Standard Platinum Resistance Thermometer (SPRT)
A Standard Platinum Resistance Thermometer (SPRT) is a high-precision temperature measurement instrument primarily used as a primary reference standard in metrology and laboratory settings. Below are its key aspects:
1. Working Principle
Resistance-Temperature Relationship: Based on the predictable change in electrical resistance of high-purity platinum wire with temperature. Platinum exhibits a highly linear and stable resistance-temperature relationship, as defined by the International Temperature Scale of 1990 (ITS-90).
Traceability: Calibrated against internationally recognized fixed points (e.g., triple point of water, gallium melting point) to ensure traceability to global standards.
2. Structure & Materials
Platinum Purity: Uses platinum wire with purity ≥99.999% to minimize impurities affecting resistance.
Encapsulation: The platinum coil is mounted in a stress-free glass or quartz holder to prevent mechanical strain and contamination.
Protective Sheath: Often housed in an inert gas-filled (e.g., helium) sheath to avoid oxidation and environmental interference.
3. Key Features
Ultra-High Accuracy: Measurement uncertainty as low as 0.001°C (millikelvin level), making it the gold standard in temperature metrology.
Wide Temperature Range: Typically covers -190°C to 962°C (aligned with ITS-90).
Long-Term Stability: Maintains calibration stability for years with proper maintenance.
4. Primary Applications
Metrological Calibration: Serves as a primary standard at national metrology institutes and calibration labs to calibrate secondary thermometers (e.g., industrial PRTs, thermocouples).
Scientific Research: Used in material science, cryogenics, and thermodynamics requiring extreme precision.
ITS-90 Implementation: Critical for realizing and maintaining the International Temperature Scale.
5. Comparison with Industrial Platinum Resistance Thermometers
| Feature | SPRT | Industrial PRT (e.g., Pt100) |
|---|
| Accuracy | 0.001°C level | 0.1°C–0.5°C level |
| Platinum Purity | ≥99.999% | ~99.9% |
| Construction | Stress-free encapsulation, inert gas | Metal sheath, mineral-insulated cable |
| Usage | Laboratories, metrology institutes | Industrial processes, field use |
| Calibration Cycle | Periodic recalibration at fixed points | Factory-calibrated, rarely rechecked |
| Cost | High | Low |
6. Operational Considerations
Avoid Mechanical Stress: Fragile platinum wire requires careful handling.
Low Measurement Current: Typically uses 1 mA current to minimize self-heating errors.
Recalibration: Requires periodic recalibration at fixed points (e.g., water triple point) via accredited labs.
7. Example Models
Summary
The SPRT is the gold standard in temperature metrology, offering unparalleled accuracy and traceability. Despite its high cost and delicate handling requirements, it remains indispensable for establishing and maintaining global temperature standards.
