc. 1, Series II, No.

3, GA4 (1956).

[121] Catalog Section 4 (Rev. July 1959), Aero Research

Instrument Company, Inc., Chicago.

[122] J: D. Sine, Pyrometer Supplies Division, Minneapolis-Honeywell Regulator Company, Philadelphia, Pa., Private communication (July 1960).

[123] Catalog No. 300, Conax Corporation, Buffalo, N.Y.

(1959).

[124] J. C. Faul, Pyro Electric, Inc., Winnetka, 111., Private communication and catalog material

(August 1960).

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Mo., Private communication and catalog material

(July 1960).

[126] E. G. Weissenberger, The Claud S. Gordon Company, Chicago, Private communication (August

1960).

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Company, Inc., Chicago.

[128] J. Zenon, Tuttle and Kift, Chicago, Private communication (June 1960).

6. Appendix

The tables given in tliis section are for thermocouples for which no NBS reference tables have

been heretofore available. Of these, the data for

the iridium 40 percent-rhodium 60 percent versus

iridium and the platinum 85 percent-iridium 15

percent versus palladium thermocouples were obtained at the National Bureau of Standards and

have been presented elsewhere in expanded form

as reference tables. Data for the other thermocouples which appear to be in current use, and for

which data have not been obtained at the National

Bureau of Standards, are presented as given in

the sources cited.

Table 1-A. Condensed calibration for the platinum 70

percent rhodium 30 percent versus platinum 94 percent

rhodium 6 percent thermocouple

Temperature Emf Temperature Emf

"C "F mv °C "F mv

0 32 0.000 900 1,652 3.969

26 77 0.007 1,000 1,832 4.83, 50 122 0.020 1,100 2, 012 5.79J 100 212 0. 056 1,200 2,192 6.81I 200 392 0. 162 1,300 2,372 7.89"

300 672 0. 419 1,400 2,552 9.000 400 752 0.790 1,500 2,732 10. 130 600 932 1.245 1,600 2, 912 11.260 600 1,112 1.796 1,700 3, 092 12.390

700 1,292 2.442 1,760 3, 182 12. 966

800 1,472 3. 162

42
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oo CO

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O

i-i l-h H-3 vO 00 Cn •P- CO

OQ fD CO >^ C3^ 4^ CO I—"

o VO

c l-h hO I-*

O 00 o> •f> CO

g

m

O 3 i-f hO h-* O VO 00 C3N p> CO o en M • o 3 n> CO Ol CO N3 I—- o o * H o e e e > U3 h-" »j I-" -P- C» CO C cr VO U) VO 1—

C3N PO 3 (-> Ul h-" VO K5 VO 00 -P- ls5 (C (D 3 o CO rt

!-h rt d O

I-i

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pj (jO UO CO U) ISi IsJ lo |nJ < 3 fD C» -f> O CO c^ 4^- o O a ^-! O o O o O O o O O o 1—

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1^

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o « 0 o dw of (jO K) h-' O VO 00 -P~ CO

rt)

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Ul 4> ON VO Ul 00

o

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CO 3^O

(13 o

d

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fD

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O I-i O fD 3 M < 3

l-h

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jo fD 3 M

l< 3

l-h

rt

cr

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3- 3^

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fD

HiO

ONo<M

I-i

rt3^

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I-i <

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d

fD03

M-3

rt3"

H03

OOod 3 I-" OQ fD O

03 3

1-^ CL d

fD 03 03 O

>{

03

033^

o

o

d

d I-"

I-* P- acr

cr fD

fD

o

03

O

d 3

cr

03 !H rt I-i H- <D rtd

rt

fDQ03cr

fD

0333

fD3

rtO3

(Da

rt3'

fD 3 rt l-h 3^ O

rt fD i-{ P3cr < rt

fD H-' fD d ^ fD M I 0) i-{ >• O -P- Ml O5-3

03 OP

fD

coH

03cr

fD

pa *^

Table 2-A. Condensed reference table for the iridium

40 percent rhodium 60 -percent versus iridium thermocouple

Temperature Emf Temperature Emf

°c ° F °C

32 Q

7710

^ 1 900 1037. 8 ""5. 513

50 10.0 0.032 2! 000 1093! 3 5. 814

100 37.8 0.126 2, 100 1148.9 6. 114

150 65. 6 0.227 2 200 1204. 4 6. 491 200 93.

3

0.337 2' 300 1260. 0 6. 708

300 148. 9 0. 576 2 400 1315. 6 7.005

400 204! 4 0.840 2^500 137L 1 7.305

500 260.0 1.122 2,600 1426.7 7. 607

600 315.

6

1.418 2 700 1482. 2 7. 914

700 371.

1

1.726 2' 800 1537.

8

8.228

800 426. 7 2.041 2 900 1593.

3

8. 545

900 482.

