This information is part of the Modelica Standard Library maintained by the Modelica Association.

Every medium model provides the following constants. For example, if a medium is declared as:

replaceable package Medium = Modelica.Media.Interfaces.PartialMedium;

then constants "Medium.mediumName", "Medium.nX", etc. are defined:

Type	Name	Description
String	mediumName	Unique name of the medium (is usually used to check whether the media in different components connected together are the same, by providing Medium.mediumName as quantity attribute of the mass flow rate in the connector)
String	substanceNames[nS]	Names of the substances that make up the medium. If only one substance is present, substanceNames = {mediumName}.
String	extraPropertiesNames[nC]	Names of the extra transported substances, outside of mass and energy balances.
Boolean	singleState	= true, if u and d are not a function of pressure, and thus only a function of a single thermal variable (temperature or enthalpy) and of Xi for a multiple substance medium. Usually, this flag is true for incompressible media. It is used in a model to determine whether 1+nXi (singleState=true) or 2+nXi (singleState=false) initial conditions have to be provided for a volume element that contains mass and energy balance.
AbsolutePressure	reference_p	Reference pressure for the medium
MassFraction	reference_X[nX]	Reference composition for the medium
AbsolutePressure	p_default	Default value for pressure of medium (for initialization)
Temperature	T_default	Default value for temperature of medium (for initialization)
SpecificEnthalpy	h_default	Default value for specific enthalpy of medium (for initialization)
MassFraction	X_default[nX]	Default value for mass fractions of medium (for initialization)
Integer	nS	number of substances contained in the medium.
Integer	nX	Size of the full mass fraction vector X nX=nS.
Integer	nXi	Number of independent mass fractions. If there is a single substance, then nXi = 0.
Boolean	reducedX	= true, if the medium has a single substance, or if the medium model has multiple substances and contains the equation sum(X) = 1. In both cases, nXi = nS - 1 (unless fixedX = true). = false, if the medium has multiple substances and does not contain the equation sum(X)=1, i.e., nXi = nX = nS (unless fixedX = true).
Boolean	fixedX	= false: the composition of the medium can vary, and is determined by nXi independent mass fractions (see reducedX above). = true: the composition of the medium is always reference_X, and nXi = 0.
FluidConstants	fluidConstants[nS]	Critical, triple, molecular and other standard data that are provided for every substance of a medium.

The record FluidConstants that is defined in PartialMedium contains the following elements

Type	Name	Description
String	iupacName	complete IUPAC name
String	casRegistryNumber	chemical abstracts sequencing number
String	chemicalFormula	Chemical formula, (brutto, nomenclature according to Hill)
String	structureFormula	Chemical structure formula
MolarMass	molarMass	molar mass

This record is extended in the partial packages further down the hierarchy (such as PartialTwoPhaseMedium or PartialMixtureMedium) and may contain some or all of the following elements

Temperature	criticalTemperature	critical temperature
AbsolutePressure	criticalPressure	critical pressure
MolarVolume	criticalMolarVolume	critical molar Volume
Real	acentricFactor	Pitzer acentric factor
Temperature	triplePointTemperature	triple point temperature
AbsolutePressure	triplePointPressure	triple point pressure
Temperature	meltingPoint	melting point at 101325 Pa
Temperature	normalBoilingPoint	normal boiling point (at 101325 Pa)
DipoleMoment	dipoleMoment	dipole moment of molecule in Debye (1 debye = 3.33564e10-30 C.m)
Boolean	hasIdealGasHeatCapacity	true if ideal gas heat capacity is available
Boolean	hasCriticalData	true if critical data are known
Boolean	hasDipoleMoment	true if a dipole moment known
Boolean	hasFundamentalEquation	true if a fundamental equation
Boolean	hasLiquidHeatCapacity	true if liquid heat capacity is available
Boolean	hasSolidHeatCapacity	true if solid heat capacity is available
Boolean	hasAccurateViscosityData	true if accurate data for a viscosity function is available
Boolean	hasAccurateConductivityData	true if accurate data for thermal conductivity is available
Boolean	hasVapourPressureCurve	true if vapour pressure data, e.g., Antoine coefficients are known
Boolean	hasAcentricFactor	true if Pitzer acentric factor is known
SpecificEnthalpy	HCRIT0	Critical specific enthalpy of the fundamental equation
SpecificEntropy	SCRIT0	Critical specific entropy of the fundamental equation
SpecificEnthalpy	deltah	Difference between specific enthalpy model (h_m) and f.eq. (h_f) (h_m - h_f)
SpecificEntropy	deltas	Difference between specific enthalpy model (s_m) and f.eq. (s_f) (s_m - s_f)