Thermocouples

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	t_{^{emperature 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}}	S_{^{ub-zero temperature ranges. Resistant to moist atmospheres}}
_{^{R and S}}	_{^{0° to +1600°}}	_{^{High temperature. Precision}}
_^B	_{^{600°C to +1700°}}	M_{^{ax. 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