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ISO 1

ISO 1 is an international standard published by the International Organization for Standardization (ISO) that defines the standard reference temperature for the specification of geometrical and dimensional properties of objects as 20 °C, according to the International Temperature Scale of 1990 (ITS-90).^[1] This temperature serves as a fixed, universally agreed-upon value to ensure unambiguous and consistent measurements in fields such as manufacturing, engineering, and metrology, where thermal expansion can affect dimensions.^[2] The standard distinguishes between a reference temperature, which is the temperature of an object assumed to be uniform and specified as part of a geometrical or dimensional property definition, and the standard reference temperature, which is the specific value of 20 °C adopted internationally for these purposes.^[1] It applies to the specification, verification, and calibration of measurands related to properties like size, location, orientation, form, and surface texture within the Geometrical Product Specifications (GPS) framework.^[2] By establishing this reference, ISO 1 minimizes errors arising from temperature variations, promoting interoperability in global standards for precision engineering and quality control.^[3] First introduced in 1951 and formally published as ISO 1 in 1975, the standard has evolved through multiple editions, with the current fourth edition (ISO 1:2022) released in June 2022 to refine definitions and align with modern metrological practices while retaining the 20 °C value established in earlier revisions.^[2]^[4] This consistency traces back to historical conventions in length metrology, where 20 °C was chosen as a practical room temperature for industrial measurements, replacing earlier national variations like 0 °C or 15.5 °C.^[4]

Introduction

Scope and Purpose

ISO 1 establishes the foundational concepts for specifying a reference temperature in the context of geometrical product specifications (GPS), applying specifically to the definition of geometrical and dimensional properties of workpieces and measuring equipment. These properties include size, form, orientation, location, run-out, and surface texture, ensuring that specifications account for thermal influences on physical objects. The standard extends to the measurand definitions used in verification and calibration processes, promoting precision in metrological assessments.^[2]^[1] The purpose of ISO 1 is to provide a uniform reference temperature framework that enables unambiguous and consistent specification of geometrical and dimensional properties, which are inherently sensitive to temperature variations due to thermal expansion. This uniformity supports the reliable definition of measurands across functional requirements, verification, and calibration activities within the GPS system, thereby facilitating interoperability and accuracy in industrial applications. As a fundamental GPS standard, it underpins the broader ISO 14638 framework for geometrical product specifications.^[2]^[1] ISO 1 applies universally whenever GPS specifications are invoked for products or equipment, serving as an essential baseline for metrological consistency. However, it does not address thermal expansion coefficients or provide detailed methods for correcting measurements at non-standard temperatures, focusing instead solely on the reference temperature specification.^[2]^[1]

Role in Geometrical Product Specifications

ISO 1 serves as a fundamental standard within the Geometrical Product Specifications (GPS) framework, providing the essential temperature basis upon which all other GPS standards define and evaluate geometrical and dimensional properties. By establishing a consistent reference for thermal conditions, it ensures that specifications across the GPS matrix—outlined in ISO 14638—are aligned, preventing discrepancies arising from environmental variations during design and production processes.^[2]^[5] This integration positions ISO 1 at the core of the GPS system, influencing chain links from population specification to verification by mandating that product properties be referenced to a standardized thermal state.^[6] In the specification chain, ISO 1 enables the unambiguous definition of nominal geometry and tolerances by assuming a reference temperature, thereby linking directly to the general GPS framework in ISO 14638. This support is critical for technical drawings and product definitions, where dimensional features must be specified without implicit temperature assumptions that could lead to misinterpretation. For instance, in manufacturing drawings, ISO 1's reference temperature prevents tolerance variations due to unstated thermal effects on materials, ensuring that designers and manufacturers apply consistent criteria for form, orientation, location, and runout.^[5]^[7] Regarding verification, ISO 1 ensures that measurands, such as linear dimensions or surface profiles, are defined with reference to the standard temperature, allowing measurements taken under non-reference conditions to be corrected or accounted for in uncertainty to eliminate ambiguity in acceptance criteria. This relation to the verification process in the GPS matrix facilitates accurate metrological assessments, promoting interoperability in global supply chains.^[2]^[5] By doing so, it underpins the duality principle of specification and verification central to GPS, reducing errors in quality control and conformance testing.

