Thermocouple Technical Guide/Reference
and FAQs
A Guide for Non-Engineers
What is a thermocouple?
A thermocouple is a temperature sensor. It is comprised
of two dissimilar metals, joined together at one end and connected
at the other end to a read output device known as a thermocouple
thermometer.
How does it work?
A thermocouple works based on the theory of the thermoelectric
effect. Electricity and changes in temperature (heat
or cold) are very closely related. Anyone with a
notebook computer sitting on their lap will attest that electricity
can produce heat, but most people are unaware of the inverse,
that heat can produce electricity! It is upon this
effect that a thermocouple operates. In 1821, Thomas
Johann Seebeck discovered that when any metal conductor is subjected
to a change in temperature, it will generate a voltage. Also
discovered was the fact that each type of metal conductor produces
a different voltage for the same change in temperature.
Why are two dissimilar metals needed in
a thermocouple?
Measuring the voltage given off by a change in temperature
of a metal conductor involves connecting another conductor to
complete an electrical circuit. This new conductor
will also experience the change in temperature and develop its
own voltage, opposing the original. When two dissimilar
metals are used each will generate a differing voltage and the
difference between the two can be measured.
How is a thermocouple used in an industrial
application?
A thermocouple-based temperature measurement system consists
of a thermocouple or probe and a thermocouple thermometer (read
output device). Extension wire is often used
if the thermocouple thermometer can not be placed close enough
to the actual thermocouple being read.
What is a probe?
A probe is a thermocouple covered with a sheath on the measuring
end (hot junction) and usually contains an industry standard
connector for connecting to a thermocouple thermometer on the
other end (cold junction). Probes have slower response
times but are more resistant to the environment and last longer.
What applications are thermocouples best
suited for?
There is no easy answer to this question. The decision
must be based on factors in the application. In general,
thermocouples are very inexpensive. The main limitation
is accuracy. Accuracy of less than 1°C can be
difficult to achieve.
How do I choose what type of sensor I should
use to measure temperature in an application?
Again there is no easy answer to this question. Application
factors are considered in the final decision. Major
factors are:
1. Temperature Range
2. Temperature Accuracy
3. Response Time
4. Environment
5. Cost
When using electrical devices additional factors may contribute
to the decision:
1. Electrical Noise
What materials are used in building thermocouples?
Although almost any two types of metal can be used to make
a thermocouple, standards have been made in the industry for
using the best suited combinations. Thermocouples
are built in a variety of industry-standard materials. These
materials combined are called thermocouple types. Each
type has a different range of measurable temperature.
These industry standards are listed below:
| Type* |
ANSI Colors
|
Materials |
Magnetic |
Temperature
Range
|
Output** |
Notes / Properties |
| K |
 |
chromel (90% nickel, 10% chromium) (+)
alumel (95% nickel, 2% aluminum, 2% manganese, 1% silicon)
(-)
|
No
Yes |
-270°C to 1372°C
(-454°F to 2500°F) |
Approx. 41 µV/°C |
Most common, best all-around type
Curie point at 354°C
|
| J |
 |
iron (+)
constantan (45% nickel, 55% copper) (-)
|
Yes
No |
-210°C to 1200°C
(-346°F to 2192°F) |
Approx. 52 µV/°C |
Iron susceptible to rust - shouldn't be used
in areas with high humidity or water content |
| T |
 |
copper (+)
constantan (45% nickel, 55% copper) (-) |
No
No |
-270°C to 400°C
(-454°F to 752°F) |
Approx. 43 µV/°C |
Use for low temperature (cryogenic) measurement. |
| S |
 |
platinum-rhodium (90%-10%) (+)
platinum (-) |
No
No |
-50°C to 1768°C
(-58°F to 3214°F) |
Approx. 10 µV/°C |
Used for high temperature readings |
| R |
 |
platinum-rhodium (87%-13%) (+)
platinum (-)
|
No
No |
-50°C to 1768°C
(-58°F to 3214°F) |
Approx. 10 µV/°C |
Used for high temperature readings |
| B |
 |
platinum-rhodium (70%-30%) (+)
platinum-rhodium (94%-6%) (-) |
No
No |
0°C to 1820 °C
(32°F to 3308°F) |
Approx. 10 µV/°C |
Used for very high temperature readings. |
| E |
 |
chromel (90% nickel, 10% chromium) (+)
constantan (45% nickel, 55% copper) (-) |
No
No |
-270°C to 1000°C
(-454°F to 1832°F) |
Approx. 68 µV/°C |
High EMF output. Seen most frequently
in the power industry. |
| N |
 |
nicrosil (14% chromium, 1.4% silicon, 84.6%
nickel) (+)
nisil (0.4% silicon, 95.6% nickel) (-) |
No
No |
-270°C to 1300°C
(-454°F to 2372°F)
|
Approx. 39 µV/°C |
Highly stable |
* ASTM E 230-03, "Standard Specification and Temperature-Electromotive
Force (EMF) Tables for Standardized Thermocouples," ASTM
International. For referenced ASTM standards, visit
the ASTM website, www.astm.org,
or contact ASTM Customer Service at service@astm.org. For
Annual Book of ASTM Standards volume information, refer to the
standard's Document Summary page on the ASTM website.
** Higher output makes a thermocouple better for measuring
lower temperatures.
International Color Codes
|
Country
|
International
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North America
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UK
Czech
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Germany
Netherlands
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Japan
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France
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National
Standard
|
IEC 584-3
|
ANSI
MC 96.1
|
BS
1843
|
DIN
43714
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JIS
C1610-1981
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NF
C42-323
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S
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E
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Standard Thermocouple Coefficents
Below is the web version of Standard Reference Database 60.
NIST reference functions and tables of thermocouple electromotive
force (emf) versus temperature have been adopted as standards
by the American Society for Testing and Materials (ASTM) and
the International Electrotechnical Commission (IEC). Distributed
by Standard Reference Data Program of the National Institute
of Standards and Technology. NIST reserves the right to charge
for access to this database in the future.
Temperature vs.Millivolts DC (EMF) Tables: Directly downloadable
from the NIST online data base on their web site in the USA:
(NIST, the National Institute of Standards and Technology, is
an agency of the U.S. Commerce Department’s Technology
Administration)
Below are links to the NIST database files (in degrees Celsius
only). (These files are in ASCII text format):
