RTD Sensor Assemblies

Contents

1. Basic Sheathed RTD Assemblies
2. Mineral Insulated RTD Assemblies (MI or MIMS)

Platinum RTD sensor assemblies are widely used across industries and laboratories for precise and reliable temperature measurement. These assemblies are typically supplied ready-to-install, combining the RTD sensing element with a protective sheath, internal wiring, end seals, and terminal options such as connector heads or blocks. A wide variety of RTD designs are available to suit applications from general-purpose industrial use to harsh and corrosive environments.

Basic Sheathed RTD Assemblies

General-purpose sheathed RTD assemblies are designed for a broad temperature range, typically from -100°C to +350°C, though custom specifications are common. These cost-effective designs typically use platinum RTD elements with a nominal resistance of 100 ohms at 0°C, complying with IEC 60751 standards—most commonly Class B, though higher-accuracy Class A and even ±0.01% tolerance elements are available.

The sensing elements are typically wire-wound pure platinum, enclosed in high-purity ceramic or glass for environmental protection. Alternatively, thin-film or thick-film RTD elements can be used, depending on application needs.

These RTD assemblies are available in single, duplex, or triplex configurations, supporting 2-wire, 3-wire, and 4-wire output formats. The 4-wire versions may be supplied in blind loop or compensated formats, making them suitable for bridge and potentiometric measurement systems. Lead wires are usually copper with high-temperature insulation such as Kapton.

Design features include:

• Minimum immersion depth: approximately 50mm
• Recommended excitation current: <5mA
• Insulation resistance: >100 MΩ at 240V (ambient conditions)

Sheath options include reduced tip diameters, thin wall sections for faster response, and pierced shrouds for air and gas measurement. Internal voids are packed with inert materials to enhance thermal conductivity and mechanical strength. Sheaths are hermetically sealed to protect against moisture, corrosion, and vibration.

Standard sheath diameters range from 1.5mm to 13mm and are available in a wide variety of lengths and bend configurations. Common sheath materials include 316 stainless steel, Inconel 600, Incoloy 800, nickel alloys, and fluoroplastic-clad options for aggressive chemical environments.

End sealing options are also broad, ranging from basic laboratory-style connectors and quick-release plugs to heavy-duty industrial heads with thermowells, head-mounted transmitters, and ruggedized protection tubes.

Mineral Insulated RTD Assemblies (MI or MIMS)

Mineral insulated RTDs offer high durability and excellent thermal performance, combining a compact, hermetically sealed sheath with magnesium oxide (MgO) powder for insulation. These assemblies closely mirror the construction of mineral insulated thermocouples, offering superior protection in harsh or high-vibration environments.

Key advantages include:

• High mechanical strength and flexibility
• Excellent insulation resistance (>100 MΩ)
• Long-term accuracy and stability
• Fast response times
• Resistance to vibration, pressure, vacuum, and corrosion

Typical operating ranges extend from -100°C to +600°C, using 100-ohm platinum RTDs (IEC 60751 Class B) in single or duplex configurations. High-accuracy elements (Class A, 1/3, 1/5, and 1/10 DIN) are available on request. Sensor elements are often platinum wire in ceramic or glass, though thin-film sensors are also supported.

As with standard sheathed RTDs, these assemblies support 2-wire, 3-wire, and 4-wire configurations. Energizing current should not exceed 5mA, and minimum immersion depth is generally 50mm.

Sheath construction is robust, typically with wall thicknesses around 15% of the outer diameter. This allows bending to a radius of 12x the diameter (or down to 4x with a mandrel). Standard diameters include 3mm, 6mm, and 8mm, and assemblies can be custom-built to suit length and shape requirements.

Common sheath materials include:

• 316 stainless steel – corrosion-resistant for general applications
• Inconel 600 – suited for extreme environments and carburizing atmospheres (but not sulphur-rich atmospheres at high temperature)
• Fluoroplastic coatings – available for corrosive or chemically aggressive settings

Seal and sheath terminations are available in a wide range of styles, matching standard industrial fittings and enclosure types.

⚠️ Important Note: One critical consideration for MI RTDs is moisture ingress. If water vapor penetrates the sheath, it can degrade insulation resistance and cause sensor failure. This is avoidable with proper manufacturing quality control and by specifying robust sealing during procurement.

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.