Steam Property Glossary

Unit labels show the defaults for the SI and US display presets and, where different, internal calculation units. Result rows for entered properties retain your selected input units. A dash (—) in the value column means PhaseGauge does not report that property for the state. For saturation results, f denotes saturated liquid, g saturated vapor, and fg the vapor-minus-liquid difference.

Use consistent units in formulas: with p in MPa and v in m³/kg, h = u + 1,000pv gives h in kJ/kg when u is in kJ/kg. The sound-speed derivative uses pressure in Pa and density in kg/m³. Property availability also follows the calculator's supported pressure and temperature ranges.

Pressurep

SI default: bar (abs) · US default: psi (abs) · Internal: MPa

Pressure is normal force per unit area. PhaseGauge calculates with absolute pressure in MPa. Inputs in barg or psig are converted by adding a fixed standard atmosphere of 101.325 kPa. If a gauge reading uses the measured local atmosphere, add that pressure yourself and enter the result in an absolute unit.

TemperatureT

SI default: °C · US default: °F · Internal: K

IF97 uses temperature in kelvin on the International Temperature Scale of 1990 (ITS-90). PhaseGauge converts Celsius using T[K] = t[°C] + 273.15, and Fahrenheit using T[K] = (t[°F] − 32) × 5/9 + 273.15. Temperature alone does not define a water state. On the saturation curve, pressure and temperature together still do not determine vapor quality; use P–x or T–x to specify it.

Specific volume and densityv / ρ

SI default: m³/kg · kg/m³ | US default: ft³/lb · lb/ft³

Specific volume is volume per unit mass; density is mass per unit volume, with ρ = 1/v. For wet steam, v = (1 − x)v_f + x v_g = v_f + x v_fg. Bulk mixture density is the reciprocal of that volume. Averaging the endpoint densities directly by quality gives the wrong result.

Vapor qualityx

Dimensionless · vapor mass fraction, 0–1

Vapor 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 is undefined for nonsaturated single-phase states. Enter x with P–x or T–x; successful P–h, P–s, h–s, T–h, and T–s solves can also return x for saturation states.

Specific internal energyu

SI default: kJ/kg · US default: BTU/lb

Specific internal energy is microscopic energy stored in the fluid per unit mass, excluding bulk kinetic and gravitational potential energy. The IF97 reference convention sets u = 0 for saturated liquid at the triple point, 273.16 K. For wet steam, u = (1 − x)u_f + x u_g. Reconcile reference offsets before combining values from different tables.

Specific enthalpyh

SI default: kJ/kg · US default: BTU/lb

Specific enthalpy is h = u + pv and is used in steady-flow energy balances. It is a state property, not heat contained in the fluid. For wet steam, h = h_f + x h_fg, where h_fg = h_g − h_f is the enthalpy of vaporization. Energy balances must account separately for heat transfer, work, kinetic energy, and potential energy where relevant.

Specific entropys

SI default: kJ/(kg·K) · US default: BTU/(lb·°F)

Specific entropy is a state property used in second-law analysis and ideal isentropic calculations. The IF97 reference convention sets s = 0 for saturated liquid at the triple point, 273.16 K. For wet steam, s = s_f + x(s_g − s_f). Constant entropy alone does not prove that a process is reversible; equipment efficiency is a separate calculation.

Isobaric and isochoric heat capacitycₚ / cᵥ

SI default: kJ/(kg·K) · US default: BTU/(lb·°F)

For a single-phase state, cₚ = (∂h/∂T)ₚ is the isobaric heat capacity and cᵥ = (∂u/∂T)ᵥ is the isochoric heat capacity. Both are local response functions and can change sharply near the critical point. Saturation results include single-phase endpoint values, with — for the bulk mixture. Two-phase cᵥ can be defined, but PhaseGauge does not calculate mixture heat capacities.

Speed of soundw

SI default: m/s · US default: ft/s

Speed of sound is the propagation speed of a small isentropic pressure disturbance in a homogeneous fluid, with w² = (∂p/∂ρ)ₛ. PhaseGauge reports single-phase and saturated-endpoint values. It does not assign a bulk value to wet steam because phase-change dynamics affect wave propagation. Finite-amplitude shocks require a separate model.

Isobaric thermal expansion coefficientβ

SI default: 1/K · US default: 1/°F

The isobaric thermal 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. Saturation results show endpoint values, with — for the bulk mixture.

Isothermal compressibilityκₜ

SI default: 1/MPa · US default: 1/psi

Isothermal compressibility is κₜ = −(1/v)(∂v/∂p)ₜ, the fractional reduction in specific volume per unit pressure increase at constant temperature. Its reciprocal is the isothermal bulk modulus. The minus sign gives a positive coefficient for an ordinary stable state. Single-phase and saturated-endpoint values are available, with — for the bulk mixture. This equilibrium derivative does not by itself provide a transient acoustic model.

Dynamic viscosityμ

SI default: μPa·s · US default: cP · Internal: Pa·s

Dynamic viscosity relates shear stress to velocity gradient for a Newtonian fluid. PhaseGauge uses the industrial form of IAPWS R12-08 without the near-critical enhancement. Within supported states, values are available up to 1173.15 K (900 °C), including saturated endpoints, but not for a bulk wet-steam mixture. For conversion, 1 cP = 1,000 μPa·s = 0.001 Pa·s.

Thermal conductivityλ

SI default: W/(m·K) · US default: BTU/(hr·ft·°F)

Thermal conductivity relates conductive heat flux to a temperature gradient: q″ = −λ∇T. Results use the industrial form of IAPWS R15-11, including critical enhancement where applicable. Within the supported pressure range, values are available up to 1173.15 K (900 °C), including saturated endpoints, with — for a bulk two-phase mixture. Choose a bulk or film state appropriate to your heat-transfer correlation.

Surface tensionσ

SI and US default: mN/m · Internal: N/m

Surface tension is interfacial free energy per unit area, or force per unit length, at the saturated liquid–vapor interface. PhaseGauge uses IAPWS R1-76(2014) from 273.16 K (0.01 °C) to below 647.096 K (373.946 °C) for saturation states, including wet-steam results. This is an interface property, not a bulk mixture average. It tends to zero toward the critical point. For conversion, 1 N/m = 1,000 mN/m.

Dielectric and ionization constantsε / pK_w

Dimensionless

The static dielectric constant ε is water's relative permittivity at the zero-frequency limit. PhaseGauge uses IAPWS R8-97 up to 873 K (599.85 °C). The ionization result pK_w = −log₁₀K_w uses IAPWS R11-24 up to 1273.15 K (1000 °C). Within supported states, both are available for single-phase states and saturated endpoints, with — for a bulk mixture. These values do not replace a solution-chemistry or frequency-dependent dielectric model, and pK_w alone does not determine a solution's pH.