Introduction to ITS-90 Realisation
Realising ITS-90 in practice requires specialised equipment, careful procedures, and thorough understanding of the physical principles involved. This simplified guide provides practical information for temperature metrology laboratories implementing ITS-90 calibrations and maintaining temperature standards.
The complexity and requirements vary significantly depending on the temperature range and level of uncertainty required. National metrology institutes maintain primary standards, while calibration laboratories typically work with secondary standards calibrated against the primary realisations.
Water Triple Point Cell
The water triple point is the most commonly realised ITS-90 fixed point, serving as the fundamental reference for platinum resistance thermometer calibrations.
🧪 Apparatus Requirements
- Triple point cell: Glass cell containing ultra-pure water sealed under vacuum
- Mantling system: Ice bath or maintenance bath for thermal control
- Standard platinum resistance thermometer (SPRT)
- Thermometry bridge or digital thermometer readout
- Standard resistors for bridge operation
🔬 Simplified Realisation Procedure
- Cell preparation: Ensure cell is clean and undamaged
- Freezing: Surround cell with ice bath and initiate freeze
- Mantle formation: Create thin ice mantle around thermometer well
- Equilibrium: Allow system to reach thermal equilibrium (typically 30+ minutes)
- Measurement: Make multiple resistance measurements over time
- Verification: Check for drift and repeatability
📊 Typical Uncertainties
| Source | Standard Uncertainty (k=1) |
|---|---|
| Cell impurities | ±0.05 mK |
| Hydrostatic pressure | ±0.02 mK |
| Resistance measurement | ±0.03 mK |
| Self-heating | ±0.02 mK |
Video Introduction: Water Triple Point Cell
Water Triple Point Cell Realisation
This video introduces Water Triple Point Cells.
Video courtesy of Isotech – shared here for information purposes only.
Gallium Melting Point Cell
The gallium melting point provides an excellent secondary reference near room temperature, offering convenient calibration capabilities with exceptional reproducibility.
🧪 Apparatus Requirements
- Gallium cell: Sealed container with high-purity gallium (99.99999%)
- Constant temperature bath or apparatus: Capable of high stability
- SPRT or industrial platinum RTD
- Precision thermometry bridge
🔬 Simplified Realisation Procedure
- Initial heating: Heat cell to ~32°C to ensure complete melting
- Controlled cooling: Cool at 0.5-1 K/min through transition
- Supercooling detection: Monitor for characteristic temperature drop
- Plateau measurement: Measure temperature during stable plateau
- Multiple cycles: Repeat to verify reproducibility
⚡ Key Considerations
- Gallium expands about 3.1% on freezing, so cells must accommodate this expansion
- Extended length plateaus can be readily achieved
Video Introduction: Gallium Fixed Point Cells
Gallium Melting Point Cells
Video introducing Gallium Cells
Video courtesy of Isotech – shared here for introducing purposes only.
Metal Fixed Points (High Temperature)
Metal fixed points (indium, tin, zinc, aluminium, silver, copper) require specialised furnaces to achieve the highest accuracy for high-temperature calibrations.
🔥 Furnace Requirements
- Excellent temperature uniformity: across cell length
- Stability: ±0.05 K during measurement
- Controlled atmosphere: For Open Cells
- Heating/cooling rate control: Typically 2-5 K/min
🔬 Cell Design Features
- Crucible material: Graphite for most metals
- Gas purging system: Remove oxygen and moisture, Open cells only
- Thermometer well design: Optimise thermal contact and minimise contamination
Demonstration: ITS-90 Fixed Point Cells
Introduction to Fixed Point Cells
This video introduces higher temperature ITS-90 cells.
Video courtesy of Isotech – shared here for information purposes only.
Platinum Resistance Thermometry
Standard Platinum Resistance Thermometers (SPRTs) are the interpolation instruments for the most widely used range of ITS-90, from 13.8 K to 1,234.93 K.
🌡️ SPRT Requirements
- Platinum purity: 99.999% minimum
- Construction: Strain-free mounting, contamination-free
- Resistance ratio: W(Ga) ≥ 1.11807 for sub-range calibrations
- Stability: Drift < 2 mK/year in normal use
🔌 Measurement Systems
- Resistance bridges: 1 ppm accuracy or better
- Four-wire measurement: Eliminate lead resistance effects
- Current reversal: Minimise thermoelectric effects
- Low measurement current: Minimise self-heating (typically 1 mA)
📊 Calibration Procedures
SPRT calibration involves measurements at defining fixed points:
- Water triple point: Fundamental reference (always required)
- Additional fixed points: Selected based on intended use range
- Coefficient calculation: Determine interpolation equation coefficients
- Verification: Check at intermediate fixed points
Overview: SPRTs in Temperature Calibration
Standard Platinum Resistance Thermometers
An introduction to SPRTs, their construction, calibration requirements, and role as the primary interpolation instruments for ITS-90 temperature measurements.
Video courtesy of Isotech – shared here for educational purposes only.
Implementation Support
Implementing ITS-90 requires careful attention to detail and often benefits from expert guidance. National metrology institutes and expert suppliers can provide training, technical support, and calibration services to help laboratories achieve the highest measurement quality.