Thermocouple Introduction & Thermometry

Thermocouples can be manufactured in many styles, from a simple welded junction joining two dissimilar metals with extension leads to Mineral Insulated Sensors with Waterproof Connecting Heads and Stainless Steel Adjustable Process Fittings. The Thermocouples described on the following pages cover many designs and are suitable for a wide range of installations.

The information below is a guide to help select the Thermocouple best suited for various applications along with some definitions.

When connecting Thermocouples in order to maintain the accuracy of any given measuring system it is imperative that the correct Extension or Compensating Cable is used. A definition of Compensating Cable and Extension Cable is given below.

Glossary of Definitions

THERMOELECTRIC (SEEBECK) EFFECT

The production of an Electromotive Force (E), due to the difference of temperature between two junctions of different metals or alloys forming part of the same circuit.

THERMOCOUPLE

A pair of conductors of dissimilar materials joined at one end and forming part of an arrangement using the thermoelectric effect for temperature measurement.

MEASURING JUNCTION

That junction which is subjected to the temperature to be measured.

REFERENCE JUNCTION

That junction of the thermocouple which is at a known (reference) temperature to which the measuring temperature is compared.

REFERENCE TEMPERATURE

For the tables of this document, the reference temperature is 0°C.

Thermocouple Type Letter Designations

The following letter designations are established for thermocouple wire combinations as follows:

Letter	Thermocouple Type	Temp Range °C
T	Copper/copper-nickel	-150 to + 350
J	Iron/copper-nickel	0 to +700
K	Nickel-chromium/nickel-aluminium	0 to +1200
N	Nickel-chromium-silicon/nickel-silicon	0 to +1250
S	Platinum 10% rhodium/platinum	0 to +1500
R	Platinum 13% rhodium/platinum	0 to +1600

When identifying noble or base metal thermocouples by their nominal alloy combinations, the positive leg will be listed first.

EXTENSION CABLES

Extension cables are manufactured from conductors having the same nominal composition as those of the corresponding thermocouple. They are designated by the letter "X" following the designation of the thermocouple, for example "JX"

COMPENSATING CABLES

Compensating cables are manufactured from conductors having a different composition from the corresponding thermocouple. They are designated by a letter "C" following the designation of the thermocouple, for example "KC". Different alloys may be used for the same thermocouple type. These are distinguished by using additional letters, for example, KCA and KCB.

Tolerance Classes for Thermocouples

Reference junction at 0°C

Types	Tolerance Class 1	Tolerance Class 2
Type T
Temperature range	-40°C to +125°C	-40°C to +133°C
Tolerance value	±0.5°C	±1°C
Temperature range	125°C to 350°C	133°C to 350°C
Tolerance value	±0.4%	±0.75%
Type J
Temperature range	-40°C to +375°C	-40°C to +333°C
Tolerance value	±1.5°C	±2.5°C
Temperature range	375°C to 750°C	333°C to 750°C
Tolerance value	±0.4%	±0.75%
Type K, Type N
Temperature range	-40°C to +375°C	-40°C to +333°C
Tolerance value	±1.5°C	±2.5°C
Temperature range	375°C to 1200°C	333°C to 1200°C
Tolerance value	±0.4%	±0.75%
Type R, Type S
Temperature range	0°C to 1100°C	0°C to +600°C
Tolerance value	±1°C	±1.5°C
Temperature range	1100°C to 1600°C	600°C to 1600°C
Tolerance value	±(1 + 0.3% of T -1100)°C	±0.25%

Thermocouple Application guide

IMMERSION DEPTH

The required depth of probe immersion at which no incorrect reading will occur due to insufficient immersion. For information on specific probe constructions please consult our office.

RESPONSE TIMES

The time in which the probe will attain a proportion of the final medium temperature. Normally quoted as the time taken to achieve a percentage of a step temperature change. The probe construction, i.e. isolated or grounded junction, application and environment will affect response times. For further information please contact our office with details of your application.

TYPE T - Copper/Copper-Nickel - 150 to 350°C

Widely used to measure low temperatures and in applications where moisture is present. May be used in oxidising, or reducing atmospheres.

TYPE J - Iron /Copper-Nickel - 0 to 700°C

May be used in oxidising atmospheres limited to below 500°C in sulphurous atmospheres. Iron conductor liable to rust at low temperatures (Ice point and below).

TYPE K - Nickel-Chromium/Nickel-Aluminium - 0 to 1200°C

Widely used thermocouple with a useful measuring range. Not recommended for Sulphur Bearing atmospheres or reducing atmospheres but good resistance to Oxidising atmosphere.

TYPE N - Nickel-Chromium-Silicon/Nickel-Silicon - 0 to 1250°C

Very good resistance to oxidising atmospheres and may be used when good stability at high temperatures is required.

TYPE R/S -

Platinum - 10% Rhodium/Platinum 0 to 1500°C

Platinum - 13% Rhodium/Platinum 0 to 1600°C

Recommended for very high temperature use in oxidising atmospheres. For some applications a secondary ceramic protective sheath is recommended.

The application guide above gives recommendations appertaining to the primary thermocouple wire. When protected by the appropriate outer sheath some of the limitations given can be overcome.

Thermocouple Ordering Codes

The Product codes given to sensors and accessories throughout this catalogue are constructed to specify the product in a logical sequence, help the user to define the sensor and ensure that the final specification contains all of the component parts required.

A typical Part No. would be:

ETC GP FL K J G D30 SL200 LL1000 LIL5 TT6 FF2

The sensor description would be: TC GP FL

Calibration Type K, Junction grounded, Sheath Diameter 3.0mm, Sheath length 200mm, Lead Length 1 metre, Lead insulation Glass Braided/Tin Copper Braided Outer, Termination STD T/C Socket with 1/8" BSP Stainless Steel Fitting.