Thermocouple Wire

Thermocouple wire is used to measure the temperature electronically. Two wires of different metal joined together at the sensing end to measure the temperature. Low voltage signal is measured in thermocouple by using wire from the point of sensing to the point of cold junction compensation (cjc end). Different combination of metal wires are used in different thermocouple types such as J, K, T, E, etc.

The four most common types are J, K, T and E. There are high temperature thermocouples R and S. Each type has a different sensitivity (gain), linearity, temperature range and environment, although the maximum temperature varies with the diameter of the wire used in the thermocouple. Thermocouple type defines the temperature range, the maximum range is also limited by the diameter of the thermocouple wire.

Understanding on how to identify the insulated thermocouple wires?

Thermocouple wire insulation identified by color codes. As stated in guidelines, that the negative lead in insulated thermocouple wire is red. The positive lead has the color of the thermocouple as well as the overall color of insulated extension grade wire. The outer jacket of thermocouple grade wire is typically brown. Usually color coded tracer thread in the white material for high temperature wire. Please click to see the "Thermocouple Wire Insulation Identification" table for information on usable temperature ranges for the insulation. You may also find other tables at this link are also available for information on the metals used in the thermocouple wire, insulation color codes, and more.

Understand about the difference between Standard and Special Limits of Error (SLE) wire.

Standard vs Special Limits of Error wires are responsible for the accuracy of the wire. Standard limits of Error are commonly used thermocouples as compare with standard “thermocouple grade” wire. Special Limits of Error is high grade of thermocouple and it is purer as compare to standard wire. Because of purity it increase the accuracy of thermocouple and hence it is also expansive than standard wire.

SLE wire is the same as standard wire with the added feature of having a little better accuracy spec. Thermocouples’ accuracy vary with different type of thermocouple. E.g. type T for the lower temperature ranges, composed of copper wire in the positive lead and constantan (copper-nickel mixture) for the negative, has good accuracy specifications. Additional information on thermocouple wire accuracy.

Difference between Thermocouple grade and Extension grade wire

Wire that is used to make the sensing point (or probe part) of the thermocouple is thermocouple grade wire. Thermocouple grade wire is mostly used to manufacture thermocouple probes.

Extension grade wire helps to extend a thermocouple signal from a probe back to the instrument reading the signal. The extension grade wire typically will have a lower ambient temperature limit in which the wire may be used. Namely, it may pass a signal representing a higher temperature as received from the probe, but the wire physically may not be exposed to higher temperatures.

Thermocouple wire may be used as extension wire, but extension grade wire may not be used in the sensing point (or probe part) of the thermocouple. At OMEGA the part numbers for extension wire normally begin with an "EX" prefix.

What should be the maximum length of thermocouple wire?

There are many reasons that can impact the usable length of a thermocouple. By using guidelines, it should be fine if the length of thermocouple wire is under 100 feet with 20 AWG or thicker wire in an area free of electromagnetic interference. The total loop resistance and preventing electrical noise getting into the signal are the main factors in determining useable thermocouple length. The resistance varies on the basis of type as well as the wire diameter and length as different thermocouple wires are made of different materials. The allowable loop resistance is affected by the input resistance of the amplifier circuit to which it is attached.

But as a guideline, typically the objective is to keep the total loop resistance under 100 ohms. Loop resistance is determined by multiplying the length in feet by the resistance per double feet (remember 1 foot length of run includes 1 foot from each of the two thermocouple wires) as shown in "Resistance Vs Wire Diameter" table for thermocouple type and gauge.

Remember in your calculations to include the probe (when used/applicable) in addition to the wire length. Another major factor in running a thermocouple wire is to keep it away from any electromagnetic fields. Thermocouple wire creates a very low voltage signal and because of this factor it should not be pass through any power wires, motors, etc. as it. A metal over braid or twisted shielded wire is commonly used to help in minimizing the noise pickup.

Bare Thermocouple Wire
A thermocouple wire without any insulation is Bare Thermocouple Wire. Normally it is sold on a spool and ordered as a pair. Usage is typically for making a small thermocouple in applications requiring special considerations.

Duplex Insulated Thermocouple Wire
Duplex Insulated Thermocouple Wire is probably the most common type of thermocouple wire. The type of thermocouple, insulation, and size are all some of the parameters considered for the application.

Metal Sheathed Thermocouple Wire
Thermocouple wire surrounded with insulation material, typically magnesium oxide, and enclosed in a metal sheath is called as Metal Sheathed Thermocouple Wire. A significant feature of Metal Sheathed Thermocouple Wire is it includes the ability to bear the vibration, high temperatures and pressures.