Overview
The International Temperature Scale of 1990 is defined by 17 fixed points - specific phase transitions of pure substances that occur at highly reproducible temperatures. These fixed points serve as the fundamental reference temperatures from which all other temperatures on ITS-90 are derived through interpolation.
Each fixed point represents an equilibrium state between different phases of a pure substance (solid, liquid, or gas) under specified conditions. The temperatures are assigned based on the best available thermodynamic data and provide the most accurate and reproducible temperature references possible.
Complete List of ITS-90 Fixed Points
The International Temperature Scale of 1990 is defined by exactly 17 fixed points. These include 15 specific equilibrium states (triple points, melting points, and freezing points) of pure substances, plus 2 equilibrium hydrogen vapor pressure points used for calibrating gas thermometers in the low temperature range. All 17 points are listed in the table below.
| Substance | State | T₉₀ (K) | t₉₀ (°C) | Notes |
|---|---|---|---|---|
| Hydrogen | V | 17.035 (range 17.025–17.045) | -256.115 (range -256.125 to -256.105) | Vapor pressure point at 33.3213 kPa |
| Hydrogen | V | 20.27 (range 20.26–20.28) | -252.88 (range -252.89 to -252.87) | Vapor pressure point at 101.292 kPa |
| Hydrogen | TP | 13.8033 | -259.3467 | Equilibrium H₂ (33.321% o-H₂) |
| Neon | TP | 24.5561 | -248.5939 | |
| Oxygen | TP | 54.3584 | -218.7916 | |
| Argon | TP | 83.8058 | -189.3442 | |
| Mercury | TP | 234.3156 | -38.8344 | |
| Water | TP | 273.16 | 0.01 | Defining point |
| Gallium | MP | 302.9146 | 29.7646 | At 101.325 kPa |
| Indium | FP | 429.7485 | 156.5985 | |
| Tin | FP | 505.078 | 231.928 | |
| Zinc | FP | 692.677 | 419.527 | |
| Aluminum | FP | 933.473 | 660.323 | |
| Silver | FP | 1234.93 | 961.78 | |
| Gold | FP | 1337.33 | 1064.18 | |
| Copper | FP | 1357.77 | 1084.62 |
State Abbreviations:
- V - Vapor pressure point
- TP - Triple point
- MP - Melting point
- FP - Freezing point
Each fixed point is realized under specific conditions of pressure and purity. Triple points occur at unique equilibrium pressures (e.g., water at 611.657 Pa), while melting and freezing points are realised at defined pressures (often near 1 atm but not always). Refer to the official ITS 90 text for exact conditions.
Key Fixed Points in Detail
Water Triple Point 0.01 °C (273.16 K)
The water triple point is the fundamental reference point of ITS-90 and is assigned the exact value of 273.16 K by definition. This point represents the unique temperature and pressure at which solid, liquid, and vapor phases of pure water coexist in equilibrium.
Pressure: 611.657 Pa
Typical Uncertainty: ± 0.1 mK
Gallium Melting Point 29.7646 °C (302.9146 K)
The gallium melting point is particularly useful as a secondary reference standard due to its convenient temperature (near room temperature) and excellent reproducibility. Gallium expands upon freezing, making it ideal for fixed-point cells.
Typical uncertainty: ± 0.25 mK
Silver Freezing Point 961.78 °C (1234.93 K)
The silver freezing point marks the upper limit of the platinum resistance thermometry range and the beginning of the radiation thermometry range. It serves as a crucial calibration point for high-temperature measurements.
Typical uncertainty: ± 2 mK
Fixed Point Realization
Realizing ITS-90 fixed points requires specialized apparatus and techniques:
Triple Point Cells
Triple point cells contain ultra-pure substances and are designed to maintain the equilibrium between three phases. They typically consist of:
- High-purity substance (>99.9999% for most applications)
- Sealed glass or metal container designed for thermal equilibrium
- Thermometer well for temperature measurement
- Special preparation procedures to ensure purity and remove contaminants
Freezing Point Cells
Freezing point cells for metal fixed points use crucibles containing high-purity metals and require:
- Controlled atmosphere (inert gas or vacuum)
- Uniform furnace heating and cooling
- Careful temperature control during phase transitions
- Specialized crucible materials (graphite, alumina, etc.)
Uncertainty Considerations
The uncertainties associated with fixed point realizations depend on various factors:
- Purity of the fixed-point material
- Hydrostatic pressure effects
- Isotopic composition (for hydrogen and helium)
- Dissolved impurities and their effect on transition temperature
- Temperature measurement system uncertainties
Practical Implementation
Fixed points are typically maintained by national metrology institutes and calibration laboratories using specialized equipment and procedures. Secondary standards calibrated against these fixed points are used to disseminate the temperature scale to users worldwide.