0 Comments

1) What is a PT100?

A PT100 is a platinum Resistance Temperature Detector (RTD) whose resistance is 100 Ω at 0 °C. Its resistance increases linearly with temperature, providing high accuracy, stability, and low drift from −200 °C to +600…850 °C (range depends on element type). Compared with thermocouples and thermistors, PT100s offer an excellent balance of precision, repeatability, and ruggedness for industrial and building applications.

2) International Standards and Curves

  • Primary standard: IEC 60751 / EN 60751

    • Nominal R₀ = 100 Ω at 0 °C

    • Temperature coefficient α = 0.00385 Ω/Ω/°C (European curve)

  • Legacy/alternative: α = 0.003916 (older U.S. curve).

    • Winsen default is IEC 60751 (α = 0.00385) unless otherwise specified.

Callendar–Van Dusen equation (IEC 60751):

For 0…+850 °C:

R(t)=R0(1+At+Bt2)R(t) = R_0 \left(1 + A t + B t^2 \right)

For −200…0 °C:

R(t)=R0[1+At+Bt2+C(t−100)t3]R(t) = R_0 \left[1 + A t + B t^2 + C(t-100)t^3 \right]

with constants:

A=3.9083×10−3,B=−5.775×10−7,C=−4.183×10−12A = 3.9083\times10^{-3},\quad B = -5.775\times10^{-7},\quad C = -4.183\times10^{-12}

Reference points (α = 0.00385):

  • 0 °C → 100.00 Ω

  • 100 °C → ≈ 138.51 Ω

  • −100 °C → ≈ 60.26 Ω

3) Tolerance Classes (Interchangeability)

Per IEC 60751 (typical limits of error at temperature t in °C):

Class Tolerance (°C) Notes
AA ±(0.10 + 0.0017|t|) Highest standard accuracy; shorter range
A ±(0.15 + 0.002|t|) High accuracy, common in process/LAB
B ±(0.30 + 0.005|t|) Cost-effective, robust
C ±(0.60 + 0.010|t|) General purpose / wider range

Example: At 100 °C, Class A ≈ ±(0.15 + 0.2) = ±0.35 °C; Class B ≈ ±0.80 °C.

4) Element Types and Operating Ranges

Element Construction Typical Range Pros Considerations
Wire-wound Platinum wire on ceramic/glass core −200…+600/850 °C Best stability & high-temp range Slightly larger, higher cost
Thin-film (chip) Sputtered Pt on ceramic substrate −50…+150/200 °C Compact, fast response, economical Narrower range, slightly more drift

5) Wiring Methods: 2-, 3-, 4-Wire

Method Description Accuracy Impact Typical Use
2-wire Sensor in series with two leads Lead resistance adds error Short leads, low-cost HVAC
3-wire Adds a third lead to compensate equal lead resistances Industry standard for field wiring PLC/DCS inputs; moderate accuracy
4-wire (Kelvin) Separate current and sense pairs Best accuracy; lead-independent Labs, custody transfer, calibration rigs

6) Electrical Excitation & Self-Heating

  • Use constant-current excitation (typ. 0.1–1.0 mA) or ratiometric bridge.

  • Self-heating: ∆T ≈ P × (K/W) where P = I²R.

    • Typical self-heating coefficients: 0.05–0.4 K/mW (depends on medium and flow).

  • Minimize error by: low excitation current, pulsed/duty-cycled measurement, and good thermal coupling to the medium.

7) Response Time & Mechanics

  • Response time (t₀.₉): function of sheath diameter, flow velocity, and installation.

    • Example: 3 mm tip in moving water can reach t₀.₉ ≈ 3–8 s; in still air significantly longer.

  • Sheath materials: SS304/316L, Inconel, PTFE-coated for corrosives.

  • Probe diameters: 3 mm / 4.5 mm / 6 mm / 8 mm common.

  • Ingress protection: up to IP65–IP68 with proper potting and cable glands.

8) Typical PT100 Resistance Table (α = 0.00385)

°C Ω °C Ω °C Ω
−50 80.31 0 100.00 50 119.40
−25 90.19 25 109.73 75 129.07
−10 95.48 40 115.54 100 138.51

(Indicative; use CVD equations for precise computation or request Winsen look-up tables.)

9) PT100 vs Thermocouple vs Thermistor

Attribute PT100 (RTD) Thermocouple (e.g., Type K) NTC Thermistor
Accuracy High (Class A/B) Moderate (needs CJC) High near set-point
Stability/Drift Excellent Good–moderate Moderate (aging)
Range −200…+600/850 °C Up to 1200–1300 °C Narrow (−40…+150 °C typical)
Linearity Good Fair Nonlinear (steep)
Signal Level 0–300 Ω (needs conditioning) μV level (needs amp/CJC) kΩ–Ω (simple divider)
Wiring 2/3/4-wire 2-wire special alloys 2-wire
Best For Precision, stability High temp, rugged Cost, fast local sensing

10) Transmitters & Interfaces

  • Head/rail transmitters: Convert PT100 to 4–20 mA (2-wire loop), 0–10 V, or digitals (HART/Modbus/IO-Link).

