Why is the steam vapor pressure not equal to atmospheric pressure at 100°C?
If you have a saturated steam stream at 100°C you find the pressure is actually 101.418kPa, or 1.0009 atm, which differs from 1atm by one tenth of one percent. Or if you set the pressure to 0barg (1atm), the temperature is not exactly 100°C. The Celsius scale has been refined over the years from its original incantation, where the freezing point of water was set at 0°C, and the boiling point of water at atmospheric pressure to 100°C. The latest temperature scale defines temperatures at a number of points such as the melting point of gallium and the triple point of hydrogen. The only water value it uses is the triple point of water or 273.16°C. On this new scale, water actually boils at 99.974°C. SysCAD implements the IFS97 steam model, which uses this revised temperature scale. In practical terms this difference is minute: it would be the difference in boiling point due to an elevation change of about 8 m.
