Thermocouples are available in different combinations of metals, each with its own unique characteristics in terms of temperature range, durability, vibration resistance, chemical resistance, and application compatibility.
• Base metal thermocouples or type K, J, T, & E are relatively low cost and therefore the most popular thermocouples, they are commonly used in a broad range of low to medium temperature applications.
• Noble metal thermocouples or type R, S, and B have greater resistance than base metal thermocouples, however they have platinum conductors, making them far more expensive. They are commonly used in high temperature applications.
The graph below shows the thermocouple characteristics and relationship of temperature vs. mV output for all main base metal and nobel thermocouple types. This shows that Thermocouple Sensors have a relatively linear output.
Reference : sterlingsensors