  • Direct-to-PLC: Many PLC/DCS AI cards accept 3-/4-wire PT100 with linearization per IEC 60751.

  • Winsen options: Compact probes with integrated 4–20 mA or RS-485/Modbus RTU, configurable span (e.g., −50…+150 °C, 0…+200 °C).

11) Installation Best Practices

  • Immersion length: ≥ 10× probe diameter (or 5× with tip-sensitive designs) to reduce stem conduction error.

  • Thermowells: Use for pressure/corrosive media and easy maintenance; select by ASME PTC 19.3 wake frequency where applicable.

  • Thermal compound: Apply in dry-well/blocks and surface sensors to improve coupling.

  • Cable routing: Shielded twisted pairs; avoid VFDs/EMI sources; bond shield at one end.

  • Environmental sealing: Choose IP67+ for washdown; potting/strain relief to prevent moisture ingress.

12) Calibration & Verification

  • Field check: Ice bath at 0 °C (distilled water + crushed ice; ensure no floating water film).

  • Lab calibration: Dry-well or fluid bath at 2–3 points (e.g., 0 °C, 100 °C, mid-scale), traceable to ITS-90.

  • Documentation: Record as-found/as-left, loop current at points, ambient conditions, and probe serials.

  • Intervals: 6–24 months depending on criticality and environment.

13) Common Sources of Error & Mitigation

  • Lead resistance: Use 3- or 4-wire; match leads in 3-wire systems.

  • Self-heating: Lower excitation current; ensure flow around the tip.

  • Stem conduction: Increase immersion depth; use spring-loaded tips in thermowells.

  • Moisture ingress: IP-rated assemblies, potted transitions, proper glands.

  • EMI/ground loops: Single-point grounding, shielded cable, isolated transmitters.

  • Mechanical stress: Avoid sharp bends; use strain reliefs; select flexible mineral-insulated (MI) cable where needed.

14) Selection Guide

  1. Temperature span (e.g., −50…+150 °C; −200…+400 °C).

  2. Tolerance class (AA/A/B) and element type (wire-wound vs thin-film).

  3. Wiring (2/3/4-wire) and cable type/length (PVC, PTFE, silicone, braided).

  4. Mechanical: sheath material (316L/Inconel), diameter (3/6/8 mm), length, process connection (NPT/G/BSP, sanitary tri-clamp, bayonet).

  5. Ingress rating (IP65–IP68) and vibration requirements.

  6. Transmitter (none / 4–20 mA / 0–10 V / RS-485 Modbus / IO-Link), power and output scaling.

  7. Compliance (RoHS, REACH, CE/UKCA; FDA/3-A for sanitary).

  8. Documentation (cal cert, material certs, wiring diagram, protocol).

15) Worked Example (100 °C Check)

Using IEC 60751, R(100 °C) for PT100 (α=0.00385):

R(100)=100×(1+A⋅100+B⋅1002)≈100×(1+0.39083−0.005775)≈138.51 ΩR(100) = 100 \times (1 + A\cdot100 + B\cdot100^2) \approx 100 \times (1 + 0.39083 – 0.005775) \approx \mathbf{138.51\ \Omega}

If your readout shows 138.0 Ω at a true 100 °C bath, the indicated error is ≈ −0.51 Ω → ≈ −0.37 °C near 100 °C (using local slope ≈ 0.385 Ω/°C).

16) FAQs

Q1: Can I substitute a PT100 with a PT1000?
A: PT1000 (1000 Ω at 0 °C) reduces lead-wire error in 2-wire systems. Ensure your instrument supports PT1000 scaling and CVD coefficients.

Q2: What excitation current should I use?
A: 0.1–1.0 mA is typical. Higher current improves SNR but increases self-heating—balance for your medium and flow.

Q3: How do I wire a 3-wire PT100?
A: Two leads on one side of the element, one on the other. The instrument measures and compensates the average of the two same-side leads (assumed equal resistance).

Q4: Do I need a thermowell?
A: Use thermowells for pressure, flow, corrosives, or frequent removal. Direct immersion yields faster response if conditions allow.

Q5: How often should I calibrate?
A: 6–24 months based on criticality and environment. High-value processes, audits, or regulatory regimes may require annual or semi-annual checks.

Contact Winsen

Tell us your temperature range, class, wiring, mechanical interface, and output type. Our engineers will propose a PT100 solution with drawings, datasheets, and lead time to match your project schedule.

Author

Leave a Reply

Your email address will not be published. Required fields are marked *