Thermocouples

1 tc chemise 120
 
tete jaune 120
 
cannes pyro 120
 

Principle

Thermocouples convert thermal potential variation into electric potential variation. An electromotive force is generated when the ends of 2 wires of dissimilar metals are joined at a temperature other than 0°C (hot junction) while the other ends (cold junction) are set at 0°C (real or compensated value).

 

Thermocouple types

The most commonly used thermocouples are:

Type

 temperature range in C°

 Applications

 K and N

 -40° to +1200°

   General purpose. High temperature
   Oxydizing environment

 J

-40° to +750°

   High temperature. Vacuum, reducing
   or inert atmospheres

T

 -40° to +350

   Sub-zero temperature ranges.
   Resistant to moist atmospheres

 R and S

 0° to +1600°

   High temperature.
   Precision

 B

600°C to +1700°

   Max. temperature higher than R & S


For extreme temperatures, other thermocouples are suitable:

 Type

  Température range in C°

 Application

C

0° à 2300°

   Very high temperature
   Brittle

 

Construction:

Thermocouple sensors may be:

  • Mineral insulated
    The thermocouple wires are insulated with mineral powder inside a flexible protecting sheath.
  • With ceramic or metallic protecting tube
    The thermocouple wires are protected by rigid insulators placed in a metallic or ceramic protecting tube.
     
  • With cable, encapsulated or not
    The thermocouple is a cable. The hot junction may be protected by a capsule. 

The nature of the insulating materials and protectors depends on the working temperature and environment. 

 

Measuring junctions:

Grounded: with single or dual element. This provides fast response with protection from the process.

Ungrounded: Improved protection from interference that may be picked up by the sheath. Response time may be slower.

Exposed: Very fast response time. Not suitable for many processes because of corrosion.

 

 

Accuracy:

Thermocouples comply with the tolerance classes of the EN 60584-2 – 07/1993 standard:

   Tolerances / temperature Tolerances / temperature
 Type

 Class 1

 Class 2

 K and N

±1.5°c from –40°C to +375°C
  or
  ±0.004.|t| from 375°C to 1000°C

±2.5°c from –40°C to +333°C
  or
  ±0.0075.|t| from 333°C to 1200°C

 J

±1.5°c from –40°C to +375°C
  or  
  ±0.004.|t| from 375°C to 1000°C

±2.5°c from –40°C to +333°C
or 
 ±0.0075.|t| from 333°C to 750°C

 T

±0.5°c from –40°C to +125°C
  or
  ±0.004.|t| from 125°C to +350°C

±1°c from –40°C to +133°C
or 
 ±0.0075.|t| from 133°C to 350°C

 R and S

 ±1°c from 0°C to +1100°C
or 
±[1+0.003.(t-1100)] 
from 1100°C to 1600°C

±1.5°c from 0°C to +600°C
 or
±0.0025.|t| from 600°C to 1600°C

 B

 -

 ±0.0025.|t| from 600°C to +1700°C

 

Applications:

Thermocouples are suitable for many applications: industrial processes, monitoring equipment, test benches... Their main advantages are:

  • A large scale of temperature ranges
  • Robustness and versatility
  • Miniaturization possibilities (little intrusion).
  • Large range of insulating materials and protectors
  • Cheapness (as far as standard thermocouples are concerned)

 

CORREGE thermocouple sensors:

   > Mineral insulated 

        - with connector                                  
        -
with cable                                          
        -
with connecting head                                            

   > Encapsulated with cable         

   > Industrial thermocouples

        - with metallic protecting tube                   
        -
with ceramic protecting tube  
        -
for non-ferrous alloys baths