Conventions¶
Units of Measurement and Reference States¶
Due to the flexibility provided by the IDAES Integrated Platform, 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 platform 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 IDAES, this occurs most frequently when information regarding the state and properties of the material, which is calculated in specialized PropertyBlocks, is used in others parts of the model.
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:
{property_name}_{basis}_{state}_{condition}
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 
Constants¶
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).
Note
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 
1.38064900e23 
\(J⋅K^{1}\) 
Elementary Charge 
elementary_charge 
1.602176634e19 
\(C\) 
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 
6.67430e11 
\(m^3⋅kg^{1}⋅s^{2}\) 
Mass of an Electron 
mass_electron 
9.1093837015e31 
\(kg\) 
Pi (Archimedes’ Constant) 
pi 
3.141592 [1] 

Planck Constant 
planck_constant 
6.62607015e34 
\(J⋅s\) 
StefanBoltzmann Constant 
stefan_constant 
5.67037442e8 
\(W⋅m^{2}⋅K^{4}\) 
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 (https://www.bipm.org/utils/common/pdf/sibrochure/SIBrochure9.pdf) and are generally defined to 9 significant figures.
Acceleration due to gravity, gravitational constant and electron mass are sourced from NIST (https://physics.nist.gov) and used the significant figures reported there.
Thermophysical and Transport Properties¶
Below is a list of all the thermophysical properties which have standardized names.
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 
Molality 
molality 
Molecular Weight 
mw 
Mole Fraction 
mole_frac 
Osmotic Pressure 
pressure_osm 
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 have standardized names.
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 have standardized names.
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 