**Calibration Interval for Standard Thermocouples:**  

The recommended calibration interval for standard thermocouples depends on their application environment and accuracy requirements:  

- **General Use**: Annual calibration is typical under stable conditions, aligning with industrial standards (e.g., IEC 60751).  

- **Harsh Environments**: In settings with extreme temperatures, vibration, or chemical exposure, recalibration every **6 months** is advised to mitigate drift caused by thermal stress or contamination.  

- **Post-Event Calibration**: Recalibrate after mechanical shock, prolonged storage, or exposure to temperatures exceeding the sensor’s rated range.  

**Calibration Temperature Points:**  

Calibration typically involves 3–5 key temperature points across the thermocouple’s operational range to ensure accuracy:  

1. **Low Range**:  

   - **0°C (Ice Bath)**: Validates the cold junction compensation and baseline accuracy.  

2. **Mid Range**:  

   - **400°C, 600°C**: Common for industrial applications (e.g., K-type thermocouples).  

3. **High Range**:  

   - **800°C, 1000°C**: Critical for high-temperature processes (e.g., furnace monitoring).  

**Calibration Procedure Highlights:**  

- **Fixed-Point Comparison**: Use a **secondary standard thermocouple** (e.g., S-type) or a calibrated platinum resistance thermometer (PRT) in a stable thermal environment (e.g., a tube furnace with a ±5°C uniformity).  

- **Reference Junction Handling**: Maintain the cold junction at **0°C** using an ice bath or compensated electronic reference.  

- **Data Collection**: Take **≥4 readings per temperature point** to ensure stability (temperature drift ≤0.2°C/min). Calculate the average and compare against the standard’s values.  

**Key Standards and Tools:**  

- **IEC 60751**: Specifies tolerance classes and stability requirements.  

- **ASTM E230**: Defines thermocouple calibration protocols.  

- **Equipment**: Tube furnaces, ice baths, and precision voltmeters (e.g., UJ33a potentiometer).  

**Note**: For thermocouples used in critical applications (e.g., aerospace or nuclear industries), additional calibration points and shorter intervals may apply. Always refer to manufacturer guidelines and industry-specific standards.