Thermocouple Wire - Thermocouple Cables and Extension Grade Wire

Thermocouple Grade Wire or Extension Grade Wire?

Thermocouple Wire (cable) is available in two grades; Thermocouple Grade wire and Extension Grade wire. Typically, thermocouple grade is used to make the actual thermocouple temperature sensor or sensing point (junction) itself (detailed information on thermocouple sensor theory is available here), whereas thermocouple extension wire (cable) is used for the connection of thermocouple sensors to associated measurement systems, usually via an industrial terminal enclosure or thermocouple connector.

Extension wire will generally be of a lower grade conductor than thermocouple wire and should not be used to make the sensor or probe itself and should only be used to extend thermocouples. However, thermocouple grade wire can be used either to make the sensing junction (sensor) or used as extension cable and will usually have a higher ambient temperature range (subject to the insulation type). Our thermocouple grade cables are identified using the thermocouple type letter e.g. KK, JJ, TT etc. and extension grades are designated with an 'x' e.g. KX, JX, TX etc.

Thermocouple Cable Identification

Insulated Thermocouple Wire uses guidelines according to ANSI color coding. You will see in the table below, that commonly, negative wires are red, the outer sheath for thermocouple grades is brown and the positive lead is color coded according to thermocouple type. For extension grade cables this positive leg color is also carried over to the outer insulation color. Fiberglass insulated wires follow the same system but have a colored tracer rather than a solid color.

As can be seen from the table above, there are two accuracy classes of wire available. Cables with a Standard Limit of Error use standard thermocouple grade wire and this accuracy grade is suitable for the majority of uses. However, certain applications require a higher accuracy and so use cables with a Special Limit of Error (SLE). These wires use a purer form of the thermocouple alloys which in turn improve the accuracy as listed above.

As a company speciliaising the manufacturer of custom built thermocouples, TC can offer cable or complete assemblies made using SLE wire.

IEC 60584-3 provides tolerances and identification for a common international system for thermocouple wire identification and tolerances based essentially on thermoelectric emf as opposed to a datum of the emf of the thermo elements against platinum. Tolerances are defined as the maximum allowable deviation in microvolts, caused by the introduction of extension or compensating extension wire into the circuit. Color codes and tolerances are well established in the United States and have been in use as a national standard for years. In other parts of the world, there are alternative color code systems and tolerances, either in use, or in the process of being implemented. One such color code system is that established by the IEC.

In the United States, color codes and tolerances are defined in ANSI 96.1 (1982) and ASTM E-230. Extension wire and compensating extension wire is indicated with the letter “X” after the thermocouple type per these standards.

The color code for thermocouple grade wire includes a brown overall jacket whereas the color code for thermocouple extension wire typically includes an overall jacket in the color of the positive wire. For some insulations, colors may appear as a stripe or trace strand. High temperature braided insulation is not normally color coded. The table above shows the color coding adopted by the ASTM and IEC.

Thermocouple Wire Insulation Materials

There is a wide choice of insulation materials, the choice of which is governed by the application and the ambient temperature wires will be exposed to:

Heat Resistant (HR) PVC is a popular choice for general use in the temperature range -22ºF to +220ºF. It is available in a wide variety of constructions from simple bellwire to shielded and armored types in virtually all of the thermocouple combinations. Flame Retardant PVC is also available.

PFA is ideal for higher temperature applications up to 480ºF when PVC is not adequate. It is also excellent for temperatures down to -100ºF.

PFA will withstand attack from virtually all known chemicals, oils and fluids. All our PFA cables are made in extruded form and are therefore gas, steam and water tight which makes them most suitable for applicatiins such as autoclaves or sterilizers. We offer a wide variety of constructions from simple twisted pairs to shielded types in virtually all of the thermocouple combinations.

Excellent for high temperature applications up to 900ºF for Fiberglass, and 1475ºF for High Temperature Fiberglass and 2200ºF for Ceramic Fiber. They are suitable for use at normal air ambient temperatures where there is a possibility of a hot spot which might damage lower rated cables such as PVC or PFA.

We offer a wide variety of constructions from simple lapped and fiberglass braided to stainless steel braided types in virtually all of the thermocouple combinations.

These cables incorporate XLPE (Cross Linked Polyethylene) compound on the conductors and Low Smoke Zero Halogen (LSZH) material on the bedding and/or outer sheath. The sheathing materials used are Halogen free and the acidic gas which is evolved during combustion is less than 0.5% in accordance with IEC 60754-1. The mechanical properties of these cables meet the requirements of IEC 60811.

Resistant to a temperature of 1380°F for at least three hours in accordance with the flame test requirements of IEC 60331. These cables are essential for situations where it is of strategic importance to ensure that the cable continues to function during a major crisis involving fire. The cable incorporates a high temperature rated Mica glass tape with a XLPE (Cross Linked Polyethylene) insulation on the conductors and Low Smoke Zero Halogen material on the bedding and/or outer sheath. Sheathing materials are Halogen free.

Thermocouple Grade Wire - Data Sheets

Thermocouple Extension Wire - Data Sheets