Thermocouple Insulation

Contents

1. Standard Insulating Materials
2. Ceramic Insulators
3. Mineral Insulated Thermocouples (MIMS)
4. Advanced Sheath Materials and Drift Minimisation
5. Summary

While thermocouples can, in some cases, operate without insulation, most real-world applications require them to be protected from the media and environment in which they are used. This protection typically involves both electrical insulation and a mechanical sheath. Protective sheaths not only insulate the thermocouple electrically but also guard against mechanical damage, chemical corrosion, and thermal stress.

As a general guideline, exposed thermocouple junctions should be reserved for the measurement of static or flowing non-corrosive gas temperatures - specifically where fast thermal response is essential. In more demanding environments, such as those involving corrosive gases or liquids, an insulated junction is more appropriate. While this may reduce response speed, it significantly improves durability and safety. For even greater resilience and speed of response under pressure or in corrosive settings, grounded (earthed) thermocouple junctions, where the wires are welded to the sheath tip, are typically preferred.

Standard Insulating Materials

A wide range of standard insulating materials is used in thermocouple construction. These are often colour-coded according to thermocouple type per IEC 60584-3 guidelines. Although there is no unified international standard for insulation materials, engineering practice has led to the adoption of six core materials:

• PVC: Suitable from -30°C to +105°C and available in various constructions.
• PFA: Extends the range from -273°C up to +250°C (or 300°C for short durations), making it ideal for extreme environments.
• Varnish-impregnated glass fibre: Operational from -50°C to +400°C.
• Unvarnished glass fibre: Extends that range to 500°C, and in some cases up to 800°C.

All standard thermocouple types can be used with these materials, making them highly versatile.

Ceramic Insulators

For higher temperature industrial applications, ceramic insulators are widely used. The options vary depending on the thermocouple type and temperature range:

• Porcelain ceramic twin-bore beads: Often used for base metal thermocouples with wires of 1 mm or more.
• Mullite (aluminium silicate): Commonly paired with Type K thermocouples, especially in furnace environments.
• High-purity alumina: Preferred for platinum-based thermocouples to minimise contamination risks.

Mineral Insulated Thermocouples (MIMS)

The most widely adopted form of thermocouple in modern industry is the mineral insulated, metal sheathed (MI or MIMS) construction. These sensors consist of thermocouple wires embedded within a compacted mineral powder, typically magnesium oxide, housed inside a seamless metal sheath. This structure provides:

• Excellent mechanical strength
• Hermetically sealed protection
• High insulation resistance
• Small, flexible form factor
• Resistance to vibration, high pressure, and aggressive environments

MIMS thermocouples operate over an extended temperature range of -200°C to +1250°C and are available with 2 to 6 cores, in diameters from 0.25 mm to 10.8 mm.

They also support a wide variety of thermocouple types and sheath materials including stainless steel, mild steel, Inconel, and Nicrotherm alloys. Measuring junctions can be:

• Exposed: Offers the fastest response
• Insulated: Electrically isolated to prevent ground loops
• Grounded: Welded to the sheath tip for fast response and mechanical durability

MIMS thermocouples are not only widely available and cost-effective but can be customised for specific environments using different sheath and termination options. They are particularly suited to extreme industrial conditions.

Advanced Sheath Materials and Drift Minimisation

Despite their advantages, MIMS thermocouples can face challenges:

• Differing thermal expansion rates between sheath materials (like stainless steel) and thermocouple wires (such as Types K and N) may lead to fatigue failure.
• Vapour diffusion of sheath material into the thermocouple wires can result in contamination, calibration drift, and performance degradation.
• Water ingress during manufacture or repair can cause reduced insulation resistance and premature failure.

To address these challenges, specialised sheath alloys have been developed:

• Nicrotherm: A high-temperature, low-drift sheath alloy optimised for use with Type N thermocouples.
• Nicrobell B: A modified Nicrosil alloy with added magnesium and niobium to improve oxidation, thermal, and corrosion resistance up to 1250°C.
• Nicrobell C: Enhances Nicrobell B with increased chromium for superior corrosion resistance.
• Nicrotherm D™: The latest development, engineered with reduced manganese and aluminium to minimise element diffusion through the insulation. It offers excellent resistance to carburising environments and delivers significantly lower drift than Inconel 600 or 310 stainless steel at high temperatures.

For very high-temperature or corrosive applications, it is generally recommended to avoid very small diameter MIMS cables, as survival and performance are closely correlated with cable size.

Summary

Thermocouple insulation plays a critical role in ensuring measurement accuracy, mechanical protection, and environmental resistance. From basic PVC and glass fibre coverings for low to medium temperatures, to high-performance ceramic and mineral-insulated constructions for extreme conditions, the choice of insulation and sheath materials must be matched to the application's temperature range, media, and mechanical demands. Advanced sheath alloys like Nicrotherm D™ now provide superior long-term stability and resistance in high-temperature or corrosive environments, ensuring reliable performance even under the most demanding conditions.

Further Reading

What are the various thermocouple types?
Explore the features and characteristics of the various thermocouple types

Thermocouple Output Tables
View EMF versus Temperature tables for all thermocouple types.

What are the thermocouple colour codes?
Explore thermocouple colour codes for cable and connectors.