RTDs (e.g., Pt100/Pt1000) and thermocouples (T/C) dominate industrial temperature sensing with different physics, ranges, accuracy, and wiring needs. Senseiot supplies both across process plants, energy equipment, and environmental monitoring. This guide helps you choose maintainable, traceable solutions early in a project.
Principles and Construction
RTDs exploit the near-linear resistance change of platinum—100 Ω at 0 °C for Pt100. Constant-current excitation yields temperature with strong linearity from roughly -200 °C to +850 °C.
Thermocouples use the Seebeck effect: a junction of dissimilar metals produces millivolts versus a reference junction. Types K, J, T, and S cover different ranges beyond +1600 °C.
Need 0.1 °C accuracy? Prefer RTD. Ultra-high temperature or fast transients? Consider T/C. Specs in the Product Catalog.
Accuracy and Long-Term Stability
IEC 60751 classes A/B/AA define RTD tolerance—Class A ±0.15 °C at 0 °C for pharma and food. Low drift; calibrate every 1–2 years.
Thermocouple accuracy is ±°C or ±%—standard K type about ±1.5 °C over 0–1100 °C. High-temperature oxidation increases drift; schedule verification.
For NIST/CNAS traceability, four-wire RTD plus precision ADC simplifies uncertainty budgets; T/C needs cold-junction compensation and type-specific tables.
Range, Response, and Dynamics
Industrial RTD probes often stop near +600 °C (+850 °C special), with time constants of seconds—unsuitable for millisecond transients.
Fine-gauge T/C wires enable T90 < 1 s for flames and molds; S/R types serve furnace hot zones.
Match sensor dynamics to process speed—slow fluids suit RTD; pulsating flows need fine T/C. See Industry Applications.
Wiring: 2/3/4-Wire RTD and Cold Junction
Two-wire RTD suffers lead resistance; three-wire is standard in plants; four-wire Kelvin sensing is for lab grade.
Thermocouples require cold-junction compensation in transmitters or PLC modules—CJC error dominates system uncertainty.
Long runs: RTD with 4–20 mA transmitters; T/C needs extension-grade wire—never plain copper.
Environment and Mechanical Robustness
RTD elements are fragile—use thermowells or armored assemblies in vibration; shield differential measurement in EMI.
Metal-sheathed T/C handles shock better—compressors and exhaust; choose 316SS or Inconel in corrosive media.
Salt fog affects insulation and junction oxidation. Senseiot recommends armored probes in chemical zones—Request a Quote for details.
Cost, Maintenance, and Replacement
RTD plus transmitter costs more upfront but calibrates less often—ideal for critical PID loops like reactors.
T/C is cheap and quick to replace for dense or disposable points; only option beyond RTD limits.
Document type, class, and cal interval on P&IDs. Compare models in the Product Catalog.
Selection Workflow
Define range and tolerance; response and mounting; output (resistance, mV, 4–20 mA, Modbus); environment (vibration, corrosion, EMI, Ex rating).
IoT edge nodes can use digital RTD modules or T/C front-end ICs to reduce analog design risk.
Senseiot offers RTD/T/C modules, transmitters, and Industry Application packages with custom sheath lengths.