Special purpose thermocouples are application specific temperature sensing devices and are categorized by the resistance to environmental factors. For abrasion resistant types, the harden tip style is common for use in industrial mixers and dyers where very coarse materials are crushed, grinded or mixed. Rock and asphalt are examples of abrasive materials. Special purpose thermocouples are used to measure materials on conveyor belts or pumps used to carry “free-flow” abrasive materials like sand. The next category is chemical resistant thermocouples which are designed to withstand corrosion from harsh chemicals. Plating and acid baths are good examples of corrosive fluids that the special purpose thermocouple must be designed to withstand and operate accurately. The disadvantage of some special purpose thermocouples can be their response time which can be greatly reduced because of the thickness of the protective tubing material. The extra material “barrier” can slow the temperature rise as the heat is conducted to the thermocouple element. An example of a special purpose thermocouple is an industrial chemical processing plant uses a 60” long, 1 inch diameter rod for a probe to test temperature readings in a superheated vat containing a highly corrosive chemical. The rod is custom machined from a 310 stainless steel bar stock and bored to insert the type K thermocouple element. The probe weighs 45 lbs. and has a flange allowing the unit to be secured to the mounting plate of the vat.

Special purpose thermocouple assemblies are the connecting accessories to the special purpose thermocouple. The assembly is the complete “turnkey” unit that includes the thermocouple element, the protective sheath, mounting bracket, extension wire or cable, and the end terminations. The special purpose thermocouple assembly takes the thermocouple element and protective tube, adds the extension wire or cable to allow it to be remotely connected to a controller. There are several types of extension wire based on stranded or solid plus the type of insulation or protection required to complete the cable. Most common insulation for the conductor is polyvinyl which can withstand temperatures up to 105°C, the next most popular is Teflon FEP with a withstand temp of 316°C. For super high temperatures, Glass Braid or Ceramic Fiber offer temperature rating of 538°C and 871°C, respectively. For additional protection, the insulted conductors can have a protective sheath or jacket incasing the wires which can also be the same material as the insulation. For extra strength, stainless steel braid can be added add another layer of protection, primarily to shield against physical force or penetration. To finalize the special purpose thermocouple assembly, the end terminations whether it is jacks or plugs need to be installed to the cable to connect to the controller.