What Is the ITS-90?

The International Temperature Scale of 1990 (ITS-90) is a global standard for temperature measurement, providing a practical framework that approximates the theoretical thermodynamic temperature scale. Developed to replace earlier scales like the International Practical Temperature Scale of 1968 (IPTS-68), ITS-90 ensures consistency and accuracy in temperature measurement across industries.

This standard was introduced to address the growing demand for a reliable and reproducible temperature scale that could accommodate scientific precision and industrial practicality. By extending the temperature range and incorporating advancements in thermometry, ITS-90 has become indispensable for applications ranging from manufacturing to cutting-edge research.

This guide will delve into the principles, defining points, and practical applications of the ITS-90 — as well reviewing the takeaways of each section of the original paper — emphasizing the standard’s importance to calibration professionals, lab managers, and engineers.

Before we walk through the sections of the original ITS-90 paper, let’s cover some basics of the ITS-90.

Understanding the Basics of ITS-90

At its core, ITS-90 is based on the Kelvin (K), the International System of Units (SI) unit for thermodynamic temperature. The Kelvin scale is anchored by the triple point of water (273.16 K, 0.01 °C, 32.018 °F), a fundamental reference that defines one Kelvin as 1/273.16 of the thermodynamic temperature of this fixed point.

Celsius (C), the temperature scale commonly used in everyday applications, is a derivative unit of Kelvin and is expressed using the equation:

t / °C = T / K — 273.15t / °C = T/ K — 273.15t / °C = T / K — 273.15

This equation ensures that one degree Celsius and one Kelvin have the same magnitude, simplifying conversions between the two scales. While Kelvin is typically used for low-temperature measurements and in scientific experiments, experts often prefer Celsius for higher temperatures in both industrial and laboratory contexts.

Core ITS-90 Concepts

The success of the ITS-90 in creating a standard temperature scale hinges on three foundational elements that ensure its practicality and reproducibility:

1. Fixed Points: Specific phase transitions (e.g., triple points, melting points) of pure substances are designated as reference temperatures. These points are universally reproducible and serve as the backbone of the scale. For example, the triple point of water (273.16 K) and the melting point of gallium (302.9146 K) are key fixed points.
2. Interpolating Thermometers: Instruments such as platinum resistance thermometers (PRTs) and radiation thermometers are used to measure temperatures between the fixed points. These devices are highly precise and consistent.
3. Interpolating Equations: To calculate temperatures between fixed points, ITS-90 employs specified mathematical models called interpolating equations. These equations ensure the scale remains accurate and applicable across its range.

By relying on fixed points, appropriate equations, and precise instruments, ITS-90 ensures temperature measurements are accurate and universally comparable.

Why ITS-90 Matters

The standardization that ITS-90 provides makes it a cornerstone for precision and consistency in temperature measurement; aligning Kelvin and Celsius simplifies calculations and ensures consistency in various contexts. For instance, ITS-90-compliant fixed-point devices help maintain temperature standards in national metrology institutes.

The temperature scale has applications outside of the laboratory, too. Calibration technicians, for example, rely on ITS-90-compliant standards to validate industrial thermometers used in manufacturing processes. In healthcare, Celsius measurements calibrated to ITS-90 ensure precise monitoring of pharmaceuticals, where deviations as small as 0.1 °C can impact outcomes. And calibration engineers in aerospace use ITS-90 to ensure materials withstand extreme temperature ranges during testing.

ITS-90 vs. IPTS-68: Differences from Earlier Scales

ITS-90 originated as a replacement for older temperature scales, namely the IPTS-68. While both scales serve as standardized frameworks, ITS-90 introduced refined reference points and calibration methods, resulting in slight differences in measured temperatures compared to IPTS-68. This refinement also brought reduced variability in resistance measurements.

Key Improvements of ITS-90 vs. IPTS-68

ITS-90 introduces significant advancements over IPTS-68:

• Expanded range: The broader coverage of ITS-90 allows precise measurements at extreme ends of the temperature spectrum.
• Enhanced accuracy: Refinements in fixed points and interpolating equations reduce uncertainties, bringing the scale closer to the thermodynamic temperature scale.
• Flexibility: Overlapping sub-ranges and device options let users customize ITS-90 implementations to their specific needs.

