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An RTD (resistance temperature detector) measures temperature through resistance variation; a thermocouple generates a measuring voltage. Each has its strengths — the wrong choice wastes time and money.
| Comparison dimension | ✅ RTD（Pt100 / Pt1000） | ✅ Thermocouple (K, N, etc.) |
|---|---|---|
| accuracy | ±0.1°C (Class AA) — best | ±1.5°C (Type K) — good |
| stability | Excellent long term, minimal drift | Drifts with time (especially >1000°C) |
| Linearity | Resistance is nearly linear with temperature | Non-linearity (requires polynomial conversion) |
| Measuring Range | -200°C ~ +850°C（typicalupper limit） | -200°C to +1350°C (Type K), up to +2320°C (Type C) |
| Response speed | Slow (platinum element has large thermal mass) | Fast (small thermal mass, especially bare junction) |
| cost | 2–3 times more expensive than a thermocouple | Cheap (typical industrial sensor £10–50) |
| wiring | Requires 3-wire or 4-wire connection (compensates lead-wire resistance) | 2-wire only, simple |
| Power consumption | Requires excitation current (1–5 mA) | Self-powered (generates its own voltage) |
| Self-heating | Excitation current causes self-heating error (approx 0.1-0.5 C) | None (passive sensor) |
| application | Why use an RTD |
|---|---|
| pharmaceutical / foodverify | Regulatory requirement of ±0.1°C accuracy (FDA 21 CFR Part 11, EU Annex 11). Only an RTD can achieve it. |
| Environmental test chambers <200°C | RTDs have excellent low-temperature accuracy, no cold-junction error, and long-term stability. |
| Laboratory reference standard | The SPRT (standard platinum resistance thermometer) defines the ITS-90 scale with traceable calibration. |
| processcontrol <600°C | Process requirement of ±0.2°C control (e.g. polymer extrusion, chemical reactors); an RTD pays for itself. |
| Long service life required | An RTD lasts 10–20 years with slight drift; a thermocouple drifts significantly within 1–2 years at high temperature. |
| application | Why use a thermocouple |
|---|---|
| High temperature >850°C | RTDs fail above 850°C; only thermocouples work at furnace temperatures (1000–1600°C). |
| Fast response required | A thermocouple responds in 0.1–1 s (bare junction); an RTD needs 5–30 s. Critical for transient measurement. |
| Vibration / mechanical shock | Thermocouples are robust (mineral-insulated sheath); RTDs are brittle (platinum element cracks easily). |
| Small probe dimensions | A thermocouple can be made 0.5 mm dia. (minimal thermal mass); an RTD is at best about 3 mm. |
| Budget constrained | Thermocouple £10–50, RTD £50–200. With 100+ sensors the cost difference is significant. |
| Long cable runs | Thermocouple extension wire is cheap (£0.50/m); 3-wire RTD cable is expensive (£5/m). Over 100 m a thermocouple saves £450. |
🔹 2-wire RTD(Not recommended — lead-wire resistance causes error): Sensor ────────┐ ────────┘ The controller applies a current and measures the voltage. Issue: lead-wire resistance adds to the RTD resistance. Error: every 1Ω of lead-wire resistance adds 0.4°C (typi…
🔹 3-wire RTD(Standard — compensates lead-wire resistance): Sensor ────────┐ ────────┤ Controller ────────┘ The third wire lets the controller measure and subtract the lead-wire resistance. Assumes the three wires have equal resistance (true when in the same cable at the same temperature). Error:…
🔹 4-wire RTD(Best — completely eliminates lead-wire resistance error): Sensor ════════════┐ ════════════┤ Controller (independent sense and force pairs) ────────────┤ ────────────┘ Current flows through one pair; voltage is measured across the other (no current = no voltage drop). Error…
| Parameter | Pt100 | Pt1000 |
|---|---|---|
| 0°C Resistance | 100.00 Ω | 1000.00 Ω |
| 100°C Resistance | 138.51 Ω | 1385.06 Ω |
| Sensitivity | 0.385 Ω/°C | 3.85 Ω/°C (signal 10 times stronger) |
| lead wireResistanceerror | Higher (lead-wire resistance is a large share of total resistance) | Lower (resistance 10 times higher, so lead-wire error is 1/10) |
| Self-heatingerror | Lower (same voltage needs less current) | Higher (more current, more I²R heating) |
| Typical applications | Standard, most commonly used, works well with 3-wire or 4-wire connection | For long cable runs (>50 m); can be 2-wire with less error than a 3-wire Pt100 |