Preferred Reference: Triple Point of Water (TPW)

The TPW is the standard for temperature calibration due to its precise and reproducible nature. It provides a highly stable reference at 0.01°C, which is critical for the calibration and cross-checking of PRTs. Using a TPW cell ensures minimal uncertainty and is recommended whenever the highest level of accuracy is required.

Alternative Reference: Shaved Ice Bath

While a TPW cell is ideal, the shaved ice bath provides a practical and accessible method to perform PRT cross-checks at 0°C. The ice bath, when properly prepared, offers a reliable reference with minimal uncertainty, suitable for most routine calibration needs.

Preparation of a Shaved Ice Bath

Materials Needed:

• Distilled or deionized water
• Shaved ice (made from distilled or deionized water)
• Insulated container (such as a Dewar flask)
• Stirrer or wooden dowel
• PRT and a calibrated reference thermometer

Abbreviated Procedure:

1. Shaving the Ice:
   • Use a commercial ice shaver or manually crush ice into fine particles. Smaller ice particles increase the surface area and enhance thermal equilibrium.
   • Use distilled or deionized water to avoid impurities that may affect the reference temperature.
2. Preparing the Ice Bath:
   • Fill the insulated container with shaved ice.
   • Gradually add distilled water to the ice while stirring to create a homogeneous slushy mixture with no visible liquid water at the top.
   • Compact the mixture to eliminate air pockets, ensuring a consistent temperature throughout the bath.
3. Stabilization:
   • Allow the mixture to stabilize for several minutes.
   • Insert the PRT and reference thermometer, ensuring they are fully immersed without touching the container walls.
4. Cross-Check Measurement:
   • Once stable, verify the temperature of the shaved ice bath to be at 0.000°C ± 0.005°C with the reference thermometer.
   • Document the reading of the PRT being check and record the resistance reading.  Using this resistance value, determine your thermometer drift.  Also, record your resistance on your control chart future reference.
     • For ease of reading, you may use an indicator with the programmed PRT coefficients to directly display temperature.  This will save you time in calculation but will skew your control chart as the probe coefficients will change over time.  Direct resistance will provide the most accurate reflection of probe drift over its lifespan.
5. Repeatability Check:
   • Repeat the process as needed to verify the stability and repeatability of the shaved ice bath and the PRT readings.

Regular cross-checks are essential for identifying drifts or errors in PRT readings that could impact critical measurements. While the triple point of water is the gold standard, a properly prepared shaved ice bath is a viable alternative for most practical purposes.  A shaved ice bath provides a convenient and reliable alternative for ensuring the integrity of temperature measurements. Adhering to standardized procedures in preparing the ice bath is crucial for achieving reliable and repeatable results.