ITS-90 enables the use of multiple fixed points, allowing metrologists to maintain accuracy in smaller temperature ranges without exposing instruments to extreme conditions.

ITS-90 Section 1: Units of Temperature

1. Units of Temperature

The unit of the fundamental physical quantity known as thermodynamic temperature, symbol T, is the Kelvin symbol K, defined as the fraction 1/273.16 of the thermodynamic temperature of the triple point of water.

Because of the way earlier temperature scales were defined, it remains common practice to express a temperature in terms of its difference from 273.15 K, the ice point. A thermodynamic temperature, T, expressed in this way is known as a Celsius temperature, symbol t, defined by (equation 1):

t / ºC = T / K — 273.15

The unit of Celsius temperature is the degree Celsius, symbol ºC, which is by definition equal in magnitude to the Kelvin. A difference of temperature may be expressed in Kelvins or degrees Celsius.

The International Temperature Scale of 1990 (ITS-90) defines both International Kelvin Temperatures, symbol T₉₀, and International Celsius Temperatures, symbol t₉₀. The relation between T₉₀ and t₉₀is the same as that between T and t, i.e. (equation 2):

t₉₀ / ºC = T₉₀ / K — 273.15

The unit of the physical quantity T₉₀ is the kelvin, symbol K, and the unit of the physical quantity t₉₀ is the degree Celsius, symbol ºC, as is the case for the thermodynamic temperature T and the Celsius temperature t.

Takeaways of “1. Units of Temperature”

The first section of ITS-90 sets up the core definitions of the units of temperature that the ITS-90 temperature scale: Kelvin (T and T₉₀) and Celsius (t and t₉₀).

ITS-90 Section 2: Principles of the International Temperature Scale of 1990 (ITS-90)

2. Principles of the International Temperature Scale of 1990 (ITS-90)

The ITS-90 extends upwards from 0.65 K to the highest temperature practicably measurable in terms of the Planck radiation law using monochromatic radiation. The ITS-90 comprises a number of ranges and sub-ranges throughout each of which temperatures T₉₀ are defined. Several of these ranges or sub-ranges overlap, and where such overlapping occurs, differing definitions of T₉₀ exist: These differing definitions have equal status. For measurements of the very highest precision, there may be detectable numerical differences between measurements made at the same temperature but in accordance with differing definitions. Similarly, even using one definition, at a temperature between defining fixed points, two acceptable interpolating instruments (e.g. resistance thermometers) may give detectably differing numerical values of T₉₀. In virtually all cases, these differences are of negligible practical importance and are at the minimum level consistent with a scale of no more than reasonable complexity; for further information on this point see ‘Supplementary information for the ITS-90’ (BIPM-1990).

The ITS-90 has been constructed in such a way that, throughout its range, any given temperature the numerical value of T₉₀ is a close approximation to the numerical value of T, according to best estimates at the time the scale was adopted. By comparison with direct measurements of thermodynamic temperatures, measurements of T₉₀ are more easily made, are more precise and are highly reproducible.

There are significant numerical differences between the values of T₉₀ and the corresponding values of T₆₈ measured on the International Practical Temperature Scale of 1968 (IPTS-68), see Figure 1 and Table 6. Similarly, there were differences between the IPTS-68 and the International Practical Temperature Scale of 1948 (IPTS-48), and between the International Temperature Scale of 1948 (ITS-48) and the International Temperature Scale of 1927 (ITS-27). See the Appendix, and, for more detailed information, ‘Supplementary Information for the ITS-90.’

Takeaways of “2. Principles of the International Temperature Scale of 1990 (ITS-90)”

The second section of ITS-90 describes the basic rules that underpin how the ITS-90 works, including describing:

• The minimum and maximum temperatures
• The fact that ranges and subranges exist within this minimum and maximum
• The fact that some ranges and subranges overlap with each other
• How to handle overlap

ITS-90 Section 3: Definition of the International Temperature Scale of 1990

3. Definition of the International Temperature Scale of 1990

Between 0.65 K and 5.0 K, T₉₀ is defined in terms of the vapor-pressure temperature relations ³He and ⁴He.

