Reference · Symbols and units

Steam Property Glossary

Definitions, symbols, units, and validity limits for every property shown in PhaseGauge results.

Pressurep: MPaPressure is normal force per unit area and an independent coordinate of fluid state. IF97 uses absolute pressure. PhaseGauge converts every pressure input to absolute MPa; barg and psig add a fixed standard atmosphere of 101.325 kPa. Use an absolute unit when a specification refers to the measured local atmosphere.
TemperatureT: KThermodynamic temperature describes thermal state on an absolute scale. IF97 uses kelvin, so PhaseGauge converts Celsius and Fahrenheit inputs without rounding the stored value. Temperature alone does not define a water state. Near saturation or the critical point, a small temperature change can produce a large phase or property change.
Specific volume and densityv / ρ: m³/kg · kg/m³Specific volume is volume per unit mass; density is mass per unit volume. For a homogeneous state, ρ = 1/v. Wet-steam specific volume is calculated from vapor quality and the two saturation endpoints, then inverted for bulk density. Averaging the endpoint densities directly by quality gives the wrong result.
Vapor qualityx: dimensionlessVapor quality is the vapor mass fraction of an equilibrium saturated mixture. Zero is saturated liquid, one is saturated vapor, and values between them are wet steam. Quality is not relative humidity or volume fraction, and it is undefined in a single-phase state. PhaseGauge accepts x only with saturation pressure or temperature.
Specific internal energyu: kJ/kgSpecific internal energy is microscopic energy stored in the fluid per unit mass, excluding bulk kinetic and gravitational potential energy. Its numerical zero follows the IF97 reference convention, so balances normally use differences between states from the same formulation. Reconcile reference offsets before combining values from different tables.
Specific enthalpyh: kJ/kgSpecific enthalpy is h = u + pv. The flow-work term makes it useful for steady-flow equipment such as boilers, turbines, valves, nozzles, and heat exchangers. Enthalpy is a state property, not heat contained in the fluid. Energy balances must account separately for heat transfer, work, kinetic energy, and potential energy where relevant.
Specific entropys: kJ/(kg·K)Specific entropy is a state property used in second-law analysis and ideal isentropic calculations. Its numerical zero follows the formulation's reference convention, so state-to-state differences carry the useful information. Constant entropy is a model assumption for equipment, not proof of reversible operation. PhaseGauge reports the equilibrium property; equipment efficiency belongs in the surrounding calculation.
Isobaric and isochoric heat capacitycₚ / cᵥ: kJ/(kg·K)The isobaric heat capacity cₚ is the temperature derivative of enthalpy at constant pressure. The isochoric heat capacity cᵥ is the temperature derivative of internal energy at constant specific volume. Both are local response functions and can change sharply near the critical point. PhaseGauge shows a dash for an equilibrium two-phase bulk mixture, where one cₚ or cᵥ value is not defined.
Speed of soundw: m/sSpeed of sound is the propagation speed of a small pressure disturbance through the equilibrium fluid. It depends on compressibility and caloric response. The value supports nozzle, relief, water-hammer, and compressible-flow calculations, but finite-amplitude shocks require a separate model. PhaseGauge does not assign a bulk value to wet steam because phase-change dynamics affect wave propagation.
Isobaric cubic expansionβ: 1/KThe isobaric cubic expansion coefficient is β = (1/v)(∂v/∂T)ₚ, the fractional change in specific volume per unit temperature rise at constant pressure. It is a local derivative. Treating β as constant across a large temperature interval is an approximation; use endpoint states near phase boundaries or the critical region.
Isothermal compressibilityκₜ: 1/MPaIsothermal compressibility is κₜ = −(1/v)(∂v/∂p)ₜ, the fractional reduction in specific volume per unit pressure increase at constant temperature. The minus sign gives a positive coefficient for an ordinary stable state. Liquids usually have small values, while dilute vapor is much more compressible. This equilibrium derivative is not a complete transient bulk-modulus model.
Dynamic viscosityμ: Pa·sDynamic viscosity relates shear stress to velocity gradient for a Newtonian fluid. PhaseGauge uses the dilute-gas and finite-density terms from IAPWS R12-08 and omits the permitted near-critical enhancement. It does not assign one viscosity to a wet-steam bulk mixture. Flow and heat-transfer correlations may also require film conditions and geometry.
Thermal conductivityλ: W/(m·K)Thermal conductivity relates conductive heat flux to a temperature gradient. Single-phase results use the IAPWS R15-11 correlation, including its critical enhancement when the required inputs are available. Choose a defensible bulk or film state when using the value in a heat-transfer correlation. PhaseGauge shows a dash for an equilibrium two-phase bulk mixture.
Surface tensionσ: N/mSurface tension is interfacial free energy per unit area, or force per unit length, at the saturated liquid–vapor interface. It is defined below the critical point while both phases and their interface exist. PhaseGauge reports it for saturation states. Droplet, bubble, boiling, and condensation models also depend on geometry, wetting, nucleation, and flow conditions.
Dielectric and ionization constantsε / pK_w: dimensionlessThe static dielectric constant ε is water's relative permittivity at the zero-frequency limit. The ionization result pK_w is the negative base-10 logarithmic measure associated with water's self-ionization equilibrium. Their supplemental IAPWS correlations cover less than the full IF97 envelope, so PhaseGauge shows a dash outside those ranges and for a bulk two-phase mixture. These values do not replace a solution-chemistry or frequency-dependent dielectric model.