What IEC 60751 covers
IEC 60751 is the international standard for industrial platinum resistance thermometers and platinum resistance temperature sensors. It defines the resistance-temperature relationship, the tolerance classes, the test methods used to verify them, and the marking and documentation that has to ship with a part. The current edition is IEC 60751:2022 (Edition 3.0), which superseded the widely-deployed 2008 edition.
In Europe the same document is published as EN 60751; in China as GB/T 30121. The technical content is identical — what changes is the legal weight in each jurisdiction. When a datasheet says "to EN 60751", "to IEC 60751" or "to DIN EN 60751", treat them as the same specification.
The four tolerance classes
The 2008 revision introduced four tolerance classes — AA, A, B and C — in place of the older Class A / Class B / 1/3B / 1/10 DIN nomenclature that older catalogues used. Class AA is the tightest, Class C the loosest. The tolerance is given as an allowable temperature error and is a function of the absolute temperature being measured, because platinum's α coefficient drifts the curve over range.
| Class | Tolerance at 0 °C | Tolerance equation (°C) | Typical use |
|---|---|---|---|
| AA | ± 0.10 °C | ± (0.10 + 0.0017 ·|t|) | Laboratory reference, calibration |
| A | ± 0.15 °C | ± (0.15 + 0.002 ·|t|) | High-precision industrial |
| B | ± 0.30 °C | ± (0.30 + 0.005 ·|t|) | General industrial, HVAC, motors |
| C | ± 0.60 °C | ± (0.60 + 0.010 ·|t|) | Low-cost monitoring, non-critical |
Read the table carefully: a Class B Pt100 has ±0.30 °C error at the ice point, but ±0.55 °C at 100 °C and ±1.30 °C at 200 °C. The temperature-dependent term is why "Class B" can pass at the bench and still struggle in a 180 °C motor winding without re-trimming the loop. Lead-wire resistance adds on top of this, before you've even seen what the cable does.
The W and F suffix system
In the 2008 revision IEC 60751 also split the tolerance classes by element construction. Wire-wound elements — platinum wire on a ceramic or glass former — carry a W prefix (W 0.1, W 0.15, W 0.3, W 0.6 for AA / A / B / C). Thin-film elements — platinum sputtered onto an alumina substrate — carry an F prefix (F 0.1 … F 0.6).
Wire-wound parts can hold tolerance across the full -200 to +600 °C window. Thin-film parts are cheaper and more shock-resistant but are typically only specified across a narrower range — most film Pt100 elements lose their stated tolerance class above +300 °C, even when the substrate itself can take the heat. WIKA notes that thin-film Pt100 is usually only quoted to Class B over the full upper range; if you need Class A at 500 °C, a wire-wound element is effectively mandatory.
What changed in 2008
Before the 2008 revision only the element — the bare resistor — had to fulfil its tolerance class. The whole assembled thermometer (element + sheath + lead system + connection head) could legally drift outside spec at the connection end. After 2008, the complete thermometer must meet its declared class at the cable end.
This is invisible to most buyers but matters in two cases. First, if you have a legacy bill-of-materials that references "Class A element", a literal re-quote against IEC 60751:2022 may push you into a slightly looser overall class because of the added lead and joint contribution. Second, when comparing a Chinese supplier's quote to a European catalogue part, check whether the tolerance applies to the element or the assembly — the difference is real money on a 4-wire long-cable build.
The Callendar-Van Dusen equation
IEC 60751 defines the platinum R(T) relationship using the Callendar-Van Dusen equation. Below 0 °C the form has four constants; above 0 °C it simplifies to a quadratic:
R(T) = R₀ · (1 + A·T + B·T²) for 0 °C ≤ T ≤ +850 °C
R(T) = R₀ · (1 + A·T + B·T² + C·(T-100)·T³) for -200 °C ≤ T ≤ 0 °C
With the standard ITS-90 coefficients A = 3.9083 × 10⁻³, B = -5.775 × 10⁻⁷ and C = -4.183 × 10⁻¹², and R₀ = 100 Ω at 0 °C. The temperature coefficient α — sometimes still quoted as "Pt100 α = 0.00385 Ω/Ω/°C" — falls out of this equation as the average slope between 0 °C and 100 °C.
Almost every Pt100 transmitter on the market has the inverse Callendar-Van Dusen equation hard-coded into firmware. When you buy a part marked "to IEC 60751 Class B α=0.00385", you are buying a guarantee that the R(T) curve will lie inside the Class B band and that a standard-α transmitter will read correctly.
Pt1000 — same standard, different R₀
IEC 60751 covers Pt1000 (1000 Ω at 0 °C) under the same tolerance class structure. The element is physically similar to a Pt100 but trimmed to ten times the resistance. The advantage is a ten-fold reduction in the relative impact of lead-wire resistance, which is why Pt1000 has displaced Pt100 in many HVAC and battery-management applications where 2-wire cabling is unavoidable.
Multiply the standard Pt100 resistance table by 10 to get the Pt1000 values — the tolerance equations apply identically. Most modern transmitters can read either with a configuration bit.
How this maps to a buying decision
For an embedded motor-winding RTD (typical: -50 to +200 °C continuous, AC 2.5 kV dielectric), Class B is the universal default; Class A makes sense if the protection algorithm needs to resolve ≤ 0.5 °C drift at the rated trip point. For laboratory reference or calibration loops, Class AA is mandatory and you should expect a calibration certificate per part.
For HVAC duct and coil sensing across -40 to +120 °C, Class B is sufficient — the sensor error is typically smaller than the duct stratification error anyway. The wire-wound vs thin-film choice is dominated by cost and vibration profile rather than tolerance.
For high-vibration motor and transformer winding embedding, our recommendation is a fully-fluoropolymer-encapsulated Pt100 assembly with the element trimmed to Class A and an AWG 24 or 26 PTFE lead. The mechanical and chemical resilience of the fluoropolymer matters more for service life than the marginal class upgrade — see our article on fluoropolymer encapsulation for the material-science reasoning.
Where 60751 stops and you start
Two things IEC 60751 does not cover. First, self-heating: the standard specifies a recommended test current (typically 1 mA for Pt100) that keeps self-heating below 0.05 °C in still air, but the production current is set by your transmitter. Second, response time: the standard defines a test method but does not impose a number — response time is dominated by the probe's thermal mass and the medium it sits in, not by anything written in the spec.
For both you have to characterise the assembled probe in the actual application. We supply sample data on request; the typical response time for a 4 × 25 mm WZP wire-wound probe immersed in flowing water is under 4 seconds (τ₆₃).