Between 3.0 K and the triple point of neon (24.5561 K), T₉₀ is defined by means of a helium gas thermometer calibrated at three experimentally realizable temperatures having assigned numerical values (defining fixed points) and using specified interpolation procedures.

Between the triple point of equilibrium hydrogen (13.8033 K) and the freezing point of silver (961.78 ºC), T₉₀ is defined by means of platinum resistance thermometers calibrated at specified sets of defining fixed points and using specified interpolation procedures.

Above the freezing point of silver (961.78 ºC), T₉₀ is defined in terms of a defining fixed point and the Planck radiation law.

The defining fixed points of the ITS-90 are listed in Table 1. The effects of pressure, arising from significant depths of immersion of the sensor or from other causes, on the temperature of most of these points are given in Table 2.

3.1. From 0.65 K to 5.0 K: Helium Vapor-Pressure Temperature Equations

In this range, T₉₀ is defined in terms of the vapor pressure p of ³He and ⁴He using equations of the form (equation 3):

The values of the constants A₀, A_i, B and C are given in Table 3 for ³He in the range of

0.65 K to 3.2 K, and for 4He in the ranges 1.25 K to 2.1768 K (the lambda point) and 2.1768 K to 5.0 K.

3.2. From 3.0 K to the Triple Point of Neon (24.5561 K): Gas Thermometer

In this range, T₉₀ is defined in terms of a ³He or a ⁴He gas thermometer of the constant-volume type that has been calibrated at three temperatures. These are the triple point of neon (24.5561 K), the triple point of equilibrium hydrogen (13.8033 K), and a temperature between 3.0 K and 5.0 K. This last temperature is determined using a ³He or a ⁴He vapor pressure thermometer as specified in section 3.1.

Table 1. Defining Fixed Points of the ITS-90

^a All substances except ³He are of natural isotopic composition, e-H₂ is hydrogen at the equilibrium concentration of the ortho- and para-molecular forms.

^b For complete definitions and advice on the realization of these various states, see ‘Supplementary Information for the ITS-90.’ The symbols have the following meanings: V: vapor pressure point; T: triple point (temperature at which the solid liquid and vapor phases are in equilibrium); G: gas thermometer point; M, F: melting point, freezing point (temperature, at a pressure of 101 325 Pa, at which the solid and liquid phases are in equilibrium).

Table 2. Effect of Pressure on the Temperatures of Some Defining Fixed Points⁺

+ The Reference pressure for melting and freezing points is the standard atmosphere (p₀=101 325 Pa). For triple points (T) the pressure effect is a consequence only of the hydrostatic head of liquid in the cell.

* Equivalent to millikelvins per standard atmosphere

** Equivalent to millikelvins per meter of liquid

Table 3. Values of the Constants for the Helium Vapor Pressure Equation 3, and the Temperature Range for Which Each Equation, Identified by Its Set of Constants, Is Valid

3.2.1. From 4.2 K to the Triple Point of Neon (24.5561 K) with ⁴He as the Thermometric Gas

In this range T₉₀ is defined by the relation (equation 4):

T₉₀ = a + b p + c p²

where p is the pressure in the gas thermometer and a, b and c are coefficients the numerical values of which are obtained from measurements made at the three defining fixed points given in section 3.2, but with the further restriction that the lowest one of these points lies between 4.2 K and 5.0 K.

3.2.2. From 3.0 K to the Triple Point of Neon (24.5561 K) with ³He or ⁴He as the Thermometric Gas

For a ³He gas thermometer, and for a ⁴He gas thermometer used below 4.2 K, the non-ideality of the gas must be accounted for explicitly, using the appropriate second virial coefficient B₃ (T₉₀) or B₄ (T₉₀). In this range T₉₀ is defined by the relation (equation 5):

T₉₀= (a + b p + c p²) / (1 + B_x(T₉₀) NIV)

where p is the pressure in the gas thermometer, a, b and c are coefficients the numerical values of which are obtained from measurements at three defining temperatures as given in section 3.2, N/V is the gas density with N being the quantity of gas and V the volume of the bulb, x is 3 or 4 according to the isotope used, and the values of the second virial coefficients are given by the relations:

