A thermocouple construction consists of two conductors welded together at the measuring point and insulated from each other along the length, which will usually have an outer protection sheath.
Materials such as PVC, silicon rubber, FEP or PTFE insulators are most commonly used in applications up to 250°C, glass fibre or ceramic is used for temperatures above 250°C. For medium range temperatures (400 to 1200°C) the thermocouple protection sheath is often a metal such as stainless steel or alloy 600 and can be a machined pocket, designed to fit into a particular process. At high temperatures normally above 1100°C ceramic sheaths are used.
Often more consideration goes into selecting and designing protection sheaths than sensors as they are more expensive and must provide protection from chemical or physical attack. Sheaths are often made to order as they are designed specifically to fit into a particular process or application.