Thermocouples are the most common temperature-sensing device. They can be made in very tough designs. They are very simple in operation and measure temperature at a point. Over different types they cover from -250C to +2500C.
The principal of operation is on the Seebeck effect. A temperature gradient along a conductor creates an EMF. If two conductors of different materials are joined at one point, an EMF is created between the open ends which is dependent upon the temperature of the junction. As T1 increases, so does V. The EMF also depends on the temperature of the open ends T2.
The junction is placed in the process, the other end (for the purposes of a standard output) is in iced water at 0C. This is called the reference junction. In the field the reference junction is usually at ambient temperature in the indicating unit and a cold junction compensation allowance is made. This is usually done through the indicator electronics.
Provided that set temperature limits are not exceeded, inexpensive compensating wires can replace thermocouple conductors for part of the circuit. They replicate the thermoelectric performance of the thermocouple wires up to a certain temperature.
Temperatures along the wires (between T1 and T2) will not affect output voltage, providing that the alloys are thermocouple wire or compensating cable. Alloy combinations are chosen which produce a high EMF, are stable at temperature and readily available.
Thermocouple output depends on the alloy mix of the conductors. No two mixes will be identical and sensor calibration is required if high accuracy is needed. The performance of an individual sensor will drift with high temperature exposure.
There are defined sensor combinations as shown in the thermocouple tables below. Lower temperature combinations are often called base metal. The most common one is type K NiCr v NiAl. These are limited to 1100C if any lifetime is expected. Above this temperature rare metals are used. Type R, Pt v PtRh(13%) is very common. These are much more expensive but can record up to 1700C (Type B) and remain stable for a reasonable lifetime.
BS EN 60584-2 defines the standard output expected for standard thermocouple types. BS EN 60584-3 defines the tolerances and limits for extension cables.
A thermocouple construction consists of two conductors welded together at the measuring point and insulated from each other along the length. It will usually have an outer protection sheath.
PVC, silicon rubber or PTFE insulators are common to 250C, above this Glass fibre or ceramic is used. The protection sheath is often a metal alloy which is inert to a particular process. At high temperatures ceramic sheaths are used.
Often more consideration goes in to selecting and designing protection sheaths than sensors as this is the layer that must withstand chemical or physical attack along with offering convenient process attachment features. This is dealt with later.
It is important that along the entire length of cable between a sensing point and either an instrument, or a transmitter, the leads used replicate the thermocouple materials in terms of the EMF generated through temperature gradients. It is clear however that for Platinum sensors, to continue the complete circuit in this precious metal would be expensive and commercially vulnerable. Extension and compensating cables are used to reduce this cost.
Extension cables use the actual thermocouple materials, but in cheaper forms. This is achieved by using cheaper insulations, wider tolerance alloys and thinner conductors (as they will see little thermal stress in their lifetime). They are produced in multi-stranded forms for ease of installation The thermoelectric performance is clearly very similar to the actual sensor.
Compensating cables use completely different alloys that happen to exhibit very similar thermo-electric properties up to a limited temperature (usually 200C ). Great care should be taken to control the temperature of the junction between the compensating cable and the actual thermocouple material to keep it below the acceptable maximum. Compensating cables come in convenient cabling forms for site installations.
Type |
Temperature Range °C (Continuous) |
Temperature Range °C (Short term) |
Tolerance Class 1 (°C) |
Tolerance Class 2 (°C) |
IEC Colour Code |
BS Colour Code |
Notes |
K |
0
to +1100 |
-180
to +1300 |
-40
to +375 ±1.5°C, 375 to 1000 ±0.004*|t|°C |
-40
to +333 ±2.5°C, 333 to 1200 ±0.0075*|t|°C |
 |
 |
The
most common sensor, wide range, good in oxidising atmosphere |
J |
0
to +700 |
-180
to + 800 |
-40
to +375 ±1.5°C, 375 to 750 ±0.004*|t|°C |
-40
to +333 ±2.5°C, 333 to 750 ±0.0075*|t|°C |
 |
 |
Good
in dry & reducing atmospheres |
N |
0
to +1100 |
-270
to +1300 |
-40
to +375°C ±1.5°C, 375 to 1000 ±0.004*|t|°C |
-40
to +333 ±2.5°C, 333 to 1200 ±0.0075*|t|°C |
 |
 |
Fairly
new to market. High stability |
R |
0
to +1600 |
-50
to +1700 |
0
to +1100 ±1.0°C, 1100 to 1600 ±(1+0.003(t-1100))*|t|°C |
0
to +600 ±1.5°C, 600 to 1600 ±0.0025*|t|°C |
 |
 |
High
temperatures. Used in UK. Usually in ceramic sheath |
S |
0
to +1600 |
-50
to+1750 |
0
to +1100 ±1.0°C, 1100 to 1600 ±(1+0.003(t-1100))*|t|°C |
0
to +600 ±1.5°C, 600 to 1600 ±0.0025*|t|°C |
|
|
High
temperatures. Usually in ceramic sheath. |
B |
+200
to +1700 |
0
to +1820 |
Not
Available |
600
to 1700 ±0.0025*|t|°C |
no
pic |
no
pic |
Very
high temperatures. Always protected by high purity ceramic |
T |
-185
to +300 |
250
to +400 |
-40
to +125 ±0.5°C, 125 to 350 ±0.004*|t|°C |
-40
to +133 ±1.0°C, 133 to 350 ±0.0075*|t|°C |
 |
 |
Cryogenic
sensor. Copes with water present. |
E |
0
to +800 |
-40
to +900 |
-40
to +375 ±1.5°C, 375 to 800 ±0.004*|t|°C |
-40
to +333 ±2.5°C, 333 to 900 ±0.0075*|t|°C |
 |
 |
High
EMF output |
Notes
Maximum temperature depends on actual application conditions and sensor design. Tolerances are sourced from BS EN 60584.2:1993
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