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RTD (Pt100) and thermocouples

Pick the sensor and enter the temperature, or the measured signal, to get ohms or millivolts from the official tables, the class tolerance, the cold-junction effect and how much the cable errs with each connection.

Pre-sizing and estimating tool: results do not replace design, calculations or reports signed by a licensed professional, nor verification against the applicable codes and standards.

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Sensor and conversion
RTD
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Sensor curve over its range

Resistance (RTD) or emf with cold junction at 0 °C (thermocouple), with the computed point.

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Sensor tolerance versus temperature

Maximum deviation allowed by the class, in ± degrees, within each class validity range.

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Method, assumptions and references

Equations used

Pt100/Pt1000: R(t) = R0 · [1 + A·t + B·t² + C·(t − 100)·t³]; A = 3.9083·10⁻³, B = −5.775·10⁻⁷, C = −4.183·10⁻¹² (C = 0 for t ≥ 0 °C)
Inverse for R ≥ R0: t = [−A + √(A² − 4B · (1 − R/R0))] ÷ (2B); below 0 °C, Newton-Raphson on the full equation
Lead error: 2-wire Δt = 2 · Rc ÷ (dR/dt); 3-wire Δt = δ · Rc ÷ (dR/dt); 4-wire Δt = 0; Rc = ρ · L ÷ A
Thermocouple: E(t) = Σ cᵢ · tⁱ (+ a0 · exp[a1 · (t − 126.9686)²] for type K above 0 °C); t = Σ dᵢ · Eⁱ (NIST inverse)
Cold junction: E(T) = E_measured + E(T_cj); error ≈ cold-junction sensor error × S(T_cj) ÷ S(T)

Assumptions and limits

  • Direct coefficients from NIST SRD 60 / Monograph 175 (same as IEC 60584-1) and inverses with a maximum error of 0.02 to 0.06 °C depending on type; checked: K at 100 °C = 4.096 mV, J at 100 °C = 5.269 mV, Pt100 at 100 °C = 138.51 Ω.
  • Copper leads at 20 °C; with 3 wires only the entered lead mismatch remains. Thermocouple extension and compensating cables are not assessed.
  • Tolerances apply to new sensors; drift from aging, contamination and transmitter error add to them.

Standards and technical documents

  • IEC 60751:2022: Industrial platinum resistance thermometers and platinum temperature sensors
  • IEC 60584-1:2013: Thermocouples — Part 1: EMF specifications and tolerances
  • NIST Monograph 175 (1993): Temperature-Electromotive Force Reference Functions and Tables for the Letter-Designated Thermocouple Types Based on the ITS-90
  • ASTM E230/E230M-23: Standard Specification for Temperature-Electromotive Force (emf) Tables for Standardized Thermocouples
  • ASTM E1137/E1137M: Standard Specification for Industrial Platinum Resistance Thermometers

U.S. regulations

  • ASME PTC 19.3-2016: Thermowells — and ASME PTC 19.3 TW-2016 for thermowell design
  • NIST Handbook 44 and ISO/IEC 17025 accreditation for calibration of temperature sensors
  • OSHA 29 CFR 1910.307: electrical equipment in hazardous (classified) locations

Frequently asked questions

How many ohms does a Pt100 read at 212 °F?

138.51 Ω per IEC 60751. At 32 °F it reads 100 Ω and at 392 °F, 175.86 Ω. Sensitivity is about 0.385 Ω/°C (0.214 Ω/°F) near freezing and falls slightly with temperature.

How many mV does a type K thermocouple produce at 212 °F?

4.096 mV with the cold junction at 32 °F. With instrument terminals at 77 °F, it measures 3.096 mV and the transmitter adds 1.000 mV of cold-junction compensation.

How much error does a 2-wire RTD have?

Each ohm of cable is worth about 2.6 °C (4.7 °F). With 150 ft of 18 AWG, both leads add about 1.92 Ω, or about 5 °C (9 °F) of error. With 3 wires, the error drops to hundredths of a degree.

What is the difference between class A and class B?

At 212 °F, class A allows ±0.35 °C (±0.63 °F) and class B ±0.80 °C (±1.44 °F). Class AA allows ±0.27 °C. Outside the validity range, treat the sensor as a wider class.