Thermocouple Reference Guide & FAQ
A guide for non-engineers.
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.
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.
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.
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.
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.
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.
Again there is no easy answer to this question. Application factors are considered in the final decision. Major factors are: Temperature Range, Temperature Accuracy, Response Time, Environment, and Cost. When using electrical devices additional factors may contribute to the decision, like Electrical Noise.
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 in the table 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 firstname.lastname@example.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||North America||UK Czech||Germany Netherlands||Japan||France|
|National Standard||IEC 584-3||ANSI MC 96.1||BS 1843||DIN 43714||JIS C1610-1981||NF C42-323|
Standard Thermocouple Coefficients
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)
NIST database files (in degrees Celsius only). (These files are in ASCII text format):