iapws-if97

iapws-if97 calculates industrial water and steam properties in Node.js and TypeScript using the IAPWS-IF97 standard published by the International Association for the Properties of Water and Steam.

It includes forward and backward state solvers, saturation solvers, and transport-property helpers.

Features

• Covers IF97 Regions 1–5, including high-temperature Region 5
• Forward, backward, and saturation solvers behind a consistent API
• Transport properties: viscosity, thermal conductivity, surface tension, dielectric constant, and ionization constant
• Verified against official IAPWS tables and published engineering references

Installation

Requires Node.js >=20.19.0. The package publishes ESM, and all documented examples use import. CommonJS is not part of the tested API.

npm install iapws-if97

Quick start

Save the following as example.mjs and run it with node example.mjs:

import { solve, solvePT, solvePx } from 'iapws-if97';

const a = solvePT(3, 300);
const b = solve({ mode: 'PH', p: 3, h: a.enthalpy });
const c = solvePx(1, 0.5);

console.log(a.enthalpy);   // kJ/kg
console.log(b.temperature); // K
console.log(c.quality);     // 0.5

Main API

Function	Input pair	Units
solvePT(p, T)	pressure, temperature	MPa, K
solvePH(p, h)	pressure, enthalpy	MPa, kJ/kg
solvePS(p, s)	pressure, entropy	MPa, kJ/(kg·K)
solveHS(h, s)	enthalpy, entropy	kJ/kg, kJ/(kg·K)
solveTH(T, h)	temperature, enthalpy	K, kJ/kg
solveTS(T, s)	temperature, entropy	K, kJ/(kg·K)
solvePx(p, x)	pressure, vapor quality	MPa, dimensionless [0, 1]
solveTx(T, x)	temperature, vapor quality	K, dimensionless [0, 1]

All numeric inputs must be finite numbers.

Unified solve()

Use solve({ mode, ... }) when the input pair is determined at runtime:

import { solve } from 'iapws-if97';

const state = solve({ mode: 'PT', p: 16, T: 823.15 });

Each mode accepts canonical short fields or their long aliases:

Mode	Short fields	Long aliases
PT	p, T	pressure, temperature
PH	p, h	pressure, enthalpy
PS	p, s	pressure, entropy
HS	h, s	enthalpy, entropy
Px	p, x	pressure, quality
Tx	T, x	temperature, quality
TH	T, h	temperature, enthalpy
TS	T, s	temperature, entropy

import type { SolveInput } from 'iapws-if97';
import { solve } from 'iapws-if97';

const shortForm: SolveInput = { mode: 'PT', p: 16, T: 823.15 };
const longForm: SolveInput = { mode: 'PT', pressure: 16, temperature: 823.15 };

const a = solve(shortForm);
const b = solve(longForm);

You can mix short and long names within one input. If both aliases for the same property are present, their values must match exactly. The exported SolveInput type is the source of truth for accepted combinations.

Solver return value

All solvers return a SteamState object containing both thermodynamic and transport properties:

type SteamState = {
  region: Region;
  pressure: number;
  temperature: number;
  specificVolume: number;
  density: number;
  internalEnergy: number;
  entropy: number;
  enthalpy: number;
  cp: number | null;
  cv: number | null;
  speedOfSound: number | null;
  quality: number | null;
  viscosity: number | null;
  thermalConductivity: number | null;
  surfaceTension: number | null;
  dielectricConstant: number | null;
  ionizationConstant: number | null;
  isobaricExpansion: number | null;
  isothermalCompressibility: number | null;
};

SteamState always uses canonical property names.

Notes:

• quality is null for single-phase states; it is only defined on the saturation line.
• In two-phase mixtures (0 < x < 1), cp, cv, speedOfSound, isobaricExpansion, isothermalCompressibility, viscosity, thermalConductivity, dielectricConstant, and ionizationConstant are null.
• Saturation endpoints (x = 0 or x = 1) still expose single-phase transport properties even though they carry Region 4 metadata.
• solvePT(p, T) is a single-phase solver. On the subcritical saturation boundary it resolves to the liquid side.
• surfaceTension is only available for Region 4 saturation states below the critical point; otherwise null.
• density is provided directly, so there is no need to invert specificVolume.
• ionizationConstant is null outside the IAPWS validity range for that correlation.

