Overview
EN ISO 80000-5:2019 / ISO 80000-5:2019 - Quantities and units - Part 5: Thermodynamics - is the internationally adopted CEN/ISO standard that defines names, symbols, definitions and SI units for thermodynamic quantities. The standard clarifies how to express thermodynamic temperature, heat, entropy, heat capacity, thermal conductivity and related quantities, and provides conversion guidance where appropriate (for example between thermodynamic temperature and the International Temperature Scale ITS‑90). EN ISO 80000-5:2019 supersedes the 2013 edition and aligns thermodynamics terminology with the ISO 80000 series.
Key topics and technical requirements
- Thermodynamic temperature (T) - defined thermodynamically as the partial derivative of internal energy with respect to entropy at constant volume and particle number; units: kelvin (K). The standard notes ITS‑90 as the practical temperature scale approximation.
- Celsius temperature (°C) - definition and relationship to thermodynamic temperature; use of the degree Celsius and rules about spacing and prefixes.
- Heat (Q) and latent heat - definitions, units (joule, J), and remarks on expressing heat transfer in terms of state function changes (e.g., TΔS, ΔH).
- Heat flow rate and flux - definitions of heat flow rate (W) and density of heat flow (W/m2), with symbols and unit expressions.
- Thermal transport properties - thermal conductivity (λ), coefficient of heat transfer (k or K), surface coefficient (h), thermal diffusivity (a), and related definitions and units (W/(m·K), m2/s).
- Thermal resistances and conductances - thermal insulance, thermal resistance (R, K/W) and thermal conductance (G, W/K), and recommended usage notes (building technology terminology).
- Mechanical/thermal expansion and compressibility - linear and volumetric expansion coefficients, isothermal and isentropic compressibilities, with definitions and units (K‑1, Pa‑1).
- Heat capacity and specific heat capacity - formulation as derivative of heat with respect to temperature and link to mass- or amount-based quantities.
Applications and who uses it
- Engineers (mechanical, chemical, HVAC, building physics) applying consistent units and symbols for thermal calculations and reports.
- Metrologists and calibration laboratories needing standard definitions for temperature scales (kelvin, ITS‑90) and measurement traceability.
- Technical writers, educators and standards bodies producing documents, textbooks and software that require unambiguous thermodynamic terminology.
- Product designers and simulation teams using thermophysical property databases, CFD and heat‑transfer models that must follow standardized quantity definitions.
Related standards
- ISO 80000 series (general quantities and units), specifically ISO 80000-1, ISO 80000-3, ISO 80000-4, which are referenced for base quantities (e.g., Boltzmann constant, length, volume, pressure) and symbol rules.
Keywords: EN ISO 80000-5:2019, ISO 80000-5:2019, thermodynamics quantities and units, thermodynamic temperature, heat capacity, thermal conductivity, SI units, ITS‑90.