**Calibration Interval for Standard Platinum Resistance Thermometers (SPRTs):**  

The recommended calibration interval for SPRTs depends on their application environment, usage frequency, and required accuracy. Key guidelines include:  

- **General Laboratory Use**: Annual recalibration is typical for high-precision SPRTs under stable conditions.  

- **Industrial or Harsh Environments**: Calibration every 6 months or shorter intervals may be necessary due to exposure to vibration, thermal cycling, or contaminants.  

- **Post-Exposure Events**: Recalibrate after mechanical shock, extreme temperature excursions (>850°C), or prolonged storage.  

**Primary Calibration Temperature Points (ITS-90 Fixed Points):**  

SPRTs are calibrated at internationally defined fixed points to ensure traceability and accuracy:  

1. **Triple Point of Water (0.01°C)**: Fundamental reference for validating SPRT stability and low-temperature performance.  

2. **Melting Point of Gallium (29.7646°C)**: Used for high-accuracy validation near ambient temperatures.  

3. **Freezing Points of Indium (156.5985°C) and Tin (231.928°C)**: Critical for mid-range industrial applications.  

4. **Zinc (419.527°C) and Aluminum (660.323°C) Freezing Points**: Essential for high-temperature calibration.  

**Additional Considerations:**  

- **Interpolation**: For intermediate temperatures, SPRTs are characterized using reference equations (e.g., Callendar-Van Dusen) between fixed points.  

- **Secondary Calibration**: Some labs use secondary standards (e.g., stirred liquid baths) for cost efficiency, but fixed-point calibration remains the gold standard.  

**Reference Standards:**  

- **IEC 60751:2008/2022**: Specifies tolerance classes (A/B) and stability requirements.  

- **ITS-90**: Defines fixed-point protocols for SPRT calibration.  

Proper calibration ensures compliance with uncertainty budgets (e.g., ±0.001°C at 0°C for Class A SPRTs) and mitigates drift caused by mechanical stress or contamination.