How to Read Steam Tables (Saturated vs Superheated)

Steam tables are the single biggest source of “where do I even start” panic in an intro thermodynamics course. Once you see that a state is just a location on a phase diagram, the tables stop being a lookup maze and become a map.

The three regions you care about

For water, every state lives in one of three places:

• Compressed (subcooled) liquid — cooler than the boiling point at that pressure.
• Saturated mixture — boiling, a liquid/vapour “dome” where liquid and vapour coexist. Quality x (0 to 1) tells you the split.
• Superheated vapour — hotter than the boiling point at that pressure.

The boundary between them is the saturation temperature Tsat at a given pressure (equivalently, the saturation pressure at a given temperature).

Step 1: find Tsat at your pressure

Open the saturation-pressure table. For P = 100 kPa:

P (kPa)	Tsat (°C)	vf (m³/kg)	vg (m³/kg)	hf (kJ/kg)	hfg (kJ/kg)	hg (kJ/kg)
100	99.61	0.001043	1.6941	417.53	2257.5	2675.0

So at 100 kPa, water boils at 99.61 °C. That number is your decision line.

Step 2: compare your temperature to Tsat

• T < Tsat → compressed liquid.
• T = Tsat → saturated mixture (you need quality x).
• T > Tsat → superheated vapour.

Example. P = 100 kPa, T = 300 °C. Since 300 > 99.61, the state is superheated. The saturation table can no longer give you h — switch to the superheated table at 100 kPa and read across to 300 °C:

h ≈ 3074.3 kJ/kg, s ≈ 8.2167 kJ/kg·K.

Example. P = 100 kPa, T = 99.61 °C, x = 0.5. You are inside the dome. Use the saturation table and the quality formula (see Quality explained).

Step 3: interpolate when your value is between rows

Tables are discrete. If you need 1 MPa and 300 °C, the superheated table lists 300 °C at 1 MPa directly:

h = 3051.2 kJ/kg, s = 7.1229 kJ/kg·K, v = 0.2579 m³/kg.

If your pressure is between two listed pressures (say 1.3 MPa), interpolate linearly between 1.0 and 1.5 MPa. The calculators on this site do exactly that linear interpolation for you — enter the two known properties and the rest falls out.

The auto-detect shortcut

The property lookup calculator takes the decision off your plate: enter P and T (or any two properties) and it compares T to Tsat internally, switches to the superheated table when needed, and tells you which region it landed in. That “switched to superheated tables” note is the same judgement call you’re learning to make by hand.