Heavy Duty Industrial Assemblies

Contents

1. Sheath Materials
2. Mechanical Fittings
3. Temperature Capabilities
4. Insert Thermocouple Assemblies
5. Protective Sheaths
   1. For Thermocouples
   2. For RTDs
6. Ceramic Sheaths for High Temperatures
7. Thermowells
8. Summary

For demanding industrial environments, heavy-duty sensor assemblies offer robust protection and high-temperature performance. These weatherproof solutions are typically paired with thick-wall protective sheaths, available in both straight and angled designs, for single or multi-point measurements.

Sheath Materials

The choice of sheath material depends on the operating environment and temperature range. Common options include:

• Cast Iron and Mild Steel
  Cost-effective for general-duty applications.
• Stainless Steel Alloys
  Provides good corrosion and oxidation resistance.
• Nickel-Chromium, Iron-Nickel, Chromium-Iron Alloys
  Selected for higher temperature or corrosive conditions.

Mechanical Fittings

Assemblies can include:

• Adjustable flanges
• Welded or adjustable bushes

These fittings allow for flexibility in installation and maintenance.

Temperature Capabilities

• Up to 1,250°C with standard metal-sheathed thermocouples.
• Up to 1,600°C using ceramic protection sheaths, available in:
• Aluminous porcelain
• Recrystallised alumina
• Mullite
• Silicon carbide

Insert Thermocouple Assemblies

Insert types are compatible with all common terminal heads and sheath configurations, including:

• Metal protection tubes
• Secondary ceramic sheaths
• Thermowells

They often feature:

• Integrated end seals
• Ceramic terminal blocks, some with spring-loaded connectors for optimal tip contact
• Optional head-mounted transmitters

Protective Sheaths

Sheaths protect the sensor from harsh environments while allowing accurate temperature sensing. The material selection is critical and varies depending on the sensor type and environment.

For Thermocouples

• Metal Tubes (up to 1,250°C) with base metal sensors.
• Alumina Liners (up to 1,600°C) with platinum thermocouples.

Common Materials and Temperature Ratings

For RTDs

RTDs are sensitive to contamination at high temperatures, especially from base metals. As such, specialised materials are used:

• 316 Stainless Steel – Corrosion-resistant
• 310 Chromium-Nickel Steel – Good in sulphur-bearing atmospheres
• Inconel 600 / Incoloy 800 – High corrosion resistance, not for sulphur at high temps
• Nickel and Nickel Alloys – For corrosive applications
• Fluoroplastic Coatings – For corrosive chemical environments

Ceramic options include recrystallised alumina, used where higher temperatures or chemical inertness are required.

Ceramic Sheaths for High Temperatures

For extreme temperatures or corrosive environments where metals fail, ceramic sheaths are used:

Standard outer diameters range from 4mm to 50mm, with lengths up to 2+ metres available.

Thermowells

Thermowells are closed-end, re-entrant protection tubes designed for inserting thermocouples or RTDs into process environments. They offer excellent mechanical protection and process isolation, allowing sensor removal without system interruption.

Construction and Design

• Typically feature a taper-threaded male process connection and a female sensor entry.
• Designed to minimise air gap for better thermal response and atmospheric sealing.
• May include a reduced bore tip for improved sensor contact and faster response.

Materials and Temperature Ratings

These materials are selected based on the same environmental constraints as for protection sheaths.

Summary

• Metal Protection Sheaths (up to ~1,250°C): Common in industrial applications using base metal thermocouples; material selection includes stainless steels, Inconel 600, Incoloy 800, and more.
• Ceramic Protection Sheaths (up to ~1,800°C): Used with platinum thermocouples or in corrosive, high-temperature environments. Recrystallised alumina, mullite, and silicon carbide each offer unique advantages.
• Insert Assemblies: Compatible with metal and ceramic sheaths, with head-mounted terminal blocks or transmitters. Spring-loaded designs improve contact and thermal transfer.
• RTD Applications: Require careful material selection due to platinum sensitivity; Inconel, Incoloy, and fluoroplastic-coated sheaths are common choices.
• Thermowells: Offer mechanical protection for sensor elements in process environments. Standardised designs available in brass, stainless steels, Inconel, and Incoloy.

Tip: Material and design choices should be driven by required temperature range, atmosphere type (oxidising, reducing, corrosive), mechanical stress, and sensor type.

Further Reading

RTD vs Thermocouple – Choosing the Right Sensor
Explore the features and characteristics of thermocouples and RTDs

RTD Output Tables
View Resistance versus Temperature tables for all Pt100 sensors.

What are the RTD colour codes?
Explore RTD colour codes and wiring configurations.