Color Coding and Insulations (continued)
*Where insulation type permits, the outer insulation layer for thermocouple grade wire is brown, the tracer is threaded through the outer layer.
All of the information related so far has been concerned with the thermoelements and their characteristics. In the normal usage of a thermocouple the thermoelements must be protected from various environments that are detrimental to the materials comprising them. Effects of attack from the environment, temperature cycling, and aging all work toward producing errors from a thermocouple.
In addition to the environmental protection required, the thermoelements must be electrically insulated from one another at all points except the measuring junction.
Insulation Color Codes
Temperature Rating F
Ceramic-Cordierite or Mullite
Compacted Magnesia (MgO)
Electrical insulation is accomplished by various dielectric materials such as varnish, plastic, inorganic fibers, and ceramic. General temperature ratings for these insulation materials are:
Temperature Ratings for Insulation
The plastic type insulations provide protection to the thermoelements from moisture or fluid contamination. They have relatively lower temperature applications compared to the inorganic fibers types. The inorganic fibers types of insulation often are furnished with moisture proofing impregnations which are burned off with exposure above 400 to 500F Hard fired ceramic insulators are used on both base metal and noble metal calibrations. Above 2400F alumina insulators are recommended for the noble metal calibrations. Beryllia and thoria insulators can be used with proper precautions to 4000 to 4200F.
If wire insulation degrades to the degree that electrical contact of the two thermoelements is at a point other than the measuring junction, the signal produced by the thermocouple will be one based on the temperature difference between the reference junction and the new junction which is a secondary junction.
Barber-Colman Company, Loves Park, IL.