ISO 80000-5:2007

Overview

ISO 80000-5:2007, “Quantities and units - Part 5: Thermodynamics”, defines the names, symbols and definitions for thermodynamic quantities and their units. Published as part of the ISO/IEC 80000 family, it standardizes thermodynamic terminology, unit usage (SI and acceptable non‑SI), conversion factors and presentation conventions to ensure consistent measurement, reporting and data exchange across science, engineering and metrology.

Key topics

• Quantity names, symbols and definitions: authoritative entries for thermodynamic quantities such as thermodynamic temperature (T), Celsius temperature (t), heat (Q), heat flow rate (Φ), areic heat flow (q), thermal conductivity (λ), expansion coefficients (linear αl, cubic αV), isothermal and isentropic compressibility (κT, κS), entropy (S), enthalpy (H), etc.
• Units and unit hierarchy: coherent SI units are listed first (e.g., kelvin K, degree Celsius °C), followed by non‑SI units accepted for use with the SI; annexes provide deprecated or regionally based units (e.g., foot–pound–second family) and conversion factors.
• Definitions and metrological references: the kelvin is defined by the triple point of water and the standard includes reference to the International Temperature Scale of 1990 (ITS‑90) for practical measurements.
• Presentation and notation rules: conventions for numerical statements (=, ≈, :=), uncertainty notation (example: 2.34782(32) m), and guidance on dimensionless units (rad, sr treated as dimensionless derived units).
• Tables and layout: quantities are presented on left‑hand pages with corresponding units and conversion factors on right‑hand pages to aid clarity and consistent use.

Practical applications

• Ensures unit consistency in technical documentation, research papers, instrumentation manuals and software (simulation, data acquisition).
• Supports metrology and calibration labs in traceable temperature and heat measurements, and in applying ITS‑90 realizations.
• Guides engineers (mechanical, chemical, HVAC, materials) and scientists when specifying sensors, reporting thermal properties (conductivity, heat transfer coefficients) or converting between unit systems.
• Useful for standards writers, educators and data interoperability efforts where unambiguous symbols and definitions are required.

Who should use it

• Metrologists, instrument manufacturers, thermal analysts, HVAC and process engineers, academic researchers, standards developers, and technical authors needing authoritative thermodynamic quantity and unit definitions.

Related standards

• ISO 80000 series (Parts 1–14) for quantities and units across disciplines
• ISO 80000-3 (Space and time), ISO 80000-4 (Mechanics)
• SI Brochure (BIPM) and ITS‑90 references for practical temperature scales

By adopting ISO 80000-5:2007, organizations achieve clearer, internationally consistent communication of thermodynamic measurements, unit conversions and technical data.