Thermocouples
Thermocouples are among the most widely used temperature sensors in industrial and scientific applications, primarily due to their wide temperature range, durability, and ease of use. However, not all thermocouples deliver the same level of precision. To help users select the right sensor for their needs, the IEC 60584‑2 standard defines “accuracy classes”—commonly Class 1, Class 2, and (for some types) Class 3—that specify allowable tolerances over a given temperature range.
What Is an Accuracy Class?
An accuracy class sets the maximum permissible error a thermocouple can exhibit at any point within its calibrated range. Each class has two components:
- Fixed-offset tolerance (in degrees Celsius): a constant error bound, regardless of temperature.
- Slope-dependent tolerance (as a fraction of the measured temperature): an error bound that grows linearly with temperature.
Common Thermocouple Types and Their Tolerances
Below is a summary of the IEC 60584‑2 tolerance limits for the three most popular thermocouple types—K (chromel–alumel), J (iron–constantan), and T (copper–constantan).
| Type | Class 1 | Class 2 | Class 3* |
| K | Class 1: ±1.5 °C or ±0.004|t| (whichever greater) | Class 2: ±2.5 °C or ±0.0075|t| (whichever greater) | Class 3: ±4.0 °C or ±0.010|t| (whichever greater) |
| J | Class 1: ±1.5 °C or ±0.004|t| (whichever greater) | Class 2: ±2.5 °C or ±0.0075|t| (whichever greater) | Class 3: ±4.0 °C or ±0.010|t| (whichever greater) |
| T | Class 1: ±1.0 °C or ±0.004|t| (whichever greater) | Class 2: ±1.5 °C or ±0.0075|t| (whichever greater) | Class 3: ±2.5 °C or ±0.010|t| (whichever greater) |
Note:
- Class 3 is less commonly specified and often replaced by “special limits” in industrial contexts.
- |t| denotes the absolute temperature in °C (i.e., the magnitude of the reading).
Choosing Between Class 1, 2, and 3
Class 1
- Tightest tolerance: Ideal for applications demanding high repeatability and minimal drift
- Typical uses: Calibration labs, aerospace testing, precision metallurgy
Class 2
- Balanced performance: Good accuracy at a lower cost than Class 1
- Typical uses: Process control in food processing, HVAC monitoring, general‐purpose industrial heating
Class 3
- Broad tolerance: Lowest cost, but only for non‐critical measurements
Typical uses: Simple monitoring tasks, rough environmental checks
Why Precision Matters
Accuracy class impacts not only the absolute error of each reading, but also the consistency when swapping probes or comparing multiple sensors. In processes where even a few degrees can mean the difference between success and scrap—such as semiconductor fabrication or pharmaceutical batch reactions—opting for Class 1 ensures:
- Reliable cross‐probe consistency: Two Class 1 probes will agree more closely than two Class 2 probes, minimizing calibration overhead.
- Lower cumulative error: In systems with multiple sensing points, small per‑sensor errors don’t add up to unacceptable drift.
Practical Considerations
When specifying a thermocouple for your system, keep in mind:
- Operating Range: Make sure your chosen type (K, J, T, etc.) covers the temperatures you need.
- Environmental Factors: High vibration, oxidizing or reducing atmospheres, and thermal cycling can all degrade accuracy over time. Consider protective sheaths or thermowells.
- Cost vs. Performance: Class 1 sensors cost more—sometimes significantly—so weigh the benefit of tighter tolerance against budget and process requirements.
Wrapping Up
Thermocouple accuracy classes provide a clear framework to match sensor performance with application demands. Whether you’re running a high‐precision laboratory furnace or simply monitoring equipment temperature, understanding the difference between Class 1, Class 2, and Class 3 helps you make an informed choice—balancing cost, precision, and reliability to keep your processes on target.
Ready to choose the right thermocouple for your project? Contact our technical team today to discuss your temperature measurement needs and explore our full range of calibrated sensors.
