Thermocouple Wires

Thermocouple wire is a type of electrical wire that is used in a thermocouple. The wire is used from the point of sensing to the point of cold junction compensation where the signal is measured. Basically a sensor for measuring temperature, it comprises two different metals that are joined together at the sensing end. The combination of the two metals used in the making of such wires depends on the wire calibration like J, K, T, E etc.


Types of Thermocouple Wires (Based on uses)

• Bare Thermocouple Wire: Bare thermocouple wire comes without any insulation. Usually it is sold on a spool and ordered as a pair. It is typically used for making smaller thermocouples which are needed in special applications.
  
  
• Duplex Insulated Thermocouple Wire: This is the most common used 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: Metal Sheathed Thermocouple Wire is the type of thermocouple wire which is covered by a layer of magnesium oxide (MgO), as the insulating material and then it is enclosed in a metallic sheath. The important characteristics of these wires include the ability to withstand vibration, high temperatures and pressures.
  
  
• High Temperature Thermocouple Cable: High temperature thermocouple cables are optimized for different industrial and commercial applications.
  
  

Different types of thermocouple (Based on metal combinations)
Thermocouples are available for a variety of applications in different fields like industrial, scientific, food temperature, medical research and many more. Their selection criterion is based on the temperature range and sensitivity requirement. There are certain other selection criteria which include the inertness of the thermocouple material itself, and its magnetic nature.

• Type K - The metals used for its combination are Nickel and Chromium (Chromel) or Nickel and Aluminum (Alumel). It is the low cost, general and most common used thermocouple. They work in the range between -200 °C to +1200 °C.
  
  
• Type E - It is Chromel or Constantan (Copper-Nickel Alloy). Well suited to cryogenic uses. It is non-magnetic in nature.
  
  
• Type J - It is mainly Iron-Constantan. The working temperature range is -40°C to +750 °C.
  
  
• Type N - It is Nicrosil (Nickel-Chromium-Silicon Alloy) or Nisil (Nickel-Silicon Alloy). Suitable for measuring very high temperatures (above 1200°C). It is due to its high stability and resistance to high temperature.

The important feature of thermocouple wires

• Conductors: The solid or stranded conductors are always metals or alloys in pairs. They are combined in a way to meet the temperature requirements of the application. The common ones are Iron-Constantan, Chromel-Alumel, Nicrosil.
  
  
• Insulations: The insulating materials used include ETFE, FEP, PFA, mica tapes, glass fiber braids, ceramic braids and combinations of these materials.
  
  
• Cable Jackets: Cable jackets are also needed to meet the temperature requirements of the applications. It include ETFE, FEP, PFA, mica tapes, glass fiber braids, ceramic braids and combinations of these materials.
  
  
• Cable Shield: It is required for additional abrasion resistance and mechanical strength. A braid of stainless steel strands can also be applied.

How are insulated thermocouple wires identified?
For identification of hot and cold junctions the insulation on thermocouple wire is color coded. The negative junction or lead in insulated thermocouple wire usually becomes red as it is the general consideration. The positive junction or lead isn't provided any separate color rather it is of the same color as of overall color of insulated extension grade wire. The color of outer jacket is typically brown. For high temperature wires there is a color coded tracer thread in the white material.