Used for their sensitivity and wide temperature range of -200ºC to 1,350ºC, type K thermocouples are considered general purpose devices because of their many uses in common, everyday applications. They usually have a high sensitivity, about 4/µv/cº. Type K exhibits high thermopower and good resistance to oxidation but remains inexpensive while lasting a long time.
Type K thermocouples are used in all sorts of different heat and temperature applications, since they have good temperature precision and a wide measurement range. This kind of thermocouple is popular and available in a wide variety of probes. Since it has such a high temperature range, type K are especially used within the steel and iron fabrication industries. They measure the metal’s chemical composition and temperature during machining or casting to know when it has melted to a certain temperature and is ready to form.
Other industrial applications, which use type K thermocouples’ temperature range, include kilns, ovens, plastic extrusion machines, pressure chambers, water tanks, heat exchangers and parts washers. Heated appliances like gas stoves also use type K thermocouples as heat sensors to keep from overheating, which is a safety concern and could cause damage. In the home, standard thermostats and temperature switches are also monitored by type K thermocouples because they are so inexpensive and easy to install.
All type K thermocouples are made of a series of thin, bendable wire constructions. Each wire is made of either chromel, which is a chromium and nickel alloy, or alumel, which is an aluminum and nickel alloy. They are joined with digital devices for fast, easy readout and always use metal or ceramic protective tubes, especially when reducing atmospheres because type K perform poorly in this application. They are considered homebody thermocouples, as opposed to uppercrust, rarified and exotic. To differentiate between these different types, the wires are always labeled with a colored connector; type K are yellow.
All thermocouple instruments are divided into temperature capacity categories. Type K, because they measure up to around 2000º C, are considered noble metal thermocouples. Because thermocouples are simple in design, fabrication and assembly processes are quick and inexpensive. The wires are all extruded, which involves heating a metal alloy preform billet and forcing it through successively smaller and smaller molds until the desired wire shape and size is accomplished. The wires are easily bendable, and can twist together without the application of heat or much force.