Steam Tables and Property Tables Calculator with Interpolation

The thermodynamic property tables you normally interpolate by hand: it finds the rows either side of your numbers and shows the interpolation.

Calculated from standard textbook formulas, with every step shown. The formulas are fixed, so the working does not need checking: what matters is the numbers you type in. Use these calculators only if you are competent to choose the inputs (loads, material strength, dimensions, supports) and check your units and assumptions. For learning and checking your own working: design and sign-off are for a qualified engineer. Drafting Central does not store the numbers you type.

How this calculator works

Thermodynamics textbooks give steam and refrigerant properties in tables, and a typical problem needs a value between two rows. This calculator finds the two rows (and for superheated steam the two pressures), then shows the linear interpolation working so you can check it against your own.

The tables are generated from the IAPWS-95 formulation for water and Helmholtz equations of state for the other fluids (through the open-source CoolProp library). Saturated tables are given by temperature or by pressure. Superheated and compressed liquid use double interpolation. There are also ideal-gas air properties (h, u, s°, Pr, vr) and gas properties at atmospheric pressure.

Formulas used

  • y = y_1 + \frac{x - x_1}{x_2 - x_1}\,(y_2 - y_1)

Worked example

With the default numbers in the calculator above:

  1. Which phase is it?: superheated vapour
  2. The table rows used (superheated tables): 1 rows
  3. Interpolate in temperature, at each table pressure: 1 pressure
  4. Interpolate in pressure: exact
  5. Specific volume: 0.23275 m³/kg
  6. Internal energy: 2710.4 kJ/kg
  7. Enthalpy: 2943.1 kJ/kg
  8. Entropy: 6.9265 kJ/kg·K

Change the numbers in the calculator to see the full working, every step, for your own case.

Common questions

How accurate is linear interpolation?

Very close where the table rows are near each other, and less exact where a property changes quickly, such as saturation pressure at low temperature.

Why do my values differ slightly from my textbook?

Textbook tables use slightly different equations or reference states. For R134a the reference state also differs, so compare differences in enthalpy and entropy, not absolute values.

What is a compressed liquid?

A liquid below its saturation temperature at the given pressure. Tables exist for high pressures. At lower pressure the usual approximation is the saturated liquid at the same temperature.

Related calculators

Further reading: Cengel and Boles, Thermodynamics: An Engineering Approach; Moran and Shapiro, Fundamentals of Engineering Thermodynamics. IAPWS-95; CoolProp.

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