Units of Measurement and Reference States

Due to the flexibility provided by the IDAES modeling framework, there is no standard set of units of measurement or standard reference state that should be used in models. Units of measurement are defined by the modeler for the 7 base quantities (time, length, mass, amount, temperature, current and luminous intensity) in each property package, and the framework makes use of this and Pyomo’s Units container to automatically determine the units of all variables and expressions within a model. Thus, all components within a model using a given property package must use units based on the units chosen for the base quantities (to ensure consistency of units). However, flowsheets may contain property packages which use different sets of base units, however users should be careful to ensure units are converted correctly where property packages interact. For more detail on defining units of measurement see Defining Units of Measurement.

Pyomo also provides convenient tools for converting between different units of measurement and checking for unit consistency, of which a few are highlighted below:

  • units.convert(var, to_units=units) - returns a Pyomo expression including the variable var and the necessary conversion factors to convert it to the desired set of units (units). This method will return an Exception if the units of var are not consistent with those requested by the user.
  • units.assert_units_consistent(object) - checks for consistency of units in object and raises an AssertionError if they are not. object may be a Block, Constraint or Expression.

The IDAES developers have generally used SI units without prefixes (i.e. Pa, not kPa) within models developed by the institute, with a default thermodynamic reference state of 298.15 K and 101325 Pa. Supercritical fluids have been consider to be part of the liquid phase, as they will be handled via pumps rather than compressors.

Standard Variable Names

In order for different models to communicate information effectively, it is necessary to have a standard naming convention for any variable that may need to be shared between different models. Within the IDAES modeling framework, this occurs most frequently with information regarding the state and properties of the material within the system, which is calculated in specialized property blocks, and then used in others parts of the model. This section of the documentation discusses the standard naming conventions used within the IDAES modeling framework.

Standard Naming Format

There are a wide range of different variables which may be of interest to modelers, and a number of different ways in which these quantities can be expressed. In order to facilitate communication between different parts of models, a naming convention has been established to standardize the naming of variables across models. Variable names within IDAES follow to the format below:


Here, property_name is the name of the quantity in question, and should be drawn from the list of standard variable names given later in this document. If a particular quantity is not included in the list of standard names, users are encouraged to contact the IDAES developers so that it can be included in a future release. This is followed by a number of qualifiers which further indicate the specific conditions under which the quantity is being calculated. These qualifiers are described below, and some examples are given at the end of this document.

Basis Qualifier

Many properties of interest to modelers are most conveniently represented on an intensive basis, that is quantity per unit amount of material. There are a number of different bases that can be used when expressing intensive quantities, and a list of standard basis qualifiers are given below.

Basis Standard Name
Mass Basis mass
Molar Basis mol
Volume Basis vol

State Qualifier

Many quantities can be calculated either for the whole or a part of a mixture. In these cases, a qualifier is added to the quantity to indicate which part of the mixture the quantity applies to. In these cases, quantities may also be indexed by a Pyomo Set.

Basis Standard Name Comments
Component comp Indexed by component list
Phase phase Indexed by phase list
Phase & Component phase_comp Indexed by phase and component list
Total Mixture   No state qualifier
Phase Standard Name
Supercritical Fluid liq
Ionic Species ion
Liquid Phase liq
Solid Phase sol
Vapor Phase vap
Multiple Phases e.g. liq1

Condition Qualifier

There are also cases where a modeler may want to calculate a quantity at some state other than the actual state of the system (e.g. at the critical point, or at equilibrium).

Basis Standard Name
Critical Point crit
Equilibrium State equil
Ideal Gas ideal
Reduced Properties red
Reference State ref


IDAES contains a library of common physical constants of use in process systems engineering models, which can be imported from idaes.core.util.constants. Below is a list of these constants with their standard names and values (SI units).


It is important to note that these constants are represented as Pyomo expressions in order to include units of measurement. As such, they can be directly included in other expressions within a model. However, if the user desires to use their value directly (e.g. to initialize a variable), the value() method must be used to extract the value of the constant from the expression.

