Thermocouple

Thermocouples are widely used as a method of measuring temperature and contain two different alloy wires joined together at the measuring (‘Hot’) end and connected to a meter or other thermocouple emf sensing device at the reference (‘Cold’) end.  Thermocouples are versatile temperature sensors, robust and relatively inexpensive and can be used across a wide range of process temperatures.  Scott Precision Wire manufactures wires for base metal thermocouple types N, K, E, J and T and for compensating types KCB and SCB/A.

 

The Thermocouple Effect
In 1821 Thomas Seebeck observed that a current would flow in a circuit made up of two different metals if one of the junctions was heated. Peltier then went on to describe the electron movements observed at the junctions and Thomson the electron movement along a wire subject to a temperature gradient during the mid 19th Century, expanding the understanding of the effect. The voltage driving the current was found to be proportional to the temperature difference between the two junctions. Placing a volt meter at the unheated junction allows the voltage (emf) of the circuit to be measured and hence the temperature of the heated junction can be inferred if the temperature of the cold junction is known.

 

Thermocouple Wire Types

Code + Positive Leg - Negative Leg
N Nicrosil Nisil
K Nicro Nial
E Nicro Constantan E
J Iron Constantan J
T Copper Constantan T
KCB Copper Constantan KCB
SCB RCB Copper Cupronic 12

Uses
Thermocouples can be used to measure temperature in many different applications from food preparation to smelting through to heavy chemical and power generation plants.  The sensing probe can take a number of forms from bare wires to fully metal sheathed versions which can be inserted into the medium to be measured.  The thermocouple effect can even be harnessed to create a low power generator by connecting multiple junctions in series in a heated environment.  The individual thermocouple types have particular characteristics which make them suitable for different applications with varying temperature ranges and accuracies available. The increasing importance of lowering emissions in cars and industry is driving greater accuracy and longevity demands in automotive and process industries.  Type N thermocouples in particular can be used to provide reliable temperature readings over a wide temperature range to ensure efficiencies are maximised and noxious gases minimised.  

 

Standards, Grades and Temperature Ranges
The most commonly used standards have two thermocouple grades as well as extension grades and compensating grades.  Thermocouple grades are ‘Special’ and ‘Standard’ (American) or Class 1 and Class 2 (ISO & BS EN).  Special and Class 1 tolerances are approximately half the Standard or Class 2 tolerances.

Common standards all within Scott Precision Wire capabilities.

  • ASTM (American Society for Testing and Materials) E 230
  • ANSI (American National Standard Institute) MC 96.1
  • IEC (European Standard by the International Electrotechnical Commission 584)-1/2/3
  • DIN (Deutsche Industrie Normen) EN 60584 -1/2
  • BS (British Standards) 4937.1041, EN 60584 - 1/2
  • NF (Norme Française) EN 60584 -1/2 - NFC 42323 - NFC 42324
  • IS (Japanese Industrial Standards) C 1602 - C 1610
  • IS (Unification of the Russian Specifications) 3044

High Temperature Wire

The maximum temperature a wire can be used up to will depend upon its diameter. Thinner wires will not withstand the full temperature range stated for the thermocouple type. Used in air typical maximum temperatures are:

Maximum Temperature by Wire Size

 Type N
Wire Dia.  °C °C °C °C °C
 3.25mm 1200  1200  870  760  370 
 1.50mm 1100 1100 650 600 370
 0.81mm 1000  1000 550 500 250
 0.50mm 1000 900 450 400 200
 0.32mm 1000 900 400 400 200
 0.25mm 900 750 400 300 150
 0.12mm 600 600 300 300 150

Protecting the thermocouple from corrosive atmospheres by enclosing them in metal and ceramic powder sheaths will improve their longevity.

Thermocouple wire circuits must contain only the thermocouple alloys all the way from the probe junction to the measuring device.  As the temperature the wires experience drops to around 200°C it is possible to use Extension grade wires.  These are made from the same alloy as the thermocouple but are calibrated to a limited temperature range.  Dropping lower still in temperature ‘Compensating’ wires can be used instead.  These are combinations of different alloys which produce the same emf profile as the thermocouple alloys over the limited temperature range they are approved for.  For instance a Type K thermocouple circuit could be made up as follows:


Probe Hot Extension Lead Cold Compensating Lead
Enviroment Temperature 1000°C - 200°C 200°C - 100°C 100°C - 20°C
Thermocouple Designation K KX KCB
Alloys Nicro - Nial Nicro - Nial Copper - Constantan KCB
Alloy Designation KP - KN KPX - KNX KPCB - KNCB

  

Testing and Calibration (Offset Thermocouple Profiles)
Scott Precision Wire has an in-house laboratory capable of testing the wires as single thermoelements or thermocouple pairs from -50°C to 1200°C and testing can be arranged externally for calibrations required outside this temperature range.  Full emf profiling is provided for all our thermocouple wires and this facility also allows us to manufacture offset emf thermocouple wires for special applications.  Test results can be supplied as either mV readings or °C deviations from the nominal thermocouple values.  The mV readings can be given for either the single thermoelement, so that different batches of wire may be matched later, or for the thermocouple pair.  We can even match a single thermoelement  you may have in stock either from readings you have or from a sample we can test.


Thermocouple Wire Production and Length / Sizes
We draw and process the wire on site here in Manchester UK.  Rod is purchased from a number of high quality sources and processed through our equipment to create the required format.  Round wires can be supplied in solid form from 3.25mm to 0.07mm diameter and bunched or stranded wires from 5mm² to 0.05mm².  Bunched and stranded wires tend to be used as cable conductors as they are more flexible and resistant to work hardening than solid wires.  Please see our Bunched section for further details. Rolled tape or slit strip can also be supplied in a range of sizes and packaging formats including coils and spools.  Thermocouple products can be supplied fully annealed, using our inert atmosphere annealers, or to a range of hardnesses and tensile strengths.