When selecting a thermocouple, accuracy is a critical factor. Two common terms used to define thermocouple accuracy are Accuracy Class and Special Limits of Error (SLE). Here’s how they differ:
1. Accuracy Class
- Defined by international standards such as IEC 60584-2, each class has specified allowable deviations in temperature measurement based on the thermocouple type..
- Thermocouples are categorized into different classes based on their accuracy levels:
- Class 1 – Highest accuracy (tightest tolerances).
- Class 2 – Standard accuracy (most commonly used).
- Class 3 – Lower accuracy, typically for extreme low temperatures.
- Accuracy is typically expressed as a percentage of the temperature range or in degrees (°C or °F).
2. Special Limits of Error (SLE)
- Defined by ASTM E230 / ANSI MC96.1 (commonly used in the U.S.).
- SLE thermocouples are manufactured to tighter tolerances than standard thermocouples.
- SLE thermocouples provide twice the accuracy of standard thermocouples within a defined temperature range.
Commonly specified when higher precision is required in industrial and laboratory applications.
| Feature | Accuracy Class (IEC 60584) | Special Limits of Error (ASTM E230) |
|---|---|---|
| Standard | IEC 60584-2 | ASTM E230 |
| Categories | Class 1, Class 2, Class 3 | Standard, Special Limits of Error |
| Accuracy | Defined in °C or % of temp | SLE offers higher accuracy than standard |
| Usage Region | International (Europe, etc.) | Primarily used in the U.S. |
Key Difference:
- Accuracy Class (IEC-based) refers to defined accuracy levels under international standards.
- Special Limits of Error (ANSI-based) refers to a tighter accuracy tolerance than standard thermocouples in U.S. standards.
Which One Should You Choose?
Use Class 1 thermocouples or SLE thermocouples when high precision is required.
Class 2 or standard thermocouples are sufficient for general industrial applications where ultra-high accuracy is not necessary.