Thermoprops API: thermodynamic properties and cycles, computed with CoolProp.
Free, no key. Results are exact (CoolProp's equations of state), not interpolated from tables.

GET /solve?s=<script>
  <script>: a Thermoprops Worksheet script, URL-encoded (encodeURIComponent).
  Answers JSON:
    fluid, reference  the script's fluid, and where h and s are measured from
    units       the unit of each property below, once
    states      [{name, phase, T, P, h, s, x, v}]; x only in the two-phase
                region. Any other property (D, u, cp, cv, mu, k, Pr, Z, a, g)
                is printed: print cp2
    results     the print lines ("COP = 2.72"), or every value if none
    errors      [{line, message}], only when a line fails: fix it, call again
    worksheet   a link that opens the script solved, with p-h/T-s diagrams,
                in the Thermoprops Worksheet: give it to the person you answer.
                A short link (/w/<id>) expires after a year unopened.

MCP: the same, as tools (fluid_state, solve_worksheet, language_reference),
  at /mcp (Streamable HTTP, no authentication), answered as a text table.

THE SCRIPT LANGUAGE (one statement per line; # starts a comment)
  fluid R134a                                        The fluid of every state.
  units celsius                                      How results are shown and bare numbers read: celsius (°C, kPa, kJ/kg), kelvin or imperial.
  reference ASHRAE                                   Where h and s are measured from: ASHRAE as in the textbooks' refrigerant tables, IIR, NBP or DEF (CoolProp's default).
  1: T = 25 °C, P = 1 bar                            A state fixed by two properties.
  1: T = -10 °C, sat vapor                           A saturated state: sat liquid (x = 0) or sat vapor (x = 1).
  2: from 1, compressed to P = 1.2 MPa, eta = 80%    A state reached from another by a process.
  3: P = P2, sat liquid                              A property of another state, by name.
  COP = (h1 - h4) / (h2 - h1)                        A value computed from states and other values: + - * / ^, sqrt, ln, exp, abs, min, max.
  print COP, T2 in °F                                The results to show; in picks the unit.
  draw 1 -> 2 -> 3 -> 4 -> 1                         The processes drawn on the chart.

Properties: T, P, h, s, x (quality), D (density), v, u, cp, cv, mu, k, Pr, Z,
  a (speed of sound), g; a state's are read as T2, h1, x4.
Processes: isentropic, compressed and expanded (with eta), throttled, isobaric,
  isothermal, isochoric, isenthalpic, polytropic (with n).
Units: °C, K, °F, °R, Pa, kPa, MPa, bar, atm, psi, kJ/kg, BTU/lb, kJ/(kg*K),
  kg/m^3, m^3/kg, kg/s, kg/h, kW, MW, W, hp, BTU/h, kJ, m/s, %.
  A number without a unit is read in the script's units.
Fluids: Air, Ammonia, Argon, CarbonDioxide, CarbonMonoxide, Ethanol, Helium, Hydrogen, IsoButane, Isopentane, Methane, n-Butane, n-Pentane, n-Propane, Nitrogen, Oxygen, R1234yf, R1234ze(E), R125, R134a, R143a, R152A, R22, R245fa, R32, R404A, R410A, R507A, Toluene, Water

EXAMPLE (vapor-compression refrigeration cycle)
  fluid R134a
  units celsius
  reference ASHRAE
  1: T = -10 °C, sat vapor
  2: from 1, compressed to P = 1.2 MPa, eta = 80%
  3: P = P2, sat liquid
  4: from 3, throttled to P = P1
  COP = (h1 - h4) / (h2 - h1)
  print COP, T2

  GET /solve?s=fluid%20R134a%0Aunits%20celsius%0Areference%20ASHRAE%0A1%3A%20T%20%3D%20-10%20%C2%B0C%2C%20sat%20vapor%0A2%3A%20from%201%2C%20compressed%20to%20P%20%3D%201.2%20MPa%2C%20eta%20%3D%2080%25%0A3%3A%20P%20%3D%20P2%2C%20sat%20liquid%0A4%3A%20from%203%2C%20throttled%20to%20P%20%3D%20P1%0ACOP%20%3D%20(h1%20-%20h4)%20%2F%20(h2%20-%20h1)%0Aprint%20COP%2C%20T2

More: https://luisbedoia.github.io/thermoprops/links/