Everything in this chapter hangs off one curve: the saturation line, where water and steam sit together at the same temperature and pressure. Give it a pressure and it hands back the temperature the water boils at; give it a temperature and it hands back the pressure. Read aloud: T-sat equals T-sub-s of p — $T_{sat} = T_s(p)$, where $p$ is the absolute pressure in kilopascals and $T_{sat}$ is the saturation temperature in degrees Celsius. The subscript sat is not decoration: it says this temperature belongs to the line and to nowhere else.

Three more columns ride beside it, all in kilojoules per kilogram. $h_f$ (h-sub-f) is the enthalpy of the saturated liquid — what a kilogram of water has absorbed on its way to the boil. $h_{fg}$ (h-f-g) is the latent heat, what that kilogram must still absorb to become vapour. $h_g$ (h-sub-g) is the dry saturated steam enthalpy, and it is simply the other two added: $h_g = h_f + h_{fg}$. Subscript f is fluid, g is gas, and fg is the journey from one to the other.

One discipline before any of it works. Steam tables are printed against absolute pressure; the Bourdon gauge on the header reads gauge pressure, zeroed against the air in the room. Absolute is gauge plus atmospheric, and on this paper atmospheric is 101.3 kPa. Skip that addition and every row you read is the wrong row — quietly, plausibly, and by about twenty degrees.