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Thermocouple Thermowell assemblies - page 2 / 7





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Thermocouple Assemblies

When it comes to Thermocouples “There is a dierence!

Thermocouples are the most common and versatile sensors used to measure temperature. They can be designed to work in any application where a temperature measurement is required ranging from minus 300 oF to over 4000 oF. Thermocouples operate on the principle of thermal emf (Electromotive Force). Thomas Seebeck observed in 1821 that two dissimilar metal wires, if joined at both ends, produce a current when the junctions are at dierent temperatures. The current produced is a function of the junction temperature and the type of metals used. This relationship between thermal emf and temperature is now known as the Seebeck eect.

The thermal emf can be measured by breaking the thermocouple loop at any point and measuring the open circuit voltage. For example, a thermocouple, made of Chromel/Alumel metals (Type K) generates about 40 microvolts per degree Celsius.

The voltage/temperature relationship varies depending on the metals used. The metal combinations used depend on the out- put voltage required and the temperature range monitored. ANSI Thermocouples are tested and graded against the American National Standards Institute standards and are available in Standard and Special Limits grades (see table below). The Special Limits grade has only half the error of the Standard grade and is recommended for high temperature applications.



+/- 4f +/- 3/4%

+/- 2f +/- 3/8%

Reducing atmosphere recommended. Iron oxides rapidly at elevated temperatures.

+/- 1-1/2f +/- 3/4%

+/- 3/4f +/- 3/8%

Can be used in oxidizing or reducing atmospheres rust and corrosion resistant. Fine for low temperatures & Cryogenic.

+/- 4f +/- 3/4%

+/- 2f +/- 3/8%

Oxidizing atmosphere recommended. Vented protection tube suggested in reducing atmosphere.

+/- 3f +/- 1/2%

+/- 3/8%

Oxidizing atmosphere recommended. Highest EMF output of thermocouples commonly used.

+/- 5f +/- 1/2%

+/- 2-1/2f +/- 1/4%

Oxidizing atmosphere recommended. Easily contaminated. The accepted laboratory standard in premium grade due to its reproducibility.

+/- 3f +/- 1/4%

+/- 1f +/- 1/10%

Same conditions as 10% above but has 13% Rhodium slightly higher emf. Mostly used in industrial applications.

+/- 4f +/- 3/4%

+/- 2f +/- 3/8%

Better Resistance to Oxidation and longer life at high temperatures than Type K.

+/- 1/2%

+/- 1/2%

Can be used in a vacuum with limited life.

Thermocouples are considered to be “tip sensitive” because they measures temperature at the junction, which is located at the end or tip of the sensor. Care should be given when considering the best assembly to t an application. To acquire accurate temperature readings careful consideration should be given to the design and sensor location in the process. (See inside back cover for “Thermocouple Installation & Maintenance Suggestions”)

GIC Thermodynamics’ sales and engineering sta will assist you with designing the best thermocouple sensor style to t your process and application. Selecting the proper sensor style is very important, as each application or process has its own specic inherit problems that require careful consideration in order to determine the sensors life, accuracy, and dependability. When choosing a sensor it is important to consider its location, temperature range, accuracy required, how rapidly the temperature cycles, heat conduction, process environment, vibration, and ease of installation. If you need additional assistance with the selection of a sensor for your application, please feel free to contact our sales and engineering sta.




-75 to 200 f



200 to 700 f


chromel (non-magnetic)

alumel (magnetic)

0 to 530 f 530 to 2300 f


chromel (non-magnetic)

constantan (non-magnetic)

0 to 600 f 600 to 1600 f


platinum 10% rhodium (non-magnetic)

platinum (non-magnetic)

0 to 1200 f 1200 to 2700 f


platinum 13% rhodium (non-magnetic)

platinum (non-magnetic)

0 to 1200 f 1200 to 2700 f


nicrosil (non-magnetic)

nisil (non-magnetic)

0 to 530 f 530 to 2300 f


platinum 30% rhodium


1000 to 3200 f

appLicaTion noTes




0 to 530 f



530 to 1400 f

ansi conductor characteristics: Temperature & Limits of error

LimiTs of error

TemperaTure range f

isa code

conducTor & c posiTive

haracTerisTics negaTive

special note:

certain characteristics and tolerances of thermocouple materials change over time and usage, including the emf vs Temperature relationship. as a result of these changes it is not recommended to recalibrate used sensors once they are out of tolerance. The useful life of a thermocouple depends on several factors including wire gauge, environment and temperature range. for longer sensor life, we recommend using the largest diameter probe possible. But consider the larger the diameter the slower the response time ( see approxi- mate mgo response time chart on page 14).

Web: http://www.gicthermodynamics.com Email: sales@gicthermodynamics.com


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