How the classes are defined
Every class is the greater of a fixed °C floor or a fraction of the measured temperature: IEC Class 1 is ±1.5 °C or 0.4% (−40 to 1000 °C); Class 2 is ±2.5 °C or 0.75% (−40 to 1200 °C); Class 3 is ±2.5 °C or 1.5% (−200 to +40 °C, cryogenic). ASTM E230 Special is ±1.1 °C or 0.4% and Standard is ±2.2 °C or 0.75%, both 0 to 1260 °C.
The percentage term governs above the crossover temperature (floor ÷ fraction): 375 °C for IEC Class 1, 333 °C for Class 2, 275 °C for ASTM Special, 293 °C for ASTM Standard. Below the crossover the fixed floor applies. IEC Class 1 is broadly comparable to ASTM Special and Class 2 to Standard, but the floors, ranges, and crossovers all differ — a purchase order should name one standard explicitly rather than assume equivalence.
| °C | IEC Class 1 | IEC Class 2 | ASTM Special | ASTM Standard |
|---|---|---|---|---|
| 0 | ±1.50 | ±2.50 | ±1.10 | ±2.20 |
| 100 | ±1.50 | ±2.50 | ±1.10 | ±2.20 |
| 200 | ±1.50 | ±2.50 | ±1.10 | ±2.20 |
| 300 | ±1.50 | ±2.50 | ±1.20 | ±2.25 |
| 400 | ±1.60 | ±3.00 | ±1.60 | ±3.00 |
| 500 | ±2.00 | ±3.75 | ±2.00 | ±3.75 |
| 600 | ±2.40 | ±4.50 | ±2.40 | ±4.50 |
| 700 | ±2.80 | ±5.25 | ±2.80 | ±5.25 |
| 800 | ±3.20 | ±6.00 | ±3.20 | ±6.00 |
| 900 | ±3.60 | ±6.75 | ±3.60 | ±6.75 |
| 1000 | ±4.00 | ±7.50 | ±4.00 | ±7.50 |
| 1100 | — | ±8.25 | ±4.40 | ±8.25 |
| 1200 | — | ±9.00 | ±4.80 | ±9.00 |
| 1260 | — | — | ±5.04 | ±9.45 |
What the tolerance does and does not promise
A tolerance class bounds the initial error of new, essentially homogeneous wire. It says nothing about in-service behaviour: EMF aging, green rot, and shunting all accumulate on top of the class band.
Per the standard's own notes, the stated tolerances apply to new wire used within the recommended temperature limits, and the EMF–temperature relationship may change with time in use — results at manufacture do not necessarily hold over an extended service period. That is why loops with tight setpoints schedule periodic comparison against a reference standard; recalibration and certificates are covered under thermocouple calibration. Note also that the percentage terms are defined on temperatures expressed in °C, and the equivalent °F tolerance is 1.8× the °C value.
For QA work: enter the as-found deviation from your comparison check in the tool above and it returns a within/out-of-class verdict against the selected band. Out-of-class sensors in critical loops should be replaced rather than offset-corrected, since the drift mechanism that moved them rarely stops. Class 1 / Special-limits constructions are listed in the Type K catalog.