Two Pt100 sensors with the same label can disagree by almost a degree if they belong to different accuracy grades. Knowing how IEC 60751 defines each grade helps you buy the right sensor and set honest expectations for every loop.
IEC 60751 sorts platinum resistance thermometers into four accuracy grades, each with its own formula and valid temperature range. This guide explains every RTD tolerance class with worked examples, a calculator and practical selection advice.
What Is an RTD Tolerance Class?
An RTD tolerance class is the accuracy grade given by IEC 60751 to an industrial platinum resistance thermometer, stating how far its resistance, expressed in degrees, may deviate from the standard curve at a given temperature. The grade applies to sensors such as the Pt100 described in what is an RTD and how it works.
Every Pt100 should read exactly 100 Ω at 0 °C and follow the Callendar Van Dusen curve above and below it. Real elements never match perfectly, so the standard defines a permitted band around the ideal value, and that band widens as temperature moves away from zero.
The class covers only the sensing element or the complete thermometer, not the transmitter, the cable or the installation. Those error sources are discussed in temperature measurement errors and must be added separately when you estimate loop accuracy.
The resistance to temperature relationship used for industrial Pt100 sensors has not changed since 1995. The 2008 and 2022 editions of IEC 60751 changed tolerance rules, testing and marking, not the curve itself.
The 4 Tolerance Grades in IEC 60751
The standard uses four grades, and each one has a formula with a fixed part and a part that grows with the absolute temperature in °C. The table below lists each RTD tolerance class formula exactly as published by IEC and repeated by manufacturers such as Reotemp and JUMO.
| Class | Tolerance, °C | Wire Wound Range | Thin Film Range |
|---|---|---|---|
| AA (W 0.1 or F 0.1) | ± (0.1 + 0.0017 |t|) | minus 50 to 250 °C | 0 to 150 °C |
| A (W 0.15 or F 0.15) | ± (0.15 + 0.002 |t|) | minus 100 to 450 °C | minus 30 to 300 °C |
| B (W 0.3 or F 0.3) | ± (0.3 + 0.005 |t|) | minus 196 to 600 °C | minus 50 to 500 °C |
| C (W 0.6 or F 0.6) | ± (0.6 + 0.01 |t|) | minus 196 to 600 °C | minus 50 to 600 °C |
Tightest standard grade, about one third of Class B at 0 °C.
Half the fixed error of Class B with a smaller slope.
The most common industrial grade and the usual default.
Widest band, rarely specified for new projects.
The ranges matter as much as the formulas, because a sensor is guaranteed to its RTD tolerance class only inside the listed window. A thin film Class A element used at 400 °C is outside its range, so the class no longer applies there, which is explained in more depth in thin film RTD and its advantages.
Wire Wound and Thin Film: Why the Ranges Differ
Since the 2008 edition, IEC 60751 marks the construction in the class name, with W for wire wound and F for film elements. A wire wound coil sits almost strain free in its ceramic former, so it keeps its calibration over a wider span than a film element bonded to a substrate.
Thin film elements are smaller, cheaper and respond faster, as noted in RTD temperature sensor working. Their platinum layer and the substrate expand differently, so the standard narrows their valid range for each grade.
Isothermal Technology explains in its paper on the 2008 changes that the standard now separates the sensing resistor from the complete thermometer. A finished probe can therefore carry a different class from the bare element inside it, once sheath, mineral insulation and leads are added.
When a datasheet only says Class A, check whether it is W or F and read the stated range. A film element bought as Class A for a 350 °C oven line will not hold that grade.
How to Calculate RTD Tolerance Class Limits
The calculation is simple: take the absolute value of the process temperature, multiply it by the slope of the grade and add the fixed part. The result is the allowed deviation in °C, and you can convert it to ohms using the sensitivity shown in RTD resistance to temperature calculator.
AA: a = 0.1, b = 0.0017
A: a = 0.15, b = 0.002
B: a = 0.3, b = 0.005
C: a = 0.6, b = 0.01
Example at t = 100 °C:
AA = 0.1 + 0.0017 × 100 = ± 0.27 °C
A = 0.15 + 0.002 × 100 = ± 0.35 °C
B = 0.3 + 0.005 × 100 = ± 0.80 °C
C = 0.6 + 0.01 × 100 = ± 1.60 °C
A measured error of 0.30 °C at 100 °C meets Class A but not Class AA
RTD Tolerance Class Calculator
Second Worked Example: Hot Oil Heater at 250 °C
A thermic fluid heater runs at 250 °C and the process engineer wants the sensor error below 1 °C. Class B gives 0.3 + 0.005 × 250 = 1.55 °C, which already fails the target before any transmitter error is added.
Class A gives 0.15 + 0.002 × 250 = 0.65 °C, and 250 °C lies inside both the wire wound and the thin film window for that grade. Class AA would give 0.525 °C, but a thin film AA element is only valid up to 150 °C, so it would have to be wire wound.
Next, add the transmitter error, for example 0.1 °C plus 0.02 percent of span, using root sum of squares as shown in measurement uncertainty in calibration. In this case a wire wound Class A sensor with a good head transmitter meets the 1 °C requirement with some margin.
What Does 1/3 DIN or 1/10 DIN Really Mean?
Many catalogues still offer 1/3 DIN and 1/10 DIN sensors, named after the old DIN 43760 Class B. The Temperature Data Instruments guide to IEC 60751 points out that these labels usually describe a tight selection of R0, the resistance at 0 °C, rather than a true accuracy grade across the whole range.
