Sheath materials range from mild and stainless steels to refractory oxides (ceramics, so called) and a variety of exotic materials including rare metals. The choice of sheath must take account of operating temperature, media characteristics, durability and other considerations including the material relationship to the type of sensor.
The application guide below provides details of various commonly specified sheath materials.
|Sheath Material||Maximum Continuous Temperature||Notes||Applications|
|Refractory Oxide recrystallised, e.g. Alumina Impervious||1750°C||Good choice for rare metal thermocouples. Good resistance to chemical attack. Mechanically strong but severe thermal shock should be avoided||Forging iron & steel. Incinerators carburizing and hardening in heat treatment. Continuous furnaces. Glass Lehrs.|
|Silicon Carbide (Porous)||1500°C||Good level of protection even in severe conditions. Good resistance to reasonable levels of thermal shock. Mechanically strong when thick wall is specified but becomes brittle when aged. Unsuitable for oxidising atmospheres but resists fluxes.||Forging iron & steel. Incinerators Billet heating, slab heating, butt welding. Soaking pits ceramic dryers.|
|Impervious Mullite||1600°C||Good choice for rare metal thermocouples under severe conditions. Resists Sulphurous and carbonaceous atmospheres. Good resistance to thermal shock should be avoided.||Forging iron & steel. Incinerators. Heat treatment. Glass flues. Continuous furnaces.|
|Mild Steel (cold drawn seamless)||600°C|
Good physical protection but prone to rapid corrosion.
|Annealing up to 500°C. Hardening pre-heaters. Baking ovens.|
|Stainless steel 25/20||1150°C|
Resists corrosion even at elevated temperature. Can be used in Sulphurous atmospheres.
|Heat treatment annealing, flues, many chemical processes. Vitreous enamelling. Corrosion resistant alternative to mild steel.|
|Inconel 600/800*||1200°C||Nickel-Chromium-Iron alloy which extends the properties of stainless steel 25/20 to higher operating temperatures. Excellent in Sulphur free atmospheres; superior corrosion resistance at higher temperatures. Good mechanical strength.||Annealing, carburizing, hardening. Iron and steel hot blast. Open hearth flue & stack. Waste heat boilers. Billet heating, slab heating. Continuous furnaces. Soaking pits. Cement exit flues & kilns. Vitreous enamelling. Glass flues and checkers. Gas superheaters. Incinerators up to 1000°C. Highly sulphurous atmospheres should be avoided above 800°C.|
|Chrome Iron||1100°C||Suitable for very adverse environments. Good mechanical strength. Resists severely corrosive and sulphurous atmospheres.||Annealing, carburizing, hardening. Iron & steel hot blast. Open hearth flue and stack. Waste heat boilers. Billet heating, slab heating. Continuous furnaces. Soaking pits. Cement exit flues & kilns. Vitreous enamelling. Glass flues and checkers. Gas superheaters. Incinerators up to 1000°C.|
|Nicrobell*||1300°C||Highly stable in vacuum and oxidising atmospheres. Corrosion resistance generally superior to stainless steels. Can be used in Sulphurous atmospheres at reduced temperatures. High operating temperature.||As Inconel plus excellent choice for vacuum furnaces and flues.|
Metallic and Non-Metallic Sheath Materials
The choice of metallic or non-metallic sheathing is mainly a function of the process temperature and process atmosphere. Ceramic (non-metallic) tubes are fragile but have a high chemical resistance; they can withstand high temperatures (up to 1800°C in some cases). Metallic tubes, most commonly stainless steels, have mechanical advantages and higher thermal conductivity; they are also generally immune to thermal shock which can easily result in the shattering of ceramic tubes. Depending on the alloy specified, metallic sheaths can be used at temperatures up to 1150°C (higher in the case of rare metals such as Platinum or Rhodium). Ceramics are superior when high purity is required to avoid sensor or product contamination at elevated temperatures (outgassing is minimal or non-existent)
Metallised ceramic tubes are available which endow the ceramic material with greater mechanical strength and surface hardness. Although ceramic based tubes generally display high electrical insulation, some types can become electrically conductive at elevated temperatures. They must therefore not be relied upon for electrical insulation under all conditions.
The temperature sensor and associated connecting wires must be electrically insulated from each other and from the sheath except when a grounded (earthed) thermoelement is specified. Such insulation can take various forms including mineral insulation, wires sleeved in suitable coverings such as glassfibre and ceramic insulators.
Ceramic Sheaths with thermocouple elements
Ceramic tubes, with their comparatively poor mechanical properties, are used when conditions exclude the use of metal, either for chemical reasons or because of excessive temperatures. Their main applications are ranges between 1000 and 1800°C. They may be in direct contact with the medium or may be used as a gastight inner sheath to separate the thermocouple from the actual metal protection tube. They should be mounted in a hanging position above 1200°C to prevent distortion or fracture due to bending stresses. Even hair-line cracks can lead to contamination of the thermocouple resulting in drift or failure. The resistance of the ceramic to temperature shock increases with its thermal conductivity and its tensile strength and is greater for a smaller thermal expansion coefficient. The wall thickness of the material is also important; thin-walled tubes are preferable to larger wall thicknesses.
Cracks are frequently produced by subjecting the protection tubes to excessively rapid temperature changes when they are quickly removed from a hot furnace. The use of an inner and outer sheath of gas-tight ceramic is therefore advisable. The outer thin-walled tube protects the inner one against temperature shock through the air between them. This lengthens the life of the assembly but results in slower response.
In the case of rare metal thermocouples the ceramic has to be of very high purity. Platinum thermocouples are very sensitive to contamination by foreign atoms. Special care must therefore be taken with fittings for high-temperature measurements to ensure that insulation and protection tube materials are of high purity. Platinum wire must be handled with great care to avoid contamination; grease and metallic contaminants will present a threat at elevated temperatures. Many refractory materials including Aluminium Oxide (Alumina) and Magnesium Oxide (used as an insulant) become electrically conductive at temperatures above 1000°C. The use of high purity materials results in better insulation at elevated temperatures; multi-bore insulators in high grade recrystallised Alumina provide the best solution for thermoelement sleeving. The insulation behaviour of ceramics mainly depends upon their alkali content; the higher the alkali content, the higher the electrical conductivity becomes at even lower temperatures (800°C plus). Ceramics of pure Alumina display the best properties.