Selection of thermocouples is based on the application, the range of temperature required, chemical resistance of the sheath material, and the mechanical resistance to abrasion and vibration. The most commonly used standards are J, K, T and E. B, S, R and K are used in steel and iron industries. Thermocouples are suitable for measuring high-temperature range applications including kilns, gas turbine exhaust, diesel engines and other industrial applications (Pollock, 1991, p. 215)
K thermocouples are appropriate for testing temperatures in processing plants such as petroleum refineries and chemical production plants. They are also appropriate for the tests of heating appliance safety.
J thermocouples are appropriate for monitoring temperatures in inert materials and vacuum operations. They get oxidized when used for low-temperature applications and thus are suitable for high-temperature monitoring processes such as the manufacture of resins and plastics
N thermocouples offer a wide range of temperature and are stable resisting oxidation making suitable for use in high-temperature applications. They are used in furnaces, ovens, and kilns. They are also used to monitor the temperature in engine exhausts and gas turbines.
It is the smallest thermocouple consisting of two pieces of dissimilar wires that are held together through a welded bead. The welded bead is easily corroded or oxidized when used with liquids. It relies on a direct connection to an electrical circuit to accurately measure the temperature. They are small with fast response time making them a suitable choice for measuring gas temperature.
Has a wire that is housed inside a metallic tube. The metallic tube serves as a sheath and is made up of materials such as stainless steel and Inconel. Inconel supports a higher temperature range while stainless steel is suitable for a wider range of chemical compositions. The tip of the probe may be grounded, ungrounded or exposed. The grounded tip keeps contact with the sheath to provide quick response time.