2

2. 361 si 000 1648 9 8.862

1,000 537^8 2.684 3, 100 1704! 4 9. 182 1,100 593.3 3.009 3,200 1760.0 9.508

1,200 648.9 3. 332 3,300 1815.6 9.839

1,300 704.4 3. 654 3,400 1871.

1

10. 176 1,400 760.0 3. 973 3,500 1926. 7 10. 522

1,500 815.6 4.287 3,600 1982.

2

10. 879

1,600 871.

1

4. 599 3,700 2037.8 11.243

1,700 926.7 4. 907 3,800 2093.3 11.610

1,800 982.2 5. 211

Table 3-A. Thermal emf of iridium 40 percent rhodium

60 percent versus copper and of copper versus iridium

Temperature Ir 40-Rh 60 Iridium

"F °C emf, mv emf, mv

32 0 0 0

50 10.0 0. 026 0.006

75 23.9 .064 .014

100 37.8 . 103 .023

ISO 65.6 .178 .049

200 93.3 .253 .084 250 121.

1

.325 .129

300 148.9 .392 .184

400 204.4 .525 .315

500 260.0 .637 .485

The iridium-rhodium is positive to copper; copper is positive to iridium

Table 4-A. Condensed reference table for the platinum

86 percent iridium 15 percent versus palladium thermocouple

Temperature Emf Temperature Emf

op °C 77MJ "F °C TTW 32 0 0 1,200 648.9 17. 054 50 10.0 0. 179 1,300 704.4 18. 947

80 26.7 0.437 1,400 760.0 20. 910

100 37.8 0.702 1,500 815.6 22.939

150 65.6 1.254 1,600 871.1 25.033

200 93.3 1.830 1,700 926.7 27. 190 300 148.9 3. 051 1,800 982.2 29.403

400 204.4 4.346 1,900 1, 037.

8

31. 670

600 260.0 5.705 2,000 1,093.3 33. 985

600 315.6 7. 130 2, 100 1, 148. 9 36. 342

700 371.

1

8.619 2,200 1, 204.

4

38. 732

800 426.7 10. 172 2, 300 1,260.0 41. 141 900 482.2 11. 787 2,400 1.315.6 43.569

1,000 537.8 13. 470 2,500 1,371.

1

46.022

1, 100 693.3 15. 228 2, 550 1, 398.

9

47. 265

\ Table 5-A. Thermal emf of platinum 85 percent-iridium

16 percent versus copper and of copper versus palladium

Temperature Pt 85-Ir 15 Palladium

°F °C emf, mv emf, mv

32 0 0.000 0.000

80 26.7 .177 .312

100 37.8 .251 .453

150 65.6 .431 .823 200 93.3 .606 1.224

250 121.1 .775 1.654

300 148.9 .938 2.116

400 204.4 1.237 3. 108

500 260.0 1.502 4.201

The platinum-iridium is positive to copper; copper is positive to palladium.

The values in the table are as determined, and some

differences exist between the sum of the emfs here and the

values of table 4-A. Reasons for these differences are not apparent, but use of the above data will introduce no appreciable errors at higher temperatures.

Table 6-A. Condensed calibration data for Geminol P and

N thermocouple

Temperature Emf Temperature Emf

"F °C mv "F "C mv

32 0 0.00 1,100 593.3 17. 11 50 10.0 .24 1,200 648.9 19.06

80 26.7 .63 1, 300 704.4 21.06

100 37.8 .90 1,400 760.0 23. 10

150 66.6 1.58 1,600 815.6 25.20

200 93.3 2.26 1,600 871.1 27. 30

300 148.9 3.66 1,700 926.7 29.46

400 204.4 5. 07 1,800 982.2 31.65

500 260.0 6. 57 1,900 1,037.8 33.86 600 315.6 8.18 2,000 1,093.3 36.08

700 371.1 9.84 2,100 1,148.9 38.32

800 426.7 11.56 2,200 1,204.4 40.54 900 482.2 13. 35 2,300 1,260.0 42.76

1,000 537.8 15.20

Table 7-A. Calibration data for iridium 60 percentrhodium 40 percent versus iridium thermocouple

Temperature Emf Temperature Emf

"F "C 77!!! °C 7715 32 0 0.00 2,000 1,093.3 5.76 200 93.3 0. 32 2,200 1,204.4 6.36

400 204.4 0.82 2,400 1,315.6 6.95

600 315.6 1.39 2,600 1,426.7 7.64

800 426.7 2.00 2,800 1,537.8 8. 14

1,000 637,8 2.62 3,000 1,648.9 8.77

1,200 648.9 3.26 3,200 1,760.0 9.42

1,400 760.0 3.90 3,400 1,871.1 10.09

1,600 871.1 4.53 3, 600 1,982.2 10.78

1,800 982.2 5.15

• The values of table 7-A are from reference [122].

43

ft U.S. GOVERNMENT PRINTING OFFICE : 1969 0—333-445
II