A 1/10 DIN element is selected to about ± 0.01 percent of R0, or roughly ± 0.03 °C at the ice point. Away from 0 °C the slope of each element still varies, so its error grows and can approach that of the parent RTD tolerance class at high temperature.
At 0 °C, Class AA works out to ± 0.1 °C, which is about one third of Class B. That is why older catalogues often sold the same tolerance under the name 1/3 DIN.
How to Choose an RTD Tolerance Class for a Loop
For most utility and monitoring points the Class B RTD tolerance class is enough, especially when used with a 3 wire or 4 wire RTD connection. Class A suits product temperatures, heat transfer calculations and energy metering, where small errors become real money.
Class AA or a calibrated sensor is worth it for reference measurements, pharmaceutical sterilisers and custody transfer temperature correction. In those services, matching the sensor curve inside a temperature transmitter removes most of the remaining interchangeability error.
Write the class on the instrument datasheet as Class A, wire wound, IEC 60751, with the range in °C. A plain request for a Pt100 almost always gets you a Class B thin film element.
Checking the RTD Tolerance Class During Calibration
You can confirm the grade by comparing the sensor with a reference thermometer in a dry block calibrator or a stirred bath at two or three points. Convert each measured resistance to temperature with the standard curve and compare the difference with the formula limit.
- Use a 4 wire measurement to remove lead resistance.
- Allow enough immersion depth, at least 15 times the sheath diameter.
- Wait for stability before reading each point.
- Keep the measuring current at 1 mA or lower to limit self heating.
- Test at 0 °C and at the top of the normal operating range.
- Record as found and as left values with uncertainty.
- Compare each error with the RTD tolerance class formula, not a fixed number.
A full procedure, including ice point checks, is covered in Pt100 RTD calibration guide. Keep the calibration uncertainty at least four times smaller than the tolerance you are checking, otherwise the result cannot prove conformity.
Common RTD Tolerance Class Mistakes
The most frequent mistake is quoting the 0 °C value as if it applied everywhere, which hides the slope term. Another is mixing up the sensor class with transmitter accuracy, as discussed in instrument accuracy and error.
- Gives a clear, testable accuracy figure for each sensor.
- Makes sensors from different makers interchangeable.
- Helps size the full loop error budget early.
- Supports audits under ISO 17025 and ISO 9001.
- Covers only the element or thermometer, not the loop.
- Valid only inside the stated range for W or F.
- Fractional DIN labels are not always true classes.
- Drift after installation is not covered by the grade.
Where Each Grade Is Typically Used
Whichever grade you pick, install it well, since immersion and response matter as much as the label, as explained in thermocouple and RTD installation precautions. A precise element in a shallow thermowell still reads the wrong temperature.
IEC 60751 Reference Paper
Comparing Pt100 Accuracy Grades on Video
RTD Tolerance Class FAQ
It is the accuracy grade that IEC 60751 assigns to a platinum resistance thermometer or its sensing element. The grade states how much the reading may deviate from the standard curve at each temperature.
IEC 60751 defines four grades, from the tight Class AA to the wide Class C, each with its own formula. Every grade also has a valid range for wire wound and thin film elements.
Class A allows plus or minus 0.15 degrees plus 0.002 times the absolute temperature in degrees Celsius. At 100 °C that gives an allowed error of 0.35 degrees.
This RTD tolerance class is valid from minus 100 to 450 °C for wire wound elements. Thin film elements hold Class A only from minus 30 to 300 degrees, so check the construction first.
A Class B Pt100 allows plus or minus 0.3 degrees plus 0.005 times the absolute temperature. That equals 0.3 degrees at the ice point and 0.8 degrees at 100 °C in practice.
It is the most common industrial grade and suits general monitoring well. For product quality loops or energy metering, a tighter grade is usually worth the small extra cost.
W means a wire wound element and F means a thin film element in the sensor marking. The letter was added in the 2008 edition of the standard to make the construction clear.
Both use the same tolerance formulas for each grade. Only the valid temperature range changes, and wire wound elements always cover a wider span than film elements.
Not always, because many suppliers use the label for a tight selection of resistance at 0 °C only. Away from the ice point the error can grow toward the parent grade.
The 2022 edition allows special classes with a declared range and an sp marking. Ask the supplier which definition applies before relying on the figure.
No, the grade covers only the sensing element or the complete thermometer assembly. Transmitter accuracy, lead wire effects and installation errors must be added separately.
Engineers usually combine all these terms by root sum of squares to estimate the overall loop accuracy. Matching the transmitter to the actual sensor curve can remove most of the sensor term.
Compare the sensor with a reference thermometer in a dry block or bath at two or three temperatures. Use a 4 wire connection and keep the measuring current at about 1 milliamp.
Then compare each error with the formula limit at that temperature. Make sure your calibration uncertainty is several times smaller than the limit being tested.
Related Articles
- What Is RTD and How It Works
- Thin Film RTD and Its Advantages
- Pt100 RTD Calibration Guide
- RTD Pt100 vs Pt1000 Differences
- RTD Resistance to Temperature Calculator
External References
- Changes to the International Standard for Industrial PRTs, Isothermal Technology
- IEC 60751 Tolerance Classes Explained, Temperature Data Instruments
- Resistance Thermometer, Wikipedia
What We Learn Today
- Each RTD tolerance class in IEC 60751 adds a fixed part to a slope times absolute temperature, so the allowed error grows away from 0 °C.
- Wire wound and thin film elements share the same formulas, but film elements hold each grade over a narrower range, like 0 to 150 °C for AA.
- Fractional labels like 1/10 DIN usually describe resistance selection at 0 °C, so check the declared range before trusting them at high temperature.