Units

• Pressure: MPa
• Temperature: K
• Specific volume: m^3/kg
• Density: kg/m^3
• Enthalpy, internal energy: kJ/kg
• Entropy, heat capacities: kJ/(kg·K)
• Speed of sound: m/s
• Quality: dimensionless, 0–1 (saturation line only)
• Viscosity: Pa·s
• Thermal conductivity: W/(m·K)
• Surface tension: N/m
• Dielectric constant: dimensionless
• Ionization constant (pKw): dimensionless
• Isobaric expansion coefficient: 1/K
• Isothermal compressibility: 1/MPa

Any field typed as number | null returns null when the property is undefined for the given state.

Advanced imports

The package root is limited to the main solvers, SteamState, SolveInput, Region, and the public error classes. Lower-level helpers are available from explicit subpaths:

Subpath	Exports
iapws-if97/transport	viscosity, thermalConductivity, surfaceTension, dielectricConstant, ionizationConstant
iapws-if97/regions	region1, region2, region3ByRhoT, region5
iapws-if97/saturation	saturationPressure, saturationTemperature
iapws-if97/boundaries	boundary23_T_to_P, boundary23_P_to_T, region3Volume, region3SatVolume
iapws-if97/detect	detectRegionPT, detectRegionPH, detectRegionPS, detectRegionHS, detectRegionTH, detectRegionTS

import { viscosity } from 'iapws-if97/transport';
import { region1 } from 'iapws-if97/regions';
import { saturationTemperature } from 'iapws-if97/saturation';
import { detectRegionPT } from 'iapws-if97/detect';

These are low-level mathematical interfaces:

• Region functions return core thermodynamic properties without density or transport-property enrichment.
• Region, boundary, and detection helpers expect callers to respect the corresponding equation domains. Detection helpers return a Region value or -1 when no valid region is found.
• thermalConductivity(T, rho) calculates the base contribution. Pass cp, cv, drhodP_T, and mu to include the IAPWS 2011 critical-enhancement term.
• surfaceTension(T) is a saturation-line property. saturationPressure(T) and saturationTemperature(p) expose the mathematical Region 4 boundary and are more permissive at endpoints than solveTx and solvePx.

Transport correlation limits

Transport correlations have validity ranges independent of the IF97 thermodynamic envelope. The library may return extrapolated values where noted:

Property	Implemented behavior
Viscosity	Uses the IAPWS 2008 dilute-gas and finite-density terms. The near-critical enhancement is omitted, as permitted for industrial use by the release.
Thermal conductivity	Full SteamState calculations include the IAPWS 2011 critical enhancement. The two-argument low-level call does not. Values above the release's temperature range are extrapolations.
Surface tension	Defined for saturation states below the critical point. The low-level helper permits 273.15 K ≤ T ≤ Tc; values below the triple point Tt = 273.16 K are extrapolations.
Dielectric constant	The IAPWS 1997 release is valid through 873 K. The low-level helper does not enforce that upper bound, so higher-temperature results are extrapolations.
Ionization constant (pKw)	Returns null outside 273.15 K ≤ T ≤ 1273.15 K. The release's 1000 MPa pressure limit describes the correlation, not the state-solver pressure range.

Saturation endpoints

The triple and critical points are Pt = 0.000611657 MPa, Tt = 273.16 K, Pc = 22.064 MPa, and Tc = 647.096 K.

• Low-level Region 4 helpers accept a wider endpoint range. The saturation state solvers use these stricter bounds:
  • solvePx(p, x) accepts Pt ≤ p < Pc
  • solveTx(T, x) accepts Tt = 273.16 K ≤ T < Tc
• The triple point is supported as a saturation-state boundary:
  • solvePx(Pt, x) is valid
  • solveTx(Tt, x) is valid
• 273.15 K is treated as a low-level extrapolation boundary only.
• The solvers do not return the exact critical point as a Region 4 state.
• solveTH(T, h) and solveTS(T, s) reject inputs within 0.001 K of the critical temperature.