Historical Development

Origins and First Edition

The International Organization for Standardization (ISO) was established in 1947 in Geneva, Switzerland, as a successor to the International Federation of the National Standardizing Associations (ISA), which had operated from 1926 until its dissolution during World War II due to geopolitical disruptions.^[8]^[9] This new body aimed to coordinate global standardization efforts, particularly in technical fields like metrology, where inconsistencies in measurement practices hindered international trade and industrial collaboration. In the realm of length measurements, thermal expansion posed a significant challenge, as materials expand or contract with temperature changes, leading to discrepancies in precision engineering and manufacturing across borders.^[4] The inaugural ISO recommendation, ISO/R 1:1951, titled "Standard reference temperature for industrial length measurements," was published on January 1, 1951, marking the organization's first formal output.^[8] This document established 20 °C as the uniform reference temperature for calibrating and verifying industrial length standards, ensuring that measurements could be consistently compared regardless of local environmental conditions. By adopting this value, which had been recommended by the International Committee for Weights and Measures (CIPM) in 1931 following deliberations on metrological uniformity, ISO addressed the practical need to mitigate errors from thermal variations in gauge blocks and other artifacts.^[4]^[3] Prior to this standardization, national practices varied widely, with countries employing different reference temperatures such as 0 °C in some metric systems, 16.67 °C (62 °F) in the United Kingdom, and approximately 16.7 °C (62 °F) in the United States, complicating cross-border exchanges in precision tools and components.^[4] The motivation for ISO/R 1:1951 stemmed from the post-war economic recovery, where harmonizing these practices was essential to facilitate global trade, enhance interoperability in engineering, and support the burgeoning fields of manufacturing and quality control. This foundational recommendation laid the groundwork for subsequent expansions in geometrical product specifications within the ISO framework.^[8]^[3]

Subsequent Revisions

The first edition of ISO 1, published in April 1975, formalized the standard reference temperature of 20 °C specifically for industrial length measurements, building on the earlier ISO recommendation from 1951 by establishing it as a full International Standard while retaining the core value and its linkage to the international metre prototype at 0 °C.^[10] This edition expanded the application beyond purely linear measures to encompass broader implications for geometrical specifications in industrial contexts, though its scope remained primarily focused on lengths.^[3] The second edition, ISO 1:2002, released on July 15, 2002, marked a significant shift by integrating the standard into the Geometrical Product Specifications (GPS) framework, broadening its scope from industrial length measurements to the specification and verification of all geometrical and dimensional properties.^[11] This revision aligned ISO 1 with ISO 14638, the GPS masterplan, ensuring consistency across related standards for macro- and microgeometry, and clarified that the 20 °C reference applies universally to geometric features unless otherwise specified.^[3]^[12] Subsequent updates continued to refine the standard's precision and applicability. The third edition, ISO 1:2016, published on September 1, 2016, retained the 20 °C value but introduced formal definitions for "reference temperature" and "standard reference temperature," refined the scope to emphasize geometrical and dimensional properties such as size, form, orientation, location, and surface texture, and allowed for alternative reference temperatures when explicitly stated in specifications.^[13]^[3] The most recent edition, the fourth edition ISO 1:2022, issued in June 2022, further aligned the 20 °C value with the International Temperature Scale of 1990 (ITS-90) and added Annex B to detail the scale's precision, noting a negligible 2.8 mK difference at 20 °C that equates to approximately 0.03 μm/m expansion for steel in most metrological applications.^[2]^[1] This edition also removed outdated references to other GPS standards in the introduction and Annex C to streamline the document.^[1] Across all editions since 1975, the emphasis on 20 °C as the standard reference temperature has remained consistent to ensure interoperability in global measurements.^[3] The development of these revisions has been led by ISO Technical Committee 213 (ISO/TC 213), "Dimensional and geometrical product specifications and verification," in collaboration with the European Committee for Standardization's Technical Committee 290 (CEN/TC 290) under the Vienna Agreement for technical harmonization between ISO and CEN.^[14]^[1] This process involves working groups, such as ISO/TC 213/WG 4, comprising experts from national metrology institutes like NIST, PTB, and INRIM, focusing on resolving ambiguities in scope, terminology, and practical application through iterative drafting and international review.^[3]

