Thermocouples operate on the 3 basic principles; the law of Intermediate Metals, the Law of Homogenous Metals and the Law of Intermediate Temperatures. The basic function of a thermocouple is to measure temperature and it does this by producing a low voltage EMF caused by 2 dissimilar metals attached at a junction. When temperature is applied, the thermocouple will provide a calibrated voltage that is used to determine the temperature based on the base metal used. Certain base metal and alloy combinations were recognized as having the best temperature range and accuracy which became the most commonly used used in the industry. The Instrument Society of America were the originators of identifying by letter designations the most commonly used base metals which became an industry standard for base metal thermocouples. Adopted by IEC in 1977, it became the de facto standard for thermocouple, design, specifications and applications within the acceptable tolerances provided by the standard references tables.

Base metal thermocouples are categorized by a letter designation type. There are 8 base metal and alloy industry standard combinations. Each combination type offers specific features and operating characteristics to fit diverse temperature measuring applications and atmospheric conditions. The most common operating characteristic considerations are temperature range, accuracy, stability, and tolerance to environmental conditions. Each letter designation thermocouple type provides distinctive operating parameters within the temperature ranges characteristic to that metal and need to be evaluated to best fit each application.

For most temperature measuring applications, it is impractical to only use base metals without an insulation sheath or protection tube. The most commonly used product construction is the base metal thermocouple assembly. The base metal thermocouple assembly incorporates several major components to make a complete assembly. Upon selection of the base metal element type, the protective tube or element housing needs to be chosen. Tubing is the most popular housing construction for base metal thermocouples. The protective tubing material will be determined by the application. The most common material for the tube or housing is stainless steel which is provided as three basic grades – 316, 310 or 304 (heat resistance categories). A distant second common material is Inconel which is also available in three temperature ranges. Once the protective material is determined, the next choice is the outside diameter of the tube (available in a number of standard OD sizes) and its length need to be defined. Upon establishing the tubing material and size, the base metal wire requires an insulator to protect it from conducting against the housing and to protect it from potential foreign materials affecting the electrical characteristics of the element. This is achieved by using industry common insulating materials which are ceramic compounds and fused silica powders. They provide excellent thermal resistance and provide the insulation as well. The next step will be to connect the base metal elements to thermocouple wire and place in side tube with insulator and ceramic compounds to fill the empty spaces of the tube. The opposite end (exposed wire) can be connected to an extension wire to a controller for the displaying of the temperature.