Thermocouple Wire

Thermocouples are widely used as a method of measuring temperature and contain two different alloy wires joined together at the measuring (‘Hot’) end.

They are then 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.

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Thermocouple Wire

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 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.

key information

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

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.

thermocouple wire types

Code+ Positive Leg- Negative Leg
NNicrosilNisil
KNicroNial
ENicroConstantan E
JIronConstantan J
TCopperConstantan T
KCBCopperConstantan KCB
SCB RCBCopperCupronic 12

high temperature wire

TypeNKEJT
Wire Dia°C°C°C°C°C
3.25mm12001200870760370
1.50mm11001100650600370
0.81mm10001000550500250
0.50mm1000900450400200
0.32mm1000900400400200
0.25mm900750400300150
0.12mm600600300300150

standards, grades and temperature ranges

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:

The high-temperature wire table displays the maximum temperature by Wire Size

SpecProbeHotCold
Environment Temp1000°C - 200°C200°C - 100°C100°C - 20°C
Thermocouple DesKKXKCB
AlloysNicro - NialNicro - NialCopper - Constantan KCB
Alloy DesignationKP - KNKPX - KNXKPCB - KNCB

protecting thermocouples

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:

testing and calibration

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.

thermocouple wire faq's

Thermocouple types refer to the combination of dissimilar metals used within the device; several combinations of thermocouple wire are established and frequently used. These include types K, J, E, T, N, S, R, B.

 

  • Type K – nickel and chromium/ nickel and aluminium
  • Type J – iron/ constantan
  • Type E – nickel and chromium/ constantan
  • Type T – copper/ constantan
  • Type N – nickel and chromium and silicon/ nickel and silicon
  • Type S – platinum and 10% rhodium/ platinum
  • Type R – platinum and 13% rhodium/ platinum
  • Type B – platinum and 6% rhodium/ platinum and 30% rhodium

Thermocouples are used to measure and monitor temperature in a wide range of industries. Thermocouple applications include: extreme heat (automobile and aerospace vehicle engines and exhausts), as well as extreme cold (cold storage and freezers). Thermocouples may be used for the temperature detection in homes and businesses as well.

A thermocouple shield protects the measurement junction of the device by providing a casing. The measurement junction – or hot end – of the thermocouple must be exposed to the temperature which it is measuring, therefore is susceptible to damage. A thermocouple shield, commonly fabricated from stainless steel, protects the thermocouple from this.

Thermocouple length – or the distance between the measuring junction and the reference junction (or the hot and cold end, respectively) – does not affect the accuracy of the thermocouple or transference of signal. Extension grade wire can be used to extend the thermocouple length, so that the reference junction can be placed where it can be safely read, whilst the measuring junction remains at the location of the temperature which it needs to measure.

A thermocouple and RTD (resistance temperature detector) both measure temperature, but do so by differing methods. A thermocouple utilises dissimilar wires to generate a temperature-dependent voltage which can be calculated to determine temperature. A RTD uses the known resistance of metals to measure the change in resistance to calculate temperature; resistance increases as temperature increases. Thermocouples are more substantial and can withstand a greater temperature range, whilst RTDs are more accurate with higher levels of repeatability.

Thermocouple burnout is failure of the device. This is caused by disruption of the input signal to the controller, leading to the controller registering a pseudo-low temperature; the controller responds by increasing heat input, which can lead to failure of the process. Thermocouple burnout may occur due to oxidation, metal wear, or using an unsuitable thermocouple type in an extreme environment.

Thermocouple advantages include: thermocouples can be used in very high and low temperatures, depending on the type; they have a quick response time; good durability; they do not require an external power source.

Thermocouple disadvantages include: thermocouples are not as accurate as RTDs; non linearity for signals; thermocouple wire may need shielding; accuracy may be reduced by corrosion of the wire.

Thermocouple wire and extension grade wire may both be used within a thermocouple device, however for differing applications. Thermocouple wire is used within the measuring junction as the probe for reading temperature; thermocouple extension wire is used to carry the signal from the probe to where the signal can be read in the reference junction.

Both wire and a probe can be used to register temperature at the measuring junction; thermocouple probes offer more protection to the device in extreme environments which may corrode wire, and the design of the sensor must be chosen with application in mind.

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featured case study

A UK Aerospace customer who manufactures high specification thermocouple assemblies for use in jet engines had a problem in that their manufacturing process caused the thermocouple EMF to drift out of International Specification’s. ... read more