Core Technical Content

Key Definitions

In ISO 1:2022, the reference temperature is defined as the temperature of an object, having a uniform temperature, specified in the definition of a geometrical or dimensional property.^[2] This concept assumes isothermal conditions for the workpiece, allowing for the precise specification of properties such as size, form, or orientation at a designated temperature to account for thermal expansion effects in metrology.^[1] The standard reference temperature is established as the internationally agreed-upon reference temperature, which serves as the default value unless otherwise specified in the technical documentation.^[2] This definition separates the conceptual term from its assigned value of 20 °C, emphasizing its role as a fixed, global benchmark for consistency in specifications.^[1] These definitions are normative within the Geometrical Product Specifications (GPS) framework, drawing on the International Vocabulary of Metrology (VIM) as outlined in JCGM 200:2012 to ensure terminological precision and alignment with basic metrological concepts.^[15] By tying geometrical and dimensional properties unambiguously to temperature, ISO 1:2022 facilitates standardized interpretation across industries, mitigating ambiguities that could arise from varying environmental conditions.^[2]

Standard Reference Temperature Value

ISO 1 specifies the standard reference temperature as t_{90} = 20^\circ \mathrm{C}, where the subscript 90 indicates that this value is realized according to the International Temperature Scale of 1990 (ITS-90). This temperature serves as the default for geometrical product specifications (GPS), ensuring that length measurements and verifications are standardized under controlled conditions. The ITS-90 provides a practical approximation to thermodynamic temperature, calibrated using fixed points such as the triple point of water.^[16] The precision of this value is anchored to the ITS-90 definition, where the triple point of water is assigned exactly t_{90} = +0.01^\circ \mathrm{C}. Consequently, 20 °C corresponds to an interval of precisely 19.99 °C above this triple point, yielding $20^\circ \mathrm{C} = 19.99^\circ \mathrm{C} + 0.01^\circ \mathrm{C}. While the ITS-90 closely approximates the thermodynamic scale, there is a small deviation of approximately 2.8 mK at 20 °C, reflecting minor differences between the practical scale and absolute thermodynamic temperature, as clarified in Annex B of the 2022 edition. This alignment maintains high accuracy for metrological applications within the GPS framework.^[16]^[17]^[1] Although 20 °C is the prescribed default, ISO 1 permits the use of non-standard reference temperatures provided they are explicitly stated in the product specifications or technical documentation. This flexibility accommodates specific industrial needs while upholding the standard as the baseline for consistency in GPS. Such provisions ensure that deviations are documented and accounted for in measurements.^[2] The establishment of this value aligns with the International System of Units (SI), as endorsed by the General Conference on Weights and Measures (CGPM), promoting global uniformity in temperature references for dimensional metrology. By integrating with the ITS-90, ISO 1 supports reproducible and comparable results across international standards and practices.

