Saturation Temperature and Pressure of Steam

Steam has exactly one degree of freedom while it is boiling. Fix the pressure and the temperature is decided for you, and fix the temperature and the pressure is decided for you, which is why the first column of every steam table is really a single curve with two labels. At 100 kPa absolute water boils at 99.606 °C, at 1 MPa it boils at 179.886 °C, and at 100 °C the vapour pressure is 101.418 kPa. Those three numbers are the whole table in miniature, and the last one is worth staring at: standard atmosphere is 101.325 kPa, so the round 100 °C boiling point everybody learns in school is about a tenth of a kilopascal off the real one. On a steam fitter's gauge the same curve reads 15 psig for 250 °F and 100 psig for 338 °F.

The engine underneath is IAPWS-IF97 Region 4, and it is genuinely two-way. Equation (29) of the release is one implicit quadratic in $\beta = (p/1\ \text{MPa})^{1/4}$ and $\vartheta = T + n_9/(T - n_{10})$, and Equations (30) and (31) are that same quadratic solved once for pressure and once for temperature. Nothing here iterates and nothing interpolates between printed rows, so a round trip through the pair comes back to the digit it started on. IF97 is the industrial formulation, released in 1997 and adopted by ASME, so the numbers this page returns are the numbers the ASME tables are printed from rather than an approximation of them. Two honest limits. Region 4 stops dead at both ends: 0 °C and 611.213 Pa at the bottom, and the critical point at 373.946 °C and 22.064 MPa at the top, above which liquid and vapour stop being different substances and there is no line left to sit on. And the pressure is ABSOLUTE. A 100 psig gauge is 114.7 psia, and feeding the gauge reading straight in is the single most common way to get a saturation temperature 30 degrees too low.