- For ³He (equation 6a)
B₃(T₉₀) / m³mol^-1= {16.69 — 336.98(T₉₀/K)^-1+ 91.04(T₉₀/K)^-2 — 13.82(T₉₀/K)^-3} 10^-6

- For ⁴He (equation 6b)
B₄(T₉₀) / m³mol^-1= {16.708 — 374.05(T₉₀/K)^-1— 383.53(T₉₀/K)^-2+ 1799.2(T₉₀/K)^-3— 4033.2(T₉₀/K)^-4+3252.8(T₉₀/K)^-5}10^-6

Table 4. The Constants A₀, A_i; B_n, B_i; C₀, C_i; D₀ and D_i In the Reference Functions of Equations 9a; 10a; and 10b Respectively

The accuracy with which T₉₀ can be realized using equations 4 and 5 depends on the design of the gas thermometer and the gas density used. Design criteria and current good practice required to achieve a selected accuracy are given in "Supplementary Information for the ITS -90.”

3.3. The Triple Point of Equilibrium Hydrogen (13.8033 K) to the Freezing Point of Silver (961.78 ºC): Platinum Resistance Thermometer

In this range, T₉₀ is defined by means of a platinum resistance thermometer calibrated at specified sets of defining fixed points, and using specified reference and deviation functions for interpolation at intervening temperatures.

No single platinum resistance thermometer can provide high accuracy, or is even likely to be usable, over all of the temperature range 13.8033 K to 961.78 ºC. The choice of temperature range, or ranges, from among those listed below for which a particular thermometer can be used is normally limited by its construction.

For practical details and current good practice, in particular concerning types of thermometers available, their acceptable operating ranges, probable accuracies, permissible leakage resistance, resistance values, and thermal treatment, see "Supplementary Information for ITS-90.” It is particularly important to take account of the appropriate heat treatments that should be followed each time a platinum resistance thermometer is subjected to a temperature above about 420 ºC.

Temperatures are determined in terms of the ratio of the resistance R(T₉₀) at a temperature T₉₀ and the resistance R(273.16 K) at the triple point of water.
This ratio, W (T₉₀), is² (equation 7):

W(T₉₀) = R(T₉₀) / R(273.16 K)

² Note that this definition of W (T₉₀) differs from the corresponding definition used in the ITS-27, ITS-48, IPTS-48, and IPTS-68: for all of these earlier scales W (T) was defined in terms of reference temperature of 0 ºC, which since 1954 has itself been defined as 273.15 K

An acceptable platinum resistance thermometer must be made from pure, strain-free platinum, and it must satisfy at least one of the following two relations (equations 8a and 8b):

W(29.7646 °C) ≥ 1.11807

W(-38.8344 °C) ≤ 0.844 235

An acceptable platinum resistance thermometer that is to be used up to the freezing point of silver must also satisfy the relation (equation 8c):

W(961.78 °C)≥ 4.2844

In each of the resistance thermometer ranges, T₉₀ is obtained from W_r (T₉₀) as given by the appropriate reference function (equations 9b or 10b), and the deviation W(T₉₀) - W_r(T₉₀). At the defining fixed points this deviation is obtained directly from the calibration of the thermometer: At intermediate temperatures, it is obtained by means of the appropriate deviation function (equations 12, 13 and 14).

-For the range 13.8033 K to 273.16 K, the following reference function is defined (equation 9a):

ln [W_r(T₉₀)] = A₀ + [[ln (T₉₀/273.16 K) + 1.5]/1.5]ⁱ

An inverse function, equivalent to equation 9a. to within 0,1 mK, is (equation 9b):

T₉₀/273.16 K = B₀ + [[W_r(T₉₀)^1/6 — 0.65]/0.35]ⁱ

The values of the constants A₀, A_i, B₀ and B_i are given in Table 4.

A thermometer may be calibrated for use throughout this range or, using progressively fewer calibration points, for ranges with low temperature limits of 24.5561 K, 54.3584 K and 83.8058 K, all having an upper limit of 273.16 K.

- For the range 0 ºC to 961.78 ºC, the following reference function is defined (equation 10a):

W_r(T₉₀) = C₀ +  [[T₉₀/K — 754.15]/481]ⁱ

An inverse function, equivalent to equation 10a to within 0.13 mK is (equation 10b):

T₉₀/K — 273.15 = D₀ + [[W_r(T₉₀) — 2,64]/1.64]ⁱ

The values of the constants C₀, C_i, D₀ and D_i are given in Table 4.

