IEC 60584 Thermocouple Tolerance Classes 1, 2, 3
A thermocouple datasheet that quotes class 1 or class 2 is citing IEC 60584, the standard that sets both the reference tables and the tolerance classes for thermocouples. Engineers often carry over RTD habits and misread these classes, but thermocouples use a different numbering and a different valid range per couple type. This page explains the IEC 60584 tolerance classes so you specify and read them correctly. It complements the site's overview of thermocouples and their types.
IEC 60584 thermocouple tolerance classes in one line: IEC 60584 defines three tolerance classes for thermocouples, class 1, class 2, and class 3, from tightest to loosest allowed error. Each class states the permitted deviation as the larger of a fixed value or a percentage of the reading, and the temperature range over which a class is valid depends on the thermocouple type. Class 1 is the tightest generally available, class 2 is the common industrial default, and class 3 covers extended low-temperature ranges for some types.
Why Thermocouple Classes Differ From RTD Classes
Thermocouples and RTDs both have tolerance classes, but they are not the same system and should not be conflated. IEC 60584 numbers thermocouple classes 1, 2, and 3, whereas the RTD standard letters its classes AA to C. More importantly, the tolerance formula differs: a thermocouple class is typically the larger of a fixed error and a percentage of the measured temperature, so the allowed error scales with the reading rather than with distance from zero. The site's page on the thermocouple covers how the sensor generates its signal, which is why its error behaves differently from an RTD.
The other difference is that the valid range of each class depends heavily on the thermocouple type. Because a type K, type J, and type S couple cover very different temperature spans, IEC 60584 assigns each type its own range over which a given class holds. A class 1 rating on one type may cover a different span than the same class on another type, so the type and the class must be read together.
The Three Classes and How They Scale
The table below orders the classes and describes how each behaves. The exact tolerance values and per-type ranges live in IEC 60584 itself, which is the authority for the numbers.
| Class | Relative tightness | Notes |
|---|---|---|
| Class 1 | Tightest | Better accuracy, narrower valid range |
| Class 2 | Standard | General industrial default |
| Class 3 | Loosest | Extends to low temperatures for some types |
The scaling matters in practice. Because the tolerance is the larger of a fixed value or a percentage of the reading, the fixed value dominates at low temperatures and the percentage dominates at high temperatures. That means a class 2 couple can have a surprisingly large absolute error at high process temperatures, which catches out engineers who assume a small room-temperature error holds everywhere. The couple type also sets which classes are even offered; not every type supports every class.
Choosing the couple type is a separate decision that constrains the achievable accuracy. The site's page on thermocouple types J, K, T, E, N covers that choice, and the type you pick determines both the usable temperature span and the tolerance classes available for it before class even enters the discussion.
Specifying and Verifying a Class
To specify a class correctly, work from your actual operating temperature and the couple type you have chosen. Compute the allowed error the class permits at that temperature using the larger-of-fixed-or-percentage rule, and confirm it meets your requirement. Because the percentage term grows with temperature, a class that satisfies you at moderate temperatures may not at the top of your range, so always check the worst-case end.
Also confirm the class is valid over your whole span for that type. A class rating that ends below your maximum temperature is not a guarantee at the top of your range, and reading a class without its type-specific valid range is a common and costly omission. The datasheet should state the type, the class, and the range together.
Finally, remember that the class describes the thermocouple wire and junction, not the whole measurement chain. Cold-junction compensation error, extension-wire mismatch, and the temperature transmitter all add to the sensor's class tolerance, so the delivered accuracy at the control system is looser than the class alone. Budget the whole chain, not just the couple, when the measurement is critical.
Frequently Asked Questions
What is the difference between class 1 and class 2 thermocouples?
Class 1 is tighter than class 2: IEC 60584 permits class 1 a smaller deviation from the true temperature, but usually over a narrower valid range than class 2. Both express the allowed error as the larger of a fixed value or a percentage of the reading, so both grow at high temperatures, with class 1 staying within a smaller band. Class 2 is the general industrial default, and class 1 is chosen where the accuracy is justified and the type supports it over your range.
Why does the thermocouple tolerance depend on the type?
Because different thermocouple types cover very different temperature spans, IEC 60584 defines the valid range of each tolerance class per type. A type K and a type S couple do not share the same usable range, so the temperature interval over which a class 1 or class 2 rating holds differs between them. That is why you must read the type, the class, and the valid range together: a class rating is meaningless for your application unless it actually covers your operating temperature for that specific couple type.
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