Errors and limits

The library throws typed errors:

Error	Meaning
OutOfRangeError	Input is outside the supported IF97 range
ConvergenceError	An internal iterative solve failed to converge
IF97Error	Base class; root solvers also use it directly for non-numeric or non-finite inputs, conflicting aliases, unsupported modes, and unsupported critical-point states

Numeric inputs to root solvers and transport helpers must be finite. Low-level region, boundary, and detection helpers rely on callers to respect their equation domains. solvePT(Pc, Tc), exact critical Region 4 states, and solveTH/solveTS inputs within 0.001 K of Tc are rejected because the required derivative properties or inverse solution are singular or ill-conditioned.

Thermodynamic validity

The thermodynamic solvers follow the piecewise IF97 industrial range:

• 273.15 K ≤ T ≤ 1073.15 K: pressure up to 100 MPa.
• 1073.15 K < T ≤ 2273.15 K (Region 5): pressure up to 50 MPa.

Transport-property limits are separate; see Transport correlation limits.

Verification

The test suite covers:

• Official IF97 verification tables (Regions 1–5)
• Backward-equation round-trip accuracy
• Temperature-led backward round trips (TH and TS)
• High-pressure Region 4 regressions
• IAPWS R11-24 ionization-constant verification values
• Coverage thresholds enforced locally
• ASME and GB/T steam-table comparisons against published values

For a clean local verification run:

npm ci
npm run test:package
npm test

test:package performs a clean build and checks that the dry-run npm tarball has no dangling source-map references. Coverage and published-table comparisons are available separately:

npm run test:coverage
npm run test:standards

Integration validation

This Mollier h-s diagram was generated with iapws-if97 as an end-to-end check of the calculation pipeline.

The standards tests check the published tables below. Both suites enforce maxRelativeError < 5e-4 (less than 0.05%).

Reference	Coverage	Points checked	Worst relative error
ASME International Steam Tables for Industrial Use, 3rd ed., Table S-2	Saturation states at 6 pressures	42	0.0315405%
GB/T 34060-2017, Table A.3(续), page 55	Superheated steam at 10 MPa and 6 temperatures	24	0.0007020%

See tests/standards-asme and tests/standards-cn for the source values and per-case comparisons.

TypeScript

The package ships with generated TypeScript declarations for the root and every public subpath.

import type { SolveInput, SteamState } from 'iapws-if97';
import { Region, solve } from 'iapws-if97';

const input: SolveInput = { mode: 'PT', p: 3, T: 300 };
const state: SteamState = solve(input);

if (state.region === Region.Region1) {
  console.log('compressed liquid');
}

References

• IAPWS official website
• IAPWS R7-97(2012), Industrial Formulation 1997
• IAPWS supplementary releases for T(p,h), T(p,s), p(h,s), Region 3 boundary equations, Tsat(h,s), and v(p,T)
• IAPWS SR4-04(2014), Region 3 p(h,s) and Region 4 Tsat(h,s)
• IAPWS R12-08, Viscosity of Ordinary Water Substance
• IAPWS R15-11, Thermal Conductivity of Ordinary Water Substance
• IAPWS R8-97, Static Dielectric Constant of Ordinary Water Substance
• IAPWS R1-76(2014), Surface Tension of Ordinary Water Substance
• IAPWS R11-24, Ionization Constant of H2O

Project origin

iapws-if97 began as the calculation engine behind the WeChat Mini Program "汽水计算器" (wxid: wx7201fd1713b524e5). Since its 2019 launch, it has served more than 20,000 users. AI-assisted review helped bring this open-source edition up to date with IAPWS R11-24 (2024). I am sharing the engine so it can reach engineers far beyond the original program.

Shuping, retired thermodynamic engineer

License

MIT

Disclaimer

This software is provided "as is", without warranty of any kind, express or implied, including but not limited to the warranties of merchantability, fitness for a particular purpose, and noninfringement. In no event shall the authors or copyright holders be liable for any claim, damages, or other liability arising from the use of this software.