A thermometer may be calibrated for use throughout this range or, using fewer calibration points, for ranges with upper limits of 660.323 ºC, 419.527 ºC, 231.928 ºC, 156.5985 ºC or 29.7646 ºC, all having a lower limit of 0 ºC.

- A thermometer may be calibrated for use in the range 234.3156 K (-38.8344 ºC) to 29.7646 ºC, the calibration being made at these temperatures and at the triple point of water. Both reference functions (equations 9 and 10) are required to cover this range.

The defining fixed points and deviation functions for the various ranges are given below, and in summary from in Table 5.

3.3.1. The Triple Point of Equilibrium Hydrogen (13.8033 K) to the Triple Point of Water (273.16 K)

The thermometer is calibrated at the triple points of equilibrium hydrogen (13.8033 K), neon (24.5561 K), oxygen (54.3584 K), argon (83.8058 K), mercury (234.3156 K), and water (273.16 K), and at two additional temperatures close to 17.0 K and 20.3 K. These last two may be determined either: by using a gas thermometer as described in Sect. 3.2, in which case the two temperatures must lie within the ranges 16.9 K to 17.1 K and 20.2 K to 20.4 K respectively; or by using the vapor pressure-temperature relation of equilibrium hydrogen, in which case the tow temperatures must lie within the ranges 17.025 K to 17.045 K and 20.26 K to 20.28 K respectively, with the precise values being determined from equations 11a and 11b respectively:

T₉₀ / K — 17.035 = (p/kPa — 33.3213) / 13.32

T₉₀ / K — 20.27 = (p/kPa — 101.292) / 30

The deviation function is ³ (equation 12):

³ This deviation function (and also those of equations 13 and 14) may be expressed in terms of W_r rather than W; for this procedure see "Supplementary Information for ITS-90."

W(T₉₀) — W_r(T₉₀) = a[W(T₉₀) —1] + b[W(T₉₀) —1]² +  [ln W(T₉₀)]ⁱ⁺ⁿ

with values for the coefficients a, b and c_i being obtained from measurements at the defining fixed points and with n = 2.

For this range and for the sub-ranges 3.3.1.1 to 3.3.1.3 the required values W_r(T₉₀) are obtained from equation 9 or from Table 1.

3.3.1.1. The Triple Point of Neon (24.5561 K) to the Triple Point of Water (273.16 K)

The thermometer is calibrated at the triple points of equilibrium hydrogen (13.8033 K), neon (24.5561 K), oxygen (54.3584 K), argon (83.8058 K), mercury (234.3156 K) and water (273.16 K).

The deviation function is given by equation 12 with values for the coefficients a, b, c₁, c₂ and c₃ being obtained from measurements at the defining fixed points and with c₄ = c₅ = n = 0.

3.3.1.2 The Triple Point of Oxygen (54.3584 K) to the Triple Point of Water (273.16 K)

The thermometer is calibrated at the triple points of oxygen (54.3584 K), argon (83.8058 K), mercury (234.3156 K) and water (273.16 K).

The deviation function is given by equation 12 with values for the coefficients a, b and c₁ being obtained from measurements at the defining fixed points, with c₂ = c₃ = c₄ = c₅ = 0 and with n = 1.

Table 5. Deviation Functions and Calibration Points for Platinum Resistance Thermometers In the Various Ranges In Which They Define T₉₀

Ranges with an upper limit of 273.16 K

Ranges with a lower limit of 0 ºC

* Calibration points 9, 12-14 are used with d = 0 for t₉₀ ≤ 660.323 ºC; the values of a, b and c thus obtained are retained for t₉₀ > 660.323 ºC with d being determined from calibration point 15.

3.3.1.3. The Triple Point of Argon (83.8058 K) to the Triple Point of Water (273.16 K)

The thermometer is calibrated at the triple points of argon (83,8058 K), mercury (234,3156 K) and water (273,16 K).

