The accuracy of a thermocouple depends on much more than just the wire junction that is doing the sensing. It is the sheath that determines whether or not a thermocouple’s reading will remain accurate over several months. If the sheath is wrong, even the best thermocouple can drift, corrode, or fail prematurely, no matter how good the sensing elements are.
What a Thermocouple Sheath Does
The sheath is an outer protective layer that wraps around the thermocouple and junction wires. The sheath protects the sensing element against whatever process is thrown at it, whether that be corrosive gases, molten steel, mechanical vibrations, high pressure, or just repeated thermal cycling. It is necessary because the thin wires used to make the junction will degrade very quickly in industrial settings.
The sheath is used in both mineral and plastic insulated designs. The sheath encloses both the wires and a densely packed insulating powder (usually magnesium oxide) to form a single sealed wire rather than two separate tubes. This construction is used in the Tempsens MI thermocouple line. It gives a compact, flexible assembly that can withstand vibrations and tight spaces.
The Sheath Material Should Be Determined by Several Factors
Temperature Range of the Process
Sheaths are made from different materials and have varying upper limits. Sheaths rated at 600degC won’t hold up to a furnace that runs closer to 1600degC. This is the first to be applied, as it will immediately reduce the options available before any other considerations.
Chemical Exposure and Corrosion Hazard
Certain processes use acidic or neutral media, sulfurous atmospheres, or oxidizing gases, which all attack metals more quickly than others. If this is not taken into account, a sheath can corrode inside-out well before it reaches its service life rating. This could lead to sensor drift and/or failure.
Mechanical Vibration and Stress
Sheaths for thermocouples that are installed near rotating machinery, high-velocity flows, or thermal shocks need to be able to withstand mechanical stress. Ceramic sheaths are brittle and can crack under mechanical stress. They should be mounted carefully to prevent bending.
Response time requirements
The reaction time of a thermocouple to temperature changes is slowed down by a thicker sheath, or by adding a protective layer. When a fast response is required, as in combustion monitoring or tight tolerance process control loops, a thin sheath or smaller diameter MI cable construction will be preferred to a bulkier installation.
Cost and service life trade-offs
Sheath materials made of exotic metals such as tantalum and rhenium are significantly more expensive than stainless steel grades. They are only necessary in extreme environments or for highly reactive atmospheres. A well-chosen nickel alloy or stainless steel sheath is sufficient for most industrial heating, monitoring, and control applications.
Thermocouple Sheath Materials
Thermocouples made of base metal (Types J, K, T, E, and N).
The most common thermocouples used in industrial settings are made from nickel, iron, or copper alloys. There are many sheath materials available, including SS316 and SS310. SS347 and SS446, and Inconel 625, 800, and 605. Type K thermocouples cover a temperature range of -200degC up to 1,260degC. However, the practical limit depends on which sheath is used. They are used in steel mills and boilers as well as chemical reactors, refineries and cement kilns.
Thermocouples made of noble metal (Types S, R, B).
These thermocouples are made from platinum or platinum-rhodium and can withstand much higher temperatures – up to 1,750degC or 1,950degC, depending on the configuration – and drift less over time than base metal types. These thermocouples, which are made of platinum, are usually housed in ceramic tubes with high purity, as metal contact can introduce impurities and affect accuracy. Tempsens provides these with Pt/Pt alloy thimble-ceramic tubes that are designed to eliminate this contamination risk.
Thermocouples made of metal (Types C D G)
Refractory thermocouples are made of exotic metals like rhenium and tungsten for the highest temperatures. Platinum, rhodium, tantalum, rhenium, molybdenum, and specialized ceramics are available as sheaths for this level. Standard stainless steel alloys or nickel alloys cannot withstand these temperatures. These metals can be more difficult to work with and are more expensive, so they are reserved for more demanding applications.
Mineral Insulated Construction (MI)
The wire pair, magnesium oxide insulation, and a drawn metal sheath are wrapped in an MI construction, regardless of the type of thermocouple. These sheaths can be made from stainless steel or other materials with diameters between 0.75mm and 8mm. The result is a flexible cable-like assembly which resists mechanical and moisture damage better than non-MI builds. However, the whole assembly must be replaced if the sensor fails because the wires and the sheath are one sealed unit.
Standards to Check
The performance of a thermocouple assembly that is built according to standards recognized by the industry can be more confidently guaranteed. Tempsens manufactures its thermocouples according to IEC 60584, ASTM E230, and ANSI MC96.1, for EMF output, material consistency, and certifications like ISO 17025. It is reasonable to check that the manufacturer’s claims about sheaths and construction are supported by these standards before placing a large order.
Final Thoughts
The choice of sheath material is not something that should be left until the type of thermocouple has been determined. The right sheath will balance the different factors, such as temperature range, chemical exposure, and mechanical stress. It is better to choose the material that best suits the conditions of your installation, rather than choosing the cheapest option or the one you are most familiar with. When you work with a manufacturer who can clearly explain the trade-offs, it is more likely that the sensor will last as long as required by the process, as opposed to one that must be replaced before its service life has ended.
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