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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From small, one off samples to large, quality controlled orders, Scott Precision Wire offer the best, not only in quality production but with ongoing technical support all backed up with a strong supply chain, huge stock and multiple years of experience.
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.
high temperature wire
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
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.
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