Thermocouple cables are an integral part of the measuring circuit. The alloy they are made from generates a voltage of its own, which is added to the sensor signal and reaches the measuring instrument as part of the temperature reading. This is why two main cable families are available: extension cables, made from the same alloys as the thermocouple, and compensating cables, made from substitute alloys designed to reproduce a similar thermoelectric response within a limited temperature range.
Using one in place of the other can introduce an error of several degrees while still producing a perfectly plausible reading, meaning that no alarm will necessarily detect the problem. In short, extension cable is required when the cable runs through high-temperature environments or outside the specified temperature range of the substitute alloy, while compensating cable is generally more cost-effective for noble-metal thermocouples and for cable runs operating close to ambient temperature.
An extension cable uses the same materials as the thermocouple sensor. A compensating cable uses different alloys selected to generate a similar electromotive force within a specified temperature range. Outside this range, the thermoelectric characteristics no longer match and the measurement error increases without any obvious warning.
The cost advantage of compensating cables comes from the materials used. With platinum thermocouples, namely types R, S and B, extending the circuit using the same alloys would cost more than the sensor itself. For type K thermocouples, both options may be suitable, depending on the cable routing and operating conditions. For type J, the issue does not arise, as the standard provides only for JX extension cable: iron and constantan are relatively inexpensive, so there is little economic benefit in using substitute alloys.
The two cable families are listed separately in the Caviterm range: thermocouple compensating cables on one side and thermocouple extension cables on the other.
The designation printed on the cable sheath provides most of the information required for identification. The first letter indicates the thermocouple type: J, K, T, E and N for base-metal thermocouples, and R, S and B for platinum-based thermocouples.
An X following the thermocouple letter identifies an extension cable, as in JX and KX. A C identifies a compensating cable and is followed by a second letter indicating the specific pair of substitute alloys used. KCA and KCB, for example, are two different compensating cables designed for the same type K thermocouple.
These designations are not interchangeable. KCB, for example, is suitable when the cable temperature remains between 0 and 100°C. If the cable runs alongside a furnace or inside a hot electrical enclosure, this temperature range may be exceeded, whereas KX can operate from -25°C to +200°C.
One detail that is often overlooked is that the cable must be selected according to the sensor, not the measuring instrument. A type J thermocouple cable connected to a type K thermocouple follows a different thermoelectric curve, even though the display may still show a plausible value.
Tolerance is specified separately for the sensor and for the cable, and both contributions affect the overall measurement uncertainty.
Thermocouple tolerances are defined by IEC 60584-1. Class 1 allows the greater of 1.5°C or 0.4% of the measured temperature, while Class 2 allows the greater of 2.5°C or 0.75%. In a furnace operating at 600°C, the difference is significant: 2.4°C compared with 4.5°C. Class 3 applies only to certain thermocouple types at low temperatures and does not apply to type J.
Thermocouple cable tolerances are instead covered by IEC 60584-3, which specifies 1.5°C for Class 1 and 2.5°C for Class 2, provided that the cable remains within its specified operating temperature range. Compensating cables are limited to Class 2. The same classes apply to type K thermocouple cable, the most widely used thermocouple type in industrial installations.
When a Class 2 sensor, a Class 2 cable and the uncertainty of the measuring instrument are considered together, the total error in a measuring loop can reach approximately seven degrees while all individual components remain within their specified tolerances. On an extrusion line, seven degrees can be enough to distinguish an acceptable profile from a rejected one. Tolerance class should therefore not be treated as a minor purchasing detail.
Under IEC 60584-3, the negative conductor is always white, while the sheath uses the same colour as the positive conductor.
| Type | Positive | Negative | Sheath |
|---|---|---|---|
| J | black | white | black |
| K | green | white | green |
| T | brown | white | brown |
| E | violet | white | violet |
| N | pink | white | pink |
| R and S | orange | white | orange |
The North American ANSI MC 96.1 standard follows a different convention: the negative conductor is always red. This is frequently encountered on imported machinery and can result in reversed polarity if the cable colours are interpreted using the wrong standard.
Older national colour codes are also still found in existing installations, so the conductor identity should always be verified when the origin of a cable is uncertain. With type K cable, a magnet provides a quick check: the negative conductor is magnetic, whereas the positive conductor is not.
For a complete overview, see the dedicated guide to thermocouple cable colours and the thermocouple cable table, including cable designations and temperature ranges.
One of the most common mistakes is extending the thermocouple circuit with whatever cable happens to be available in stock. Adding a section of copper wire creates two additional junctions at the terminal block. These junctions act as parasitic thermocouples: as long as the terminal block remains close to ambient temperature, the resulting error may be limited, but when the enclosure temperature rises to 50°C the error can become significant.
Another common problem is reversed polarity, which can usually be identified when the displayed temperature decreases as the process temperature rises.
Connectors also need to be considered. A thermocouple connector must be manufactured from the appropriate alloy; otherwise, the thermoelectric circuit is interrupted at precisely the point where it appears to remain continuous.
Cable routing is equally important. A signal cable installed parallel to a power cable can pick up electrical interference, and with thermocouple signals measuring only a few tens of microvolts per degree, this interference can become visible on the display.
A twisted pair and a shield grounded at one end only are sufficient in most installations. Where this is not enough, the cable route should be reviewed before replacing the cable, as explained in the dedicated article on electrical cable interference.
Caviterm has been manufacturing thermocouple cables in Provaglio d’Iseo, Italy, since 1981, using alloys available with Special ½ tolerance classification and maintaining a warehouse organised to support prompt delivery.
The range includes both extension and compensating cables for the most widely used thermocouple types, with insulation materials ranging from PVC to fibreglass and standard products designed for operating temperatures of up to 400°C. Caviterm’s quality management system is certified to ISO 9001:2015.
When a standard cable is not suitable for the application, the technical department can develop a solution according to customer specifications. This may include reproducing an obsolete cable from an existing sample, developing special shielding configurations or manufacturing cables with a number of pairs different from standard versions.
Customers can start by exploring the complete thermocouple cable range and contact Caviterm’s technical team to assess the most suitable solution based on the installation data.
No. Thermocouples use different combinations of alloys, and these differences affect their measuring range, sensitivity and resistance to different process atmospheres. A type J and a type K thermocouple connected to the same instrument will produce different readings at the same temperature because they generate different thermoelectric voltages.
Type J uses iron and constantan and can operate at temperatures of around 750°C while remaining relatively cost-effective. Type K uses nickel-chromium and nickel-aluminium alloys, can operate up to approximately 1,200°C and offers better performance in oxidising atmospheres.
The cable must be selected accordingly: JX for type J, and KX or a suitable compensating cable for type K.
A type K thermocouple uses nickel-chromium for the positive conductor and nickel-aluminium for the negative conductor. It is the most widely used thermocouple type in industrial applications thanks to its broad measuring range and relatively low cost compared with platinum thermocouples.
A thermistor is a semiconductor component whose electrical resistance changes with temperature. It generally operates over relatively limited temperature ranges and provides high sensitivity.
A thermocouple generates a thermoelectric voltage and can operate over much wider temperature ranges, including temperatures above 1,000°C, although its sensitivity is lower.
Not without introducing measurement error. Where the copper conductors meet the thermocouple alloys, additional junctions are created. These generate their own thermoelectric voltage, and the resulting error increases as the temperature of the terminal block rises.
Under IEC 60584-3, the negative conductor is white. Under the ANSI standard, it is red. If the cable markings are no longer legible, the negative conductor of a type K cable can also be identified using a magnet.
