A Resistance Temperature Detector (RTD) measures temperature by measuring the electrical resistance of a pure metal element — resistance increases predictably and near-linearly with temperature. Platinum RTDs (Pt100 and Pt1000) are defined by IEC 60751 with a temperature coefficient of resistance (TCR) of 0.00385 Ω/Ω/°C — the most accurate and stable temperature sensors for industrial and laboratory use from −200°C to +850°C. PT100 has a resistance of 100 Ω at 0°C; PT1000 has 1,000 Ω — PT1000 is preferred in applications with long cable runs or high-noise environments as the larger signal reduces cable resistance effects.
The PT100/PT1000 sensing element is a coiled platinum wire wound on a ceramic mandrel, or a thin-film platinum track deposited on a ceramic substrate — the thin-film type is faster and more compact. The element is typically installed in a stainless steel protection tube (Pt insert). A 4-wire connection is used for highest accuracy: two wires carry the measurement current; two separate wires measure the voltage across the element only — eliminating lead resistance error entirely. 3-wire connection is standard for most industrial applications (lead resistance compensation by Wheatstone bridge). Head-mounted transmitters convert the Ω reading to 4–20 mA, providing a low-impedance, EMI-immune signal for long cable runs.
Challenges
Lead Resistance Error in 2-Wire RTD Circuits (Long Cable Runs in Cold Climates)
Cable resistance adds directly to the 100 Ω sensor reading — 1 Ω of cable resistance represents 2.5°C error in 2-wire circuit.
Self-Heating Error from Excessive Measurement Current (>1 mA)
Measurement current flowing through the element generates I²R heat — self-heating error of 0.1–0.5°C per mW in still media.
Vibration-Induced Strain on Wire-Wound Elements Changing Resistance
Mechanical vibration strains platinum wire winding — resistance changes without temperature change; drift and failure.
Moisture Ingress into Connection Head Causing Insulation Resistance Leakage
Water in the connection head causes leakage current between leads and to ground — measurement shift; insulation resistance <100 MΩ causes error.
PT100 vs PT1000 Selection Error on Long Cable Runs
Using PT100 with 2-wire connection on 50 m cable at 0.2 Ω/m = 10 Ω added = +25°C error — PT1000 reduces this to +2.5°C.
Solutions
- 4-Wire RTD Circuit for All High-Accuracy Measurements (±0.1°C or Better): 4-wire circuit eliminates lead resistance error entirely — mandatory for laboratory, calibration, and precision control service.
- Maximum Excitation Current 1 mA for Standard Elements (0.5 mA for Highest Accuracy): Limit excitation current per IEC 60751 — minimum self-heating; accurate reading in still-air and low-thermal-mass service.
- Thin-Film PT100 Element (Fast Response, Low Vibration Sensitivity) for Vibrating Machinery: Thin-film element is mechanically robust — deposited on ceramic substrate; no wire to fatigue under vibration.
- Hermetically Sealed Connection Head (IP 68) + Gel-Filled Terminal Block: IP 68 head and filled terminal block prevent moisture ingress — insulation resistance maintained >100 MΩ.
- PT1000 for All Cable Runs >20 m: Higher nominal resistance means lead resistance represents a smaller percentage of signal — 10× less sensitive to cable resistance than PT100.
Applications
- Pharmaceutical: PT100 in thermowell on bioreactors, autoclaves, and WFI systems — UKAS-calibrated; 21 CFR Part 11; ±0.1°C accuracy.
- Food & Beverage: Sanitary PT100 (Tri-Clamp, Ra <0.8 µm) on pasteurisers, CIP systems, and fermenters — 3-A certified.
- HVAC: PT100 or PT1000 for room temperature, duct air temperature, and chilled/hot water measurement.
- Precision Laboratory: PT1000 4-wire connection with low-noise transmitter — reference temperature measurement ±0.01°C.
- Chemical Process: PT100 in SS thermowell on reactors, heat exchangers, and distillation columns — −200°C to +600°C range.
Models & Capacities
RTD / PT100 / PT1000 — Models & Specifications | |||||||||||
Model / Type | Element Type | Nominal Resistance | Temp Range [°C] | IEC 60751 Class | Accuracy [°C] | Wire Config. | Probe OD [mm] | Probe L [mm] | Protection Tube Material | Output | Weight [kg] |
PT100 Class A (wire-wound) | Wire-wound coil | 100 Ω at 0°C | −200 to +850 | Class A | ±(0.15+0.002|T|)°C | 2,3,4-wire | 3–12 | 50–500 | SS 316L | Ω / 4–20 mA via TX | 0.1 |
PT100 Class B (standard) | Wire-wound coil | 100 Ω at 0°C | −200 to +850 | Class B | ±(0.30+0.005|T|)°C | 2,3,4-wire | 3–12 | 50–2,000 | SS 316L / Inconel | Ω / 4–20 mA | 0.1 |
PT100 Class AA (high accuracy) | Wire-wound coil | 100 Ω at 0°C | −50 to +250 | Class AA | ±(0.10+0.0017|T|)°C | 4-wire preferred | 3–8 | 50–300 | SS 316L / Pt sheath | Ω / 4–20 mA | 0.05 |
PT1000 (long cable runs) | Thin-film | 1,000 Ω at 0°C | −50 to +400 | Class B / A | ±(0.30+0.005|T|)°C | 2,3-wire | 3–8 | 30–300 | SS 316L | Ω / 4–20 mA | 0.05 |
PT100 Sanitary (hygienic) | Thin-film | 100 Ω at 0°C | −50 to +200 | Class A / B | ±(0.15+0.002|T|)°C | 3,4-wire | 4–8 | 50–300 | SS 316L Ra <0.8 µm | 4–20 mA + HART | 0.2 |
PT100 High-Temp (>500°C) | Wire-wound | 100 Ω at 0°C | 0 to +850 | Class B | ±(0.30+0.005|T|)°C | 3,4-wire | 6–12 | 100–1,000 | Inconel 600; ceramic | 4–20 mA | 0.3 |
PT100 Head-Mounted TX | Thin-film + TX | 100/1000 Ω | −200 to +600 | Class A | ±0.1°C (TX adds ±0.5°C) | 3,4-wire to TX | 4–12 | 50–500 | SS 316L | 4–20 mA + HART/FF | 0.3 |
Ni100 / Cu100 (non-Pt) | Wire-wound | 100 Ω at 0°C | −60 to +180 | DIN 43760 | ±0.3°C | 2,3-wire | 3–8 | 50–300 | SS 304 | Ω | 0.1 |