Applications and Implications

Impact on Measurements and Verification

In metrology, the standard reference temperature of 20 °C defined by ISO 1:2022 plays a critical role in ensuring dimensional accuracy by providing a consistent baseline for correcting thermal expansion effects in measurements. Materials exhibit linear thermal expansion or contraction when subjected to temperatures deviating from this reference, altering their geometrical and dimensional properties. For instance, common engineering materials like steel have a coefficient of thermal expansion (\alpha) of approximately $12 \times 10^{-6} /^\circ\mathrm{C}, resulting in dimensional changes on the order of 12 μm per meter per degree Celsius deviation from 20 °C.^[18]^[19] These effects must be quantified and compensated to maintain traceability and precision in verification processes.^[3] Verification of geometrical product specifications (GPS) requires adjusting measured values (measurands) to the 20 °C reference using material-specific thermal expansion coefficients, as outlined in ISO 1:2022 and supporting technical reports like ISO/TR 16015. This correction process involves calculating the dimensional deviation based on the temperature difference (\Delta T) between the measurement environment and 20 °C, applying the formula \Delta L = L \times \alpha \times \Delta T, where L is the nominal length. For small deviations (e.g., less than 1 °C), the impact may be negligible in low-precision applications, but in high-precision metrology—such as coordinate measuring machines (CMMs) or gage block calibrations—it becomes essential to avoid errors exceeding acceptable tolerances.^[1]^[19] Failure to apply these corrections can introduce systematic uncertainties, particularly for workpieces with large dimensions or high \alpha values.^[3] A practical example illustrates this: for a 1 m steel part measured at 21 °C, the temperature deviation is \Delta T = 1^\circ\mathrm{C}, yielding a correction of \Delta L \approx 1 \times (12 \times 10^{-6}) \times 1 = 12 \, \mu\mathrm{m}. This adjustment ensures the reported length aligns with the 20 °C reference, preserving interoperability in GPS verification.^[19]^[20] Calibration of measuring equipment further underscores the implications of the 20 °C reference, as most instruments—such as micrometers, CMMs, and interferometers—are calibrated at this temperature to minimize thermal influences on their scales and components. Deviations from 20 °C necessitate additional uncertainty contributions in calibration certificates, with traceability ensured through the International Temperature Scale of 1990 (ITS-90) for accurate temperature determinations during use.^[1]^[3] This approach allows for reliable verification even in non-ideal environmental conditions, provided corrections are rigorously applied.^[19]

Industrial and Metrological Uses

In manufacturing, ISO 1 establishes the standard reference temperature of 20 °C for specifying geometrical and dimensional tolerances in CAD models and engineering drawings, ensuring that production processes account for thermal variations to avoid defective parts. This is particularly critical in industries like automotive and aerospace, where precision components such as engine parts or airframe assemblies must meet tight tolerances; shop floor temperatures deviating from 20 °C could otherwise lead to rejects due to unintended dimensional changes during fabrication or assembly. By standardizing this reference, manufacturers can maintain consistency across design and production stages, minimizing waste and rework in high-volume environments.^[3]^[21] In metrology, ISO 1 is integral to the operation of coordinate measuring machines (CMMs) and precision gauges, where measurements are calibrated and reported at 20 °C to ensure traceability and accuracy. Metrology laboratories typically maintain controlled environments at 20 ± 0.5 °C with stable humidity to comply with the standard, as most measuring systems are designed to yield results referenced to this temperature, reducing errors from thermal expansion in workpieces or instruments. This practice supports reliable verification of manufactured parts, enabling quality assurance in sectors demanding high precision, such as aerospace component inspection.^[1]^[22]^[23] Annex A of ISO 1 provides guidance on specifying the reference temperature to align with functional requirements, such as assembly fit at operational temperatures, versus verification needs that default to 20 °C; it permits non-standard values when justified by the workpiece's intended use, provided they are clearly documented to avoid ambiguity. For instance, components expected to function at elevated temperatures may use a tailored reference to better reflect real-world performance, while verification remains at the standard to leverage existing metrology infrastructure. In high-accuracy contexts, Annex B references the International Temperature Scale of 1990 (ITS-90) for precise temperature realization during such measurements.^[3]^[1] The adoption of ISO 1 reduces interpretive discrepancies in specifications, fostering unambiguous communication in international supply chains and aligning with ISO 9001 quality management principles by standardizing measurement practices to enhance overall process reliability and certification compliance. This standardization minimizes risks associated with cross-border manufacturing, where varying environmental conditions could otherwise compromise product quality and interoperability.^[24]^[3]

