Temperature Scales

Contents

1. What is Temperature?
2. The International Temperature Scale of 1990 (ITS-90)
3. The Fixed Points of ITS-90
4. Summary

What is Temperature?

Understanding how temperature is defined is a key foundation for thermocouple and resistance thermometry. Although the concept may appear simple, temperature is one of the more complex physical quantities to define and measure precisely.

In measurement science, a reference system of units is required to enable accurate comparisons. Temperature is one of the seven base quantities defined by the International System of Units (SI). However, unlike other base quantities such as length or mass, temperature cannot be measured directly. It must be inferred from other measurable physical properties such as resistance, pressure, or voltage.

The standard unit of temperature is the kelvin (K), defined as 1/273.16 of the thermodynamic temperature of the triple point of water — the unique condition where water exists simultaneously in solid, liquid, and vapor phases. This corresponds to 0.01°C on the Celsius scale. The Celsius and Kelvin scales have identical intervals; the only difference is the starting point, with 0 K equivalent to -273.15°C.

Traditionally, the ice point (0°C) and the steam point (100°C) have been used as convenient reference temperatures. However, in precision measurement, these reference points can vary slightly due to factors such as atmospheric pressure and air saturation. For example, the triple point of water is defined in the absence of air, while the ice point involves air-saturated water under standard pressure.

Modern research has introduced alternative fixed points, such as the triple point of gallium (~30°C), which can offer more reproducibility under laboratory conditions. These developments help refine the accuracy and reliability of temperature measurement.

The International Temperature Scale of 1990 (ITS-90)

The evolution of international temperature scales has played a crucial role in modern thermometry. The first widely adopted standard, ITS-27, was introduced in 1927 and provided a framework for consistent temperature measurement using fixed points and calibrated sensors. It spanned from -200°C (just below the boiling point of oxygen) to 1,065°C (the freezing point of gold), with platinum resistance thermometers (PRTs) used up to 445°C and Pt-10%Rh vs Pt thermocouples (Type S) specified for higher temperatures.

In 1968, this was replaced by IPTS-68, which introduced improved interpolation formulas and a broader temperature range. However, challenges remained, especially in the 630–961°C region where thermocouples introduced interpolation errors and discontinuities.

On 1 January 1990, the International Temperature Scale of 1990 (ITS-90) was adopted, representing a significant leap forward in accuracy and usability. Key improvements included:

• Extension of platinum resistance thermometry up to 961.78°C (freezing point of silver), eliminating the need for thermocouples in this range
• Phasing out of thermocouples (e.g., Type S) as defining instruments due to limited reproducibility (±0.2°C), replaced by more precise PRTs
• Removal of outdated fixed points, including the boiling points of neon, oxygen, and water
• Expansion of the temperature range down to 0.65 K using vapor pressure thermometry
• More accurate interpolation equations for improved precision between fixed points
• Introduction of sub-ranges, allowing PRTs to be calibrated over limited spans, reducing exposure to extreme temperatures

ITS-90 divides the temperature scale into five overlapping ranges, each with its own measurement technique:

• 0.65 K to 5 K using vapor pressure of helium
• 3 K to 24.5561 K via constant volume gas thermometry
• 13.8033 K to 273.16 K using ratio thermometry with triple points
• 0°C to 961.78°C using calibrated platinum resistance thermometers
• Above 961.78°C using radiation thermometry based on Planck’s law

While thermocouples such as Types S, R, and B are no longer part of the defining scale, they are still used as secondary standards and for practical temperature measurements across industry.

ITS-90 is not a perfect representation of thermodynamic temperature, but it is the most accurate and stable scale ever implemented, forming the basis for temperature calibration and traceability worldwide.

The Fixed Points of ITS-90

Equilibrium State	t90 / K	t90 / °C
Triple point of hydrogen	13.8033	-259.3467
Boiling point of hydrogen at a pressure of 33321.3 Pa	17.035	-256.115
Boiling point of hydrogen at a pressure of 101292 Pa	20.27	-252.88
Triple point of neon	24.5561	-248.5939
Triple point of oxygen	54.3584	-218.7916
Triple point of argon	83.8058	-189.3442
Triple point of mercury	234.3156	-38.8344
Triple point of water	273.16	0.01
Melting point of gallium	302.9146	29.7646
Freezing point of indium	429.7485	156.5985
Freezing point of tin	505.078	231.928
Freezing point of zinc	692.677	419.527
Freezing point of aluminium	933.473	660.323
Freezing point of silver	1234.93	961.78
Freezing point of gold	1337.33	1064.18
Freezing point of copper	1357.77	1084.62

The Fixed Points adopted in the ITS-90

Summary

• Temperature cannot be measured directly and must be inferred through related physical properties.
• The kelvin is the SI unit of temperature, defined using the triple point of water.
• The ITS-90 scale provides an internationally accepted, highly accurate method for temperature measurement across a wide range.
• Resistance thermometry has largely replaced thermocouples in high-precision applications due to its superior reproducibility.

Further Reading

What is a thermocouple and how does it work?
Discover the working principles of thermocouples and how they generate EMF.

What is a PRT?
Explore how PRTs (RTD / Pt100) sensors work and the common styles

How to calibrate thermocouples
Read about the various methods of calibrating a thermocouple.