The deviation function is (equation 13):

W(T₉₀) — W_r(T₉₀) = a[W(T₉₀) — 1] + b[W(T₉₀) — 1]In W(T₉₀)

with the values of a and b being obtained from measurements at the defining fixed points.

3.3.2. From 0 ºC to the Freezing Point of Silver (961.78 ºC)

The thermometer is calibrated at the triple point of water (0.01 ºC), and at the freezing points of tin (231.928 ºC), zinc (419.527 ºC), aluminum (660.323 ºC) and silver (961.78 ºC).

The deviation function is (equation 14):

W(T₉₀) — W_r(T₉₀) = a[W(T₉₀ ) — 1] + b[W(T₉₀) — 1]² + c[W(T₉₀) — 1]³ + d[W(T₉₀) — W(660.323 °C)]²

For temperatures below the freezing point of aluminum d = 0, with the values of a, b and c being determined from the measured deviations from W_r(T₉₀) at the freezing points of tin, zinc and aluminum. From the freezing point of aluminum to the freezing point of silver the above values of a, b and c are retained and the value of d is determined from the measured deviation from W_r(T₉₀) at the freezing point of silver.

For this range and for the sub-ranges 3.3.2.1 to 3.3.2.5 the required values for W_r(T₉₀) are obtained from equation 10a or from Table 1.

3.3.2.1. From 0 ºC to the Freezing Point of Aluminum (660.323 ºC)

The thermometer is calibrated at the triple point of water (0.01 ºC), and at the freezing points of tin (231.928 ºC), zinc (419.527 ºC) and aluminum (660.323 ºC).

The deviation function is given by equation 14, with the values of a, b and c being determined from measurements at the defining fixed points and with d = 0.

3.3.2.2. From 0 ºC to the Freezing Point of Zinc (419.527 ºC)

The thermometer is calibrated at the triple point of water (0.01 ºC), and at the freezing points of tin (231.928 ºC). and zinc (419.527 ºC).

The deviation function is given by equation 14, with the values of a and b being obtained from measurements at the defining fixed points and with c = d = 0.

3.3.2.3. From 0 ºC to the Freezing Point of Tin (231.928 ºC)

The thermometer is calibrated at the triple point of water (0.01 ºC), and at the freezing points of indium (156.5985 ºC) and tin (231.928 ºC).

The deviation function is given by equation 14, with the values of a and b being obtained from measurements at the defining fixed points and with c = d = 0.

3.3.2.4. From 0 ºC to the Freezing Point of Indium (156,5985 ºC)

The thermometer is calibrated at the triple point of water (0.01 ºC), and at the freezing point of indium (156.5985 ºC).

The deviation function is given by equation 14 with the value of a being obtained from measurements at the defining fixed points and with b = c = d = 0.

3.3.2.5. From 0 ºC to the Melting Point of Gallium (29.7646 ºC)

The thermometer is calibrated at the triple point of water (0.01 ºC), and the melting point of gallium (29.7646 ºC).

The deviation function is given by equation 14 with the value of a being obtained from measurements at the defining fixed points and with b = c = d = 0.

3.3.3. The Triple Point of Mercury (-38.8344 ºC) to the Melting Point of Gallium (29.7646 ºC)

The thermometer is calibrated at the triple points of mercury (-38.8344 ºC), and water (0.01 ºC), and at the melting point of gallium (29.7646 ºC).

The deviation function is given by equation 14 with the values of a and b being obtained from measurements at the defining fixed points and with c = d = 0.

The required values of W_r(T₉₀) are obtained from equations 9a and 10a for measurements below and above 273.16 K respectively, or from Table 1.

3.4. The Range Above the Freezing Point of Silver (961.78 ºC): Planck Radiation Law

Above the freezing point of silver, the temperature T₉₀ is defined by the equation (equation 15):

(L _λ (T₉₀)) / (L _λ [T₉₀(X)]) = (exp(c₂[λ T₉₀(X)]^-1) — 1) / (exp(c₂[λ T₉₀]^-1) — 1)

where T₉₀(X) refers to any one of the silver {T₉₀(Ag) = 1234.93 K}, the gold {T₉₀(Au) = 1337.33 K} or the copper {T₉₀(Cu) = 1357.77 K} freezing points⁴ and in which L _λ (T₉₀) and L _λ [T₉₀(X)] are the spectral concentrations of the radiance of a blackbody at the wavelength (in vacuo) λ at T₉₀ and at T₉₀(X) respectively, and c₂ = 0.014388 m · K.