Integration with GPS Standards

ISO 1 serves as a fundamental GPS standard within the Geometrical Product Specifications framework, establishing the reference temperature as a foundational element that applies across all specification and verification processes.^[2] As defined in the GPS framework (ISO 14638), ISO 1 provides the temperature basis for the matrix model, positioned among the specification operator aspects to support defining geometrical and dimensional properties in the chains of standards.^[5] This integration ensures that temperature effects are consistently accounted for in the specification of product features, linking to core GPS elements such as geometrical tolerancing in ISO 1101.^[25] The standard underpins the chain links in the GPS matrix for key properties, including size as defined in ISO 14405, form tolerances per ISO 1101, and surface texture parameters in ISO 21920, thereby ensuring uniformity in how thermal conditions influence these specifications.^[25] It supports default assumptions in related standards, such as the tolerancing principles in ISO 8015, where the reference temperature of 20 °C from ISO 1 applies unless otherwise specified.^[26] This interconnected chain reinforces the coherence of GPS by referencing all properties back to a standardized thermal condition, minimizing variability in product verification. In the 2022 edition of ISO 1, specific references to other GPS standards in the introduction and Annex C (relation to the GPS matrix model) were removed to prevent obsolescence as the GPS framework evolves, while maintaining its core role as the temperature foundation.^[1] This update streamlines the document without altering its integrative function within the broader GPS system.

Comparison to Other Temperature References

Prior to the establishment of ISO 1, metrological practices for dimensional measurements often relied on 0 °C as a reference temperature, particularly for the International Prototype Meter, due to its reproducibility at the freezing point of water, though this proved impractical for industrial applications owing to the challenges of maintaining such low temperatures and the resulting large thermal expansion corrections.^[4] In the United States, national practices included temperatures around 16.7 °C (equivalent to 62 °F) for calibrating surveying tapes, reflecting preferences tied to the U.S. survey foot definition, but these varied approaches led to inconsistencies in international comparisons and were eventually deemed obsolete for geometrical product specifications (GPS).^[3] Within the ISO framework, ISO 1 establishes 20 °C specifically for geometrical and dimensional properties in GPS, distinguishing it from other standards like ISO 14644-1, which governs cleanroom classifications and recommends operational temperature ranges of 18–25 °C to ensure air cleanliness and personnel comfort, without designating a fixed reference for dimensional measurements. Similarly, standards for thermometry, such as those aligned with ISO guidelines for precision instruments, often calibrate at 20 °C to match ambient conditions, but ISO 1 uniquely mandates this value for specification and verification of geometric tolerances, avoiding the broader ranges used in environmental controls.^[3] Non-ISO examples further illustrate the convergence on 20 °C; for instance, the Bureau International des Poids et Mesures (BIPM), through its historical recommendations via the Comité International des Poids et Mesures (CIPM), endorses 20 °C as the reference for length metrology to facilitate global traceability.^[4] In the U.S., standards for dimensional inspection, such as those for gauge blocks and precision measurement, adopt 20 °C as the default reference temperature, aligning with international practices to minimize variability in verification processes.^[3] The selection of 20 °C in ISO 1 reflects its advantages as an average room temperature in temperate climates, reducing the need for extensive thermal corrections compared to extremes like 0 °C, which could introduce errors up to several parts per million for materials with typical coefficients of thermal expansion, thereby enhancing practicality for manufacturing and quality assurance. This choice, formalized in 1931 by the CIPM and codified in ISO 1 since 1975, promotes consistency across global supply chains without requiring specialized environmental controls.^[4]

References

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Specifying a reference temperature allows the geometrical and dimensional properties of a physical object to be unambiguously stated. v. © ISO 2022 – All rights ...
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### Summary of ISO 1:2022
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This International Standard specifies the standard reference temperature for geometrical product specifications.
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