IEC 60751 RTD Tolerance Classes AA, A, B, C
When a Pt100 datasheet says class A or class B, it is quoting IEC 60751, and the class sets how far the sensor's reading may legally stray from the true temperature. Many engineers pick a class by habit without knowing that the allowed error grows with temperature and that the classes trade cost for tightness. This page explains the IEC 60751 tolerance classes and what each one guarantees, so you specify the right one on purpose. It complements the site's overview of the Pt100 RTD itself.
IEC 60751 RTD tolerance classes in one line: IEC 60751 defines four tolerance classes for platinum RTDs, AA, A, B, and C, in order of loosening allowed error. Each class states the permitted deviation as a formula that grows with temperature away from zero, so the same sensor is most accurate near 0 degrees C and drifts wider at temperature extremes. Class AA is tightest, class B is the common industrial default, and the class also defines the temperature range over which it is valid.
What a Tolerance Class Actually Guarantees
A tolerance class is a promise about maximum error, not a fixed number. IEC 60751 expresses each class as a formula in which the allowed deviation is smallest at 0 degrees C and increases as the temperature moves away from zero in either direction. That is why a class B sensor that is well within tolerance at room temperature can still be permitted a larger error at several hundred degrees. The class caps the error across the range; it does not promise the same error everywhere. The site's page on the Pt100 RTD covers how the sensor works, while this page fixes what the class on its label asserts.
The ordering is straightforward: class AA is the tightest, then A, then B, then C, each allowing progressively more error for progressively lower cost. Class B is the workhorse of general industrial temperature measurement because it balances accuracy against price; the tighter classes are specified where the measurement genuinely justifies them, such as in custody transfer or a tightly controlled process.
The Classes and Their Trade-offs
The table below orders the classes and notes the trade-off each represents. The exact tolerance formulas are defined in IEC 60751 itself; treat the standard as the authority for the numbers.
| Class | Relative tightness | Typical use |
|---|---|---|
| AA | Tightest | Precision and reference measurement |
| A | Tight | Better process control loops |
| B | Standard | General industrial default |
| C | Loosest | Non-critical or wide-range service |
Two things constrain a class beyond its tolerance formula. First, each class is valid only over a defined temperature range, and the tighter classes cover a narrower range, so a class AA sensor may not carry its class rating across the full span a class B sensor does. Second, wiring matters: the tolerance describes the sensor element, and a poor connection or a two-wire arrangement adds lead-resistance error on top, which is why precise RTD measurement uses three- or four-wire connections.
The class also interacts with the transmitter. A tight sensor class feeding a coarse temperature transmitter wastes the sensor's accuracy, and a loose class feeding a precise transmitter is limited by the sensor. Match the class to the measurement need and to the rest of the loop rather than specifying the tightest class reflexively.
Reading and Specifying the Class
When you read a class off a datasheet, check three things: the class letter, the temperature range over which that class is guaranteed, and whether the quoted accuracy is the sensor element alone or the assembled sensor with its transmitter. A class stated without its valid range is incomplete, because the same class means a different absolute error at different temperatures.
Specifying is the mirror image. Decide the worst-case error you can tolerate at your actual operating temperature, not at 0 degrees C, then pick the class whose formula stays within that error across your range. Because the allowed error widens with temperature, a class that looks adequate at room temperature can fail your requirement at process temperature, which is the most common specification slip.
Class is one input to overall measurement quality, alongside comparing sensor technologies. The site's page on the difference between an RTD and a thermocouple helps decide whether a Pt100 is even the right sensor before you argue about its tolerance class, since a thermocouple follows a different tolerance standard entirely.
Frequently Asked Questions
What is the difference between class A and class B RTDs?
Class A is tighter than class B: IEC 60751 permits class A a smaller deviation from the true temperature than class B across the range. Both express the allowed error as a formula that grows with temperature away from 0 degrees C, so both are most accurate near zero and drift wider at extremes, but class A stays within a smaller error band. Class B is the general industrial default for cost reasons, while class A is chosen where the loop genuinely needs the extra accuracy.
Does an RTD's accuracy change with temperature?
Yes. The IEC 60751 tolerance for every class is defined as a formula in which the allowed error is smallest at 0 degrees C and increases as the temperature moves away from zero. So a sensor that is well within tolerance at room temperature is permitted a larger absolute error at high or low process temperatures. This is why you should specify a class against the worst-case error you can accept at your actual operating temperature, not at zero, or you may find the class inadequate in service.
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