For practical details and current good practice for optical pyrometry, see "Supplementary Information for the ITS-90" (BIPM-1990).

⁴ The T₉₀ values of the freezing points of silver, gold and copper are believed to be self-consistent to such a degree that the substitution of any one of them in place of one of the other two as the reference temperature T₉₀ (X) will not result in significant differences in the measured values of T₉₀.

Takeaways of “3. Definition of the International Temperature Scale of 1990”

The third section of ITS-90 describes the actual temperature ranges that make up the temperature scale. The ranges include:

• From 0.65 K: helium vapor-pressure temperature equations
• From 3.0 K to the triple point of neon (24.5561 K): gas thermometer
• From 4.2 K to the triple point of neon (24.5561 K) with ⁴He as the thermometric gas
• From 3.0 K to the triple point of neon (24.5561 K) with ³He or ⁴He as the thermometric gas
• The triple point of equilibrium hydrogen (13.8033 K) to the freezing point of silver (961.78 ºC): platinum resistance thermometer
• The triple point of equilibrium hydrogen (13.8033 K) to the triple point of water (273.16 K)
• The triple point of Oxygen (54.3584 K) to the triple point of water (273.16 K)
• The triple point of Argon (83.8058 K) to the triple point of water (273.16 K)
• From 0 ºC to the freezing point of silver (961.78 ºC)
• From 0 ºC to the freezing point of aluminum (660.323 ºC)
• From 0 ºC to the freezing point of zinc (419.527 ºC)
• From 0 ºC to the freezing point of tin (231.928 ºC)
• From 0 ºC to the freezing point of indium (156,5985 ºC)
• From 0 ºC to the melting point of gallium (29.7646 ºC)
• The triple point of Mercury (-38.8344 ºC) to the melting point of gallium (29.7646 ºC)
• The range above the freezing point of silver (961.78 ºC): Planck Radiation Law

ITS-90 Section 4: Supplementary Information and Differences from Earlier Scales

4. Supplementary Information and Differences from Earlier Scales

The apparatus, methods and procedures that will serve to realize the ITS-90 are given in "Supplementary Information for the ITS-90." This document also gives an account of the earlier International Temperature Scales and the numerical differences between successive scales that include, where practicable, mathematical functions for differences T₉₀—T₆₈. A number of useful approximations to the ITS-90 are given in "Techniques for Approximating the ITS-90."

These two documents have been prepared by the Comité Consultatif de Thermométrie and are published by the BIPM; they are revised and updated periodically. The differences T₉₀— T₆₈ are shown in Figure 1 and Table 6. The number of significant figures given in Table 6 allows smooth interpolations to be made.

However, the reproducibility of the IPTS-68 is, in many areas, substantially worse than is implied by this number.

Table 6. Differences Between ITS-90 and EPT-76, and Between ITS-90 and IPTS-68 for Specified Values of T₉₀and t₉₀.

(T₉₀—T₇₆) / mK

(T₉₀—T₆₈) / K

(t₉₀— t₆₈) / ºC

* A discontinuity in the first derivative of (t₉₀—t₆₈) occurs at a temperature of t₉₀ = 630.6 ºC, at which (t₉₀—t₆₈) = -0.125 ºC

Takeaways and Applications of “4. Supplementary Information and Differences from Earlier Scales”

The fourth section of the ITS-90 provides a table listing out the differences between ITS-90, EPT-76, and IPTS-68.

Calibration Tools for ITS-90

ITS-90 remains a cornerstone of modern metrology, bridging the gap between theoretical precision and practical application. Calibration professionals and engineers can ensure measurement accuracy by adopting tools that help with ITS-90-compliance. 

Fluke Calibration offers a range of products, from water triple-point cells to precision resistance thermometers, to meet these needs.

Related Resources

• Resistance Table Generator

Related Products

• Fluke Calibration 5627A PRT
• Fluke 5626, 5628 | Secondary Standard PRTs
• Fluke Triple Point of Water Cells