ISO 80000-5: Quantities and Units of Thermodynamics

ISO 80000-5 codifies the thermodynamic quantities that describe how heat, work, and matter interact. Use this reference when aligning laboratory protocols, building energy models, or process simulators with the International System of Units (SI).

Start with the foundational language in ISO 80000-1 and the notation guidelines from ISO 80000-2 so every variable and index in your thermodynamic equations is compliant. When your studies involve motion or control volumes, revisit the ISO 80000-3 definitions for space and time and pair them with ISO 80000-4 to keep force and energy terminology synchronised.

Need a panorama before diving deep? Scan the ISO 80000 overview and the part-by-part quick tables to understand how the thermodynamics chapter interfaces with electromagnetism, light, and physical chemistry volumes. Apply the definitions immediately with calculators such as the Celsius to kelvin converter or the specific heat energy calculator to reinforce unit discipline with practical numbers, then read the specific heat capacity explainer for the full ISO 80000 treatment of c values. For the constant-based definition that underpins these conversions, spend a moment with the kelvin base unit guide so every calculation traces back to the Boltzmann constant.

When your heat-transfer studies require dimensionless diagnostics, jump to the Nusselt number explainer and its companion guides on the Reynolds number and Mach number to keep convection, inertia, and compressibility assumptions aligned with ISO 80000 terminology.

What ISO 80000-5 covers

Equilibrium states

Explains how temperature, pressure, and specific volume describe the macroscopic condition of closed and open systems in thermodynamic balance.

Energy accounting

Clarifies the relationships between internal energy, enthalpy, and heat so energy balances match the SI joule framework.

Entropy and potentials

Defines entropy, specific entropy, and related potentials that drive irreversibility analysis and efficiency metrics.

Material response

Covers heat capacities, compressibilities, and expansion coefficients that engineers need for property tables and simulations.

Review the SI base units Connect with mechanical energy

Principal ISO 80000-5 quantities and unit definitions

Use this table as a ready reference when documenting laboratory measurements, configuring building automation systems, or publishing energy audits. Each entry highlights the coherent SI unit required by ISO 80000-5 together with typical applications that reinforce good practice.

Quantity Symbol Coherent unit ISO 80000-5 definition Example application
Thermodynamic temperature T kelvin (K) Absolute temperature measured from the Boltzmann constant definition; anchors all other thermal quantities in ISO 80000-5. Calibrating temperature sensors or setting process control ranges for cryogenic storage systems.
Celsius temperature t degree Celsius (°C) Offset temperature scale defined by t = T − 273.15; used for everyday applications while retaining kelvin-based increments. Communicating HVAC set points while maintaining traceability to absolute temperature measurements.
Thermal energy (heat) Q joule (J) Energy transferred across the boundary of a system due to temperature difference; distinct from work in energy balances. Estimating boiler loads or heat exchanger duties in process design documents.
Internal energy U joule (J) Total microscopic energy stored within a system, including molecular kinetic and potential contributions. Computing changes in internal energy to size compressors and turbines using the first law of thermodynamics.
Enthalpy H joule (J) State function H = U + pV that simplifies open-system energy balances where flow work is present. Tabulating specific enthalpy values for steam cycles and refrigeration loops.
Entropy S joule per kelvin (J/K) Measure of dispersal of energy at a given temperature; quantifies irreversibility and drives second-law analyses. Assessing turbine efficiency by comparing actual entropy change with reversible benchmarks.
Specific entropy s joule per kilogram kelvin (J/(kg·K)) Entropy normalised by mass to support property tables and unit-mass energy calculations. Using refrigerant tables to track entropy changes per kilogram in vapor-compression systems.
Heat capacity C joule per kelvin (J/K) Energy required to raise the temperature of a system by one kelvin at specified constraints. Designing calorimetry experiments that measure constant-pressure heat capacity of new materials.
Specific heat capacity (constant pressure) c_p joule per kilogram kelvin (J/(kg·K)) Heat capacity per unit mass for processes at constant pressure, relevant for most gas flow calculations. Sizing air handling equipment that relies on c_p values to predict heating and cooling loads.
Specific heat capacity (constant volume) c_v joule per kilogram kelvin (J/(kg·K)) Heat capacity per unit mass measured under constant volume conditions, often used with ideal gas relations. Calculating internal energy changes in sealed combustion chambers or cryogenic vessels.
Volumetric thermal expansion coefficient α per kelvin (1/K) Fractional change in volume per unit temperature increase at constant pressure. Checking expansion allowances in storage tanks and piping systems for liquids sensitive to temperature.
Isothermal compressibility κ_T per pascal (1/Pa) Relative change in volume with pressure at constant temperature, used for fluid property characterisation. Modeling high-pressure gas storage where compressibility impacts mass inventory calculations.
Gibbs free energy G joule (J) Thermodynamic potential G = H − TS that predicts spontaneous direction and equilibrium in processes at constant temperature and pressure. Evaluating chemical reaction feasibility or phase changes in process simulation software.

Why these definitions matter

Coherent thermodynamic units underpin energy efficiency claims, emissions reporting, and the safety cases behind heating and cooling technologies. These takeaways explain how ISO 80000-5 keeps cross-disciplinary projects aligned.

Kelvin-first temperature scale

ISO 80000-5 locks temperature measurements to the kelvin defined via the Boltzmann constant, guaranteeing that Celsius, Fahrenheit, and Rankine conversions derive from a single absolute reference. Share this with operators using our <a class="underline decoration-dotted underline-offset-4 transition hover:text-white" href="/units-and-measures/degree-celsius-definition-history-and-applications/">degree Celsius explainer</a> so human-readable logs stay aligned with SI requirements.

Energy bookkeeping with enthalpy and internal energy

Clear definitions of U and H prevent double-counting work and flow terms, keeping heat balance spreadsheets and digital twins aligned with the first law.

Entropy as a compliance metric

By standardising entropy units, the part ensures that efficiency claims, exergy audits, and environmental reports compare apples to apples across vendors and regulators.

Implementation checklist

Converting thermodynamic theory into everyday practice requires consistent documentation, calibrated instruments, and shared reference materials. Use this checklist to embed ISO 80000-5 in operations, maintenance, and analytics workflows.

  1. Audit temperature references

    List every sensor, historian tag, and report that logs temperature. Confirm that raw data is stored in kelvin or clearly documents the offset from absolute zero before conversion to Celsius or Fahrenheit displays.

  2. Normalise energy balances

    Update process flow diagrams, building energy models, and spreadsheet calculators to distinguish between heat, work, internal energy, and enthalpy, citing ISO 80000-5 terminology in annotations.

  3. Publish property tables

    Create or refresh thermophysical property tables that include specific heat capacities, compressibilities, and expansion coefficients with coherent SI units for engineering and maintenance teams.

Reinforce adoption with training sessions that cite the SI base unit guide and worked examples pulled from the ideal gas pressure calculator or the water heater recovery time tool.

Recommended follow-up reading

Expand beyond thermodynamics with these in-depth explainers from across the Units & Measures library. Each one reinforces the terminology shared across ISO 80000.

Practice with calculators

Bring the terminology to life by testing assumptions with interactive tools. These calculators reinforce the kelvin scale, the joule-based energy bookkeeping, and the entropy-aware analyses defined by ISO 80000-5.

Continue exploring thermal topics across CalcSimpler by checking our newsroom for energy technology coverage or by visiting the Science hub for adjacent research explainers.