ISO 80000-5:2019

Overview

ISO 80000-5:2019 - Quantities and units - Part 5: Thermodynamics defines names, symbols, definitions and units for thermodynamic quantities and, where appropriate, gives conversion factors. This second edition (2019) updates and simplifies the table of quantities from the 2007 edition and clarifies several physical definitions. The standard is part of the ISO 80000 series and aligns thermodynamic quantities with the International System of Quantities (ISQ) and SI units.

Key topics

• Scope and purpose
  • Standardizes thermodynamic terminology, symbols and units (Table 1).
  • Supports consistent use across metrology, engineering, research and documentation.
• Fundamental quantities
  • Thermodynamic temperature (T, K) - definition via partial derivative of internal energy w.r.t. entropy at constant volume and particles; Kelvin is a base quantity in ISQ.
  • Celsius temperature (°C) - defined relative to thermodynamic temperature; rules on symbol use and that prefixes are not allowed for °C.
  • References to the International Temperature Scale of 1990 (ITS‑90) for practical measurements and guidance on conversion to thermodynamic temperature.
• Transport and thermal properties
  • Heat (Q), latent heat, heat flow rate, and density of heat flow.
  • Thermal conductivity (λ), thermal diffusivity (a), surface heat-transfer coefficient (h) and related coefficients.
  • Thermal resistance (R), conductance (G), insulance (M) and conventions used in building technology.
• Capacity and energy
  • Heat capacity (C) and specific heat capacities (cp, cV, csat).
  • Enthalpy (H), internal energy (U), Helmholtz (A) and Gibbs (G) functions, and their specific forms (per mass).
• Mechanical–thermodynamic links
  • Compressibilities (isothermal ϰT, isentropic ϰS), expansion coefficients (linear, cubic) and ratio of specific heats (γ).
• Entropy (S) - Boltzmann definition S = k ln W and specific entropy (s).
• Practical notation rules - symbol formatting and cross-references to other ISO 80000 parts.

Applications and users

ISO 80000-5:2019 is intended for:

• Metrologists and national measurement institutes for temperature and thermal property calibration.
• Mechanical, chemical and HVAC engineers for thermodynamic calculations, simulation and product specifications.
• Building energy modelers and architects using thermal resistance/conductance conventions.
• Instrument and software developers implementing unit libraries, unit conversion and documentation.
• Educators, authors and standards writers for consistent terminology and symbols in textbooks and standards.

Related standards

• ISO 80000-1 - General quantities and units (ISQ/SI relationships)
• ISO 80000-3, -4, -9 - Cross-referenced parts for space, mechanics, mass and amount of substance
• IEC/ISO 80000 series - complementary parts on quantities and units

Keywords: ISO 80000-5:2019, thermodynamics quantities and units, thermodynamic temperature, entropy, enthalpy, heat capacity, thermal conductivity, ITS-90, SI units.