Thermocouple Sensors

A thermocouple is a junction between two different metals that produces a voltage related to a temperature difference.

A thermocouple is a temperature-measuring device consisting of two dissimilar conductors that contact each other at one or more spots. It produces a voltage when the temperature of one of the spots differs from the reference temperature at other parts of the circuit.

Principle of Operation

Thermocouples are based on the principle that two wires made of dissimilar materials connected at either end will generate a potential between the two ends that is a function of the materials and temperature difference between the two ends (also called the Seebeck Effect).

Types of Thermocouple

Before discussing the various types of thermocouples, it should be noted that a thermocouple is often enclosed in a protective sheath to isolate it from the local atmosphere. This protective sheath drastically reduces the effects of corrosion.

Type K Thermocouple (Nickel-Chromium / Nickel-Alumel):

The type K is the most common type of thermocouple. It’s inexpensive, accurate, reliable, and has a wide temperature range. The type K is commonly found in nuclear applications because of its relative radiation hardness. Maximum continuous temperature is around 1,100C.

Type K Temperature Range:

Thermocouple grade wire, –454 to 2,300F (–270 to 1260C)

Extension wire, 32 to 392F (0 to 200C)

Type K Accuracy (whichever is greater):

Standard: +/- 2.2C or +/- .75%

Special Limits of Error: +/- 1.1C or 0.4%

Type J Thermocouple (Iron/Constantan):

The type J is also very common. It has a smaller temperature range and a shorter lifespan at higher temperatures than the Type K. It is equivalent to the Type K in terms of expense and reliability.

Type J Temperature Range:

Thermocouple grade wire, -346 to 1,400F (-210 to 760C)

Extension wire, 32 to 392F (0 to 200C)

Type J Accuracy (whichever is greater):

Standard: +/- 2.2C or +/- .75%

Special Limits of Error: +/- 1.1C or 0.4%

Consideration for bare wire type J thermocouple applications:

The Type J Is Well Suited To Oxidizing Atmospheres

Type T Thermocouple (Copper/Constantan):

The Type T is a very stable thermocouple and is often used in extremely low temperature applications such as cryogenics or ultra low freezers. It is found in other laboratory environments as well. The type T has excellent repeatability between –380F to 392F (–200C to 200C)..

Type T Temperature Range:

Thermocouple grade wire, -454 to 700F (-270 to 370C)

Extension wire, 32 to 392F (0 to 200C)

Type T Accuracy (whichever is greater):

Standard: +/- 1.0C or +/- .75%

Special Limits of Error: +/- 0.5C or 0.4%

Consideration for bare wire type T thermocouple applications:

The Type T Is Well Suited To Oxidizing Atmospheres

Type E Thermocouple (Nickel-Chromium/Constantan):

The Type E has a stronger signal & higher accuracy than the Type K or Type J at moderate temperature ranges of 1,000F and lower. The type E is also more stable than the type K, which adds to its accuracy.

Type E Temperature Range:

Thermocouple grade wire, -454 to 1600F (-270 to 870C)

Extension wire, 32 to 392F (0 to 200C)

Type E Accuracy (whichever is greater):

Standard: +/- 1.7C or +/- 0.5%

Special Limits of Error: +/- 1.0C or 0.4%

Consideration for bare wire type E thermocouple applications:

In oxiding or inert atmospheres the operating range is roughly –418F to 1,652F (–250C to 900C).

Type N Thermocouple (Nicrosil / Nisil):

The Type N shares the same accuracy and temperature limits as the Type K. The type N is slightly more expensive. The type N has better repeatability between 572F to 932F (300C to 500C) compared to the type K.

Type N Temperature Range:

Maximum continuous operating temperature: up to 2,300F (1,260C)

Short term use: 2,336F (1,280C)

Thermocouple grade wire, -454 to 2300F (-270 to 1,260C)

Extension wire, 32 to 392F (0 to 200C)

Type N Accuracy (whichever is greater):

Standard: +/- 2.2C or +/- .75%

Special Limits of Error: +/- 1.1C or 0.4%

Consideration for bare wire type E thermocouple applications:

The type N holds up better to oxidation at high temperatures when compared to the type K.

Type S Thermocouple (Platinum Rhodium - 10% / Platinum):

The Type S is used in very high temperature applications. It is commonly found in the BioTech and Pharmaceutical industries. It is sometimes used in lower temperature applications because of its high accuracy and stability. The type S is often used with a ceramic protection tube.

Type S Temperature Range:

Maximum continuous operating temperature: up to 2,912F (1600C)

Short term use: up to 3,092F (1,700C)

Thermocouple grade wire, -58 to 2700F (-50 to 1480C)

Extension wire, 32 to 392F (0 to 200C)

Accuracy (whichever is greater):

Standard: +/- 1.5C or +/- .25%

Special Limits of Error: +/- 0.6C or 0.1%

Consideration for bare wire type J thermocouple applications:

The Type S can be used in inert and oxidizing atmospheres up to 2,912F (1600C) continuously and up 3,092F (1,700C) for short term use.

Type R Thermocouple (Platinum Rhodium -13% / Platinum):

The Type R is used in very high temperature applications. It has a higher percentage of Rhodium than the Type S, which makes it more expensive. The Type R is very similar to the Type S in terms of performance. It is sometimes used in lower temperature applications because of its high accuracy and stability. Type R has a slightly higher output and improved stability over the type S.

Type R Temperature Range:

Thermocouple grade wire, -58 to 2700F (-50 to 1480C)

Extension wire, 32 to 392F (0 to 200C)

Accuracy (whichever is greater):

Standard: +/- 1.5C or +/- .25%

Special Limits of Error: +/- 0.6C or 0.1%

Type B Thermocouple (Platinum Rhodium – 30% / Platinum Rhodium – 6%):

The Type B thermocouple is used in extremely high temperature applications. It has the highest temperature limit of all of the thermocouples listed above. It maintains a high level of accuracy and stability at very high temperatures. The type B has a lower output than the other noble metals (type R & type S) at temperatures below 1,112F (600C).

Type B Temperature Range:

Thermocouple grade wire, 32 to 3100F (0 to 1700C)

Extension wire, 32 to 212F (0 to 100C)

Accuracy (whichever is greater):

Standard: +/- 0.5%

Special Limits of Error: +/- 0.25%

Advantages

It is rugged in construction

Covers a wide temperature range

Using extension leads and compensating cables, long transmission distances for temperature measurement possible. This is most suitable for temperature measurement of industrial furnaces

Comparatively cheaper in cost

Calibration can be easily checked

Offers good reproducibility

High speed of response

Satisfactory measurement accuracy

Limitations

For accurate temperature measurements, cold junction compensation is necessary

The emf induced versus temperature characteristics is somewhat nonlinear

Stray voltage pickup is possible

In many applications, amplification of signal is required

Applications

Type B, S, R and K thermocouples are used extensively in the steel and iron industries to monitor temperatures and chemistry throughout the steel making process.
Gas-fed heating appliances such as ovens & water heaters.
In the testing of prototype electrical and mechanical apparatus.
Steel industry
Gas appliance safety
Thermopile radiation sensors
Manufacturing
Power production
Thermoelectric cooling
Process plants
Thermocouple as vacuum gauge