You are here → Home > Guide to Themocouple and Resistance Thermometry > Installing Thermocouples and RTDs

Installing Thermocouples and RTDs

First published 1993. Latest revision 2025.

In many applications, thermocouples and RTDs require protection from harsh environments to ensure long-term performance and accuracy. While some setups may not need protective sheaths, most benefit from metal or ceramic protection tubes, or thermowells designed to relevant standards (e.g. BS 2765, BS 1041, or industry-specific specifications for chemical and petrochemical use).

Installation Guidelines: Thermocouples

Immersion Depth and Sensor Integrity

For accurate readings, thermocouples should be immersed to a depth of at least 10 times the diameter of the sensor. Thermoelements must be properly insulated from each other and from temperature gradients outside the measurement zone. Avoid cold working the wires, as this can affect calibration.

Environmental Considerations

Contaminants such as oils (especially those containing sulphur), phosphorus, and low melting-point compounds can quickly degrade thermocouple performance. Keep the installation area as clean as possible.

Service Life and Calibration

Base metal thermocouples are generally not intended for long-term use in demanding applications. Regular replacement is recommended—typically every 6–12 months—depending on operating conditions. Periodic in-situ calibration checks can help maintain accuracy.

Correct Cabling and Polarity

Always follow the correct polarity and color coding for thermocouple and extension or compensating cables. Avoid mixing different metals in connections. Use properly matched cables and connectors for optimal signal integrity.

Junction Placement and Cable Routing

Position the transition between thermocouple wire and extension cable away from heat sources. Never form thermo-junctions using compensating cable. Even with extension cable, this is not recommended.

Reference Junctions

Reference (cold) junctions should be maintained in an isothermal environment. Compensation can be handled electronically or with temperature indicators.

Connections and Signal Integrity

Low-level thermocouple signals demand tight, clean, oxide-free electrical connections. Protect wiring from moisture and mechanical stress, as these can cause errors from leakage or induced EMFs.

EMI and Cable Protection

In environments with electrical interference (e.g. contactors or AC noise), use screened or braided cable with proper grounding. Shielded cables, metal or plastic conduit, and moulded or armored insulation are recommended in industrial settings.

Installation Tests

Use water immersion, gas pressurization, leak detection (e.g. helium), or resistance checks to verify integrity. For long cable runs, avoid using excessively thin wires—modern instruments generally tolerate up to 100 ohms resistance, but higher values may lead to measurement drift.

Installation Guidelines: RTDs

Sensor Design and Handling

RTD elements represent a trade-off between measurement precision and mechanical durability. Avoid excessive vibration and mechanical shock during installation—particularly with long probes or thermowells with oversized bores, which can reduce thermal contact and increase failure risk.

Immersion Depth and Heat Conduction

Though RTDs are stem-sensitive, correct immersion depth is still crucial for accurate readings (see IEC 60751). Also account for heat conduction along the sheath or leads, which can skew measurements.

Sheath Requirements

Use appropriate protective sheaths to guard against contamination, especially platinum poisoning. Ensure sufficient oxygen presence inside the sheath to prevent metal reduction and contamination.

Correct Wiring and Interference Mitigation

Follow wiring color codes and terminal markings. Use screened, twisted cable with continuous braiding or conductive conduit in electrically noisy environments. Avoid running sensor cables parallel to power lines.

Cable Construction and Resistance Issues

Single-strand wire can fail suddenly, while multi-strand wires can develop stray resistances—particularly problematic in bridge circuits. Route cables to minimize the impact of ambient temperature changes on resistance. For mechanical protection, use braided or armored cable.

Lead Resistance and Wiring Configuration

Use 3- or 4-wire configurations for longer runs to balance lead resistances and maintain accuracy. Two-wire setups are prone to increased error as cable length increases. Ensure total loop resistance remains within your instrument’s tolerance—typically around 100 ohms.

Circuit Isolation and Thermoelectric Effects

Circuits should generally be floating or use resistive earthing to avoid static issues. Avoid mixed metals in wiring to prevent thermoelectric voltages (a concern unless AC excitation is used).

Connection Quality and Maintenance

Use high-quality RTD connectors and weather-sealed junction boxes. Secure all connections to guard against vibration, corrosion, or temperature cycling. Poor or unstable links will introduce resistance variation—directly impacting your measurements.

Note: The information in this guide is provided for general informational and educational purposes only. While we aim for accuracy, all data, examples, and recommendations are provided “as is” without warranty of any kind. Standards, specifications, and best practices may change over time, so always confirm current requirements before use.

Need help or have a question? We’re here to assist — feel free to contact us.