Constant Standard Name Value Units
Acceleration due to Gravity acceleration_gravity 9.80665 \(m⋅s^{-2}\)
Avogadro’s Number avogadro_number 6.02214076e23 \(mol^{-1}\)
Boltzmann Constant boltzmann_constant 1.38064900e-23 \(J⋅K^{-1}\)
Elementary Charge elementary_charge 1.602176634e-19 \(C\)
Faraday’s Constant faraday_constant 96485.33212 \(C⋅mol^{-1}\)
Gas Constant gas_constant 8.314462618 \(J⋅mol^{-1}⋅K^{-1}\)
Newtonian Constant of Gravitation gravitational_constant 6.67430e-11 \(m^3⋅kg^{-1}⋅s^{-2}\)
Mass of an Electron mass_electron 9.1093837015e-31 \(kg\)
Pi (Archimedes’ Constant) pi 3.141592 [1]  
Planck Constant planck_constant 6.62607015e-34 \(J⋅s\)
Stefan-Boltzmann Constant stefan_constant 5.67037442e-8 \(W⋅m^{-2}⋅K^{-4}\)
Speed of Light in a Vacuum speed_light 299792458 \(m⋅s^{-1}\)

[1] pi imported from the Python math library and is available to machine precision.

Values for fundamental constants and derived constants are drawn from the definitions of SI units ( and are generally defined to 9 significant figures.

Acceleration due to gravity, gravitational constant and electron mass are sourced from NIST ( and used the significant figures reported there.

Thermophysical and Transport Properties

Below is a list of all the thermophysical properties which currently have a standard name associated with them in the IDAES framework.

Variable Standard Name
Activity act
Activity Coefficient act_coeff
Bubble Pressure pressure_bubble
Bubble Temperature temperature_bubble
Compressibility Factor compress_fact
Concentration conc
Density dens
Dew Pressure pressure_dew
Dew Temperature temperature_dew
Diffusivity diffus
Diffusion Coefficient (binary) diffus_binary
Enthalpy enth
Entropy entr
Fugacity fug
Fugacity Coefficient fug_coeff
Gibbs Energy energy_gibbs
Heat Capacity (const. P) cp
Heat Capacity (const. V) cv
Heat Capacity Ratio heat_capacity_ratio
Helmholtz Energy energy_helmholtz
Henry’s Constant henry
Internal Energy energy_internal
Mass Fraction mass_frac
Material Flow flow
Molecular Weight mw
Mole Fraction mole_frac
pH pH
Pressure pressure
Speed of Sound speed_sound
Surface Tension surf_tens
Temperature temperature
Thermal Conductivity therm_cond
Vapor Pressure pressure_sat
Viscosity (dynamic) visc_d
Viscosity (kinematic) visc_k
Vapor Fraction vap_frac
Volume Fraction vol_frac

Reaction Properties

Below is a list of all the reaction properties which currently have a standard name associated with them in the IDAES framework.

Variable Standard Name
Activation Energy energy_activation
Arrhenius Coefficient arrhenius
Heat of Reaction dh_rxn
Entropy of Reaction ds_rxn
Equilibrium Constant k_eq
Reaction Rate reaction_rate
Rate constant k_rxn
Solubility Constant k_sol

Solid Properties

Below is a list of all the properties of solid materials which currently have a standard name associated with them in the IDAES framework.

Variable Standard Name
Min. Fluidization Velocity velocity_mf
Min. Fluidization Voidage voidage_mf
Particle Size particle_dia
Pore Size pore_dia
Porosity particle_porosity
Specific Surface Area area_{basis}
Sphericity sphericity
Tortuosity tort
Voidage bulk_voidage

Naming Examples

Below are some examples of the IDAES naming convention in use.

Variable Name Meaning
enth Specific enthalpy of the entire mixture (across all phases)
flow_comp[“H2O”] Total flow of H2O (across all phases)
entr_phase[“liq”] Specific entropy of the liquid phase mixture
conc_phase_comp[“liq”, “H2O”] Concentration of H2O in the liquid phase
temperature_red Reduced temperature
pressure_crit Critical pressure