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Thermo couple General technical data
overview
In electronics, thermocouples are a widely used kind of temperature sensor. They are cheap, interchangeable, have standard connectors and can measure a wide range of temperatures. The main limitation is accuracy, system errors of less than 1°C can be difficult to achieve. The thermocouple is one of the simplest of all sensors. It consists of two wires of dissimilar metals joined near the measurement point. The output is a small voltage measured between the two wires.
How they work
In 1822, an Estonian physician named Thomas Seebeck discovered (accidentally) that the junction between two metals generates a voltage which is a function of temperature. A thermocouple circuit has at least two junctions: the measurement junction and a reference junction.
 
 
Typically, the reference junction is created where the two wires connect to the measuring device. This second junction it is really two junctions: one for each of the two wires, but because they are assumed to be at the same temperature (isothermal) they are considered as one (thermal) junction. It is the point where the metals change - from the thermocouple metals to what ever metals are used in the measuring device - typically copper. The output voltage is related to the temperature difference between the measurement and the reference junctions. This is phenomena is known as the Seebeck effect. (See the Thermocouple Calculator to get a feel for the magnitude of the Seebeck voltage). In practice the Seebeck voltage is made up of two components: the Peltier voltage generated at the junctions, plus the Thomson voltage generated in the wires by the temperature gradient.
The Peltier voltage is proportional to the temperature of each junction while the Thomson voltage is proportional to the square of the temperature difference between the two junctions. It is the Thomson voltage that accounts for most of the observed voltage and non-linearity in thermocouple response. Each thermocouple type has its characteristic Seebeck voltage curve. The curve is dependent on the metals, their purity, their homogeneity and their crystal structure. In the case of alloys, the ratio of constituents and their distribution in the wire is also important. These potential inhomogeneous characteristics of metal are why thick wire thermocouples can be more accurate in high temperature applications, when the thermocouple metals and their impurities become more mobile by diffusion.
The advantages and disadvantages of thermocouples
They can be very rugged , are immune to shock and vibration , are useful over a wide temperature range , are simple to manufactured, require no excitation power, there is no self heating and they can be made very small. No other temperature sensor provides this degree of versatility. robustness, wide temperature range and unique properties.
On the down side, the thermocouple produces
relative low output signal that is non-linear . These characteristics require a sensitive and stable measuring device that is able provide reference junction compensation and linearization Also the low signal level demands that a higher level of care be taken when installing to minimise potential noise sources. The measuring hardware requires good noise rejection capability. Ground loops can be a problem with non-isolated systems, unless the common mode range and rejection is adequate.

Types of thermocouple
About 13 'standard' thermocouple types are commonly used. Eight have been given an internationally recognised letter type designators. The letter type designator refers to the emf table, not the composition of the metals - so any thermocouple that matches the emf table within the defined tolerances may receive that table's letter designator. Some of the non-recognised thermocouples may excel in particular niche applications and have gained a degree of acceptance for this reason, as well as due to effective marketing by the alloy manufacturer. Some of these have been given letter type designators by their manufacturers that have been partially accepted by industry. Each thermocouple type has characteristics that can be matched to applications. Industry generally prefers K and N types because of their suitability to high temperatures, while others often prefer the T type due to its sensitivity, low cost and ease of use. A table of standard thermocouple types is presented below. The table also shows the temperature range for extension grade wire in brackets.
 

Type

Positive Material

Negative Material

Accuracy***
Class 2

Range °C
(extension)

Comments

B

Pt, 30%Rh

Pt, 6%Rh

0.5%
>800°C

50 to 1820
(1 to 100)

Good at high temperatures, no reference junction compensation required.

C**

W, 5%Re

W, 26%Re

1%
>425°C

0 to 2315
(0 to 870)

Very high temperature use, brittle

D**

W, 3%Re

W, 25%Re

1%
>425°C

0 to 2315
(0 to 260)

Very high temperature use, brittle

E

Ni, 10%Cr

Cu, 45%Ni

0.5% or 1.7°C

-270 to 1000
(0 to 200)

General purpose, low and medium temperatures

G**

W

W, 26%Re

1%
>425°C

0 to 2315
(0 to 260)

Very high temperature use, brittle

J

Fe

Cu, 45%Ni

0.75% or 2.2°C

-210 to 1200
(0 to 200)

High temperature, reducing environment

K*

Ni, 10%Cr

Ni, 2%Al
2%Mn
1%Si

0.75% or 2.2°C

-270 to 1372
(0 to 80)

General purpose high temperature, oxidizing environment

M**

Ni

Ni, 18%Mo

0.75% or 2.2°C

-50 to 1410

.

N*

Ni, 14%Cr
1.5%Si

Ni,
4.5%Si
0.1%Mg

0.75% or 2.2°C

-270 to 1300
(0 to 200)

Relatively new type as a superior replacement for K Type.

P**

Platinel II

Platinel II

1.0%

0 to 1395

A more stable but expensive substitute for K & N types

R

Pt, 13%Rh

Pt

0.25% or 1.5°C

-50 to 1768
(0 to 50)

Precision, high temperature

S

Pt, 10%Rh

Pt

0.25% or 1.5°C

-50 to 1768
(0 to 50)

Precision, high temperature

T*

Cu

Cu, 45%Ni

0.75% or 1.0°C

-270 to 400
(-60 to 100)

Good general purpose, low temperature, tolerant to moisture.

* Most commonly used thermocouple types, ** Not ANSI recognized types. *** See IEC 584-2 for more details. Materials codes:- Al = Aluminum, Cr = Chromium, Cu = Copper, Mg = Magnesium, Mo = Molybdenum, Ni = Nickel, Pt = Platinum, Re = Rhenium, Rh = Rhodium, Si = Silicon, W = Tungsten

 
Thermocouple mounting
Here are four common ways in which thermocouples are mounted with in a stainless steel or Inconel sheath and electrically insulated with mineral oxides. Each of the methods has its advantages and disadvantages.
Sealed and Isolated from Sheath:
Good relatively trouble-free arrangement. The principal reason for not using this arrangement for all applications is its sluggish response time - the typical time constant is 75 seconds.
Sealed and Grounded to Sheath:
Can cause ground loops and other noise injection, but provides a reasonable time constant (40 seconds) and a sealed enclosure.
Exposed Bead:
Faster response time constant (typically 15 seconds), but lacks mechanical and chemical protection, and electrical isolation from material being measured. The porous insulating mineral oxides must be sealed.
Exposed Fast Response:
Fastest response time constant (typically 2 seconds), depending on the gauge of junction wire. In addition to problems of the exposed bead type, the protruding and light construction makes the thermocouple more prone to physical damage.
Thermocouple Color Codes:
Thermocouple wiring is color coded by thermocouple types. Different countries utilize different color coding. Jacket coloring is sometimes a colored stripe instead of a solid color as shown.
 

United States ASTM:
 

British BS1843: 1952:

British BS4937: Part 30: 1993:
 

French NFE:

German DIN:
 
 
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