Triple point pressure of water
| Value | 611.657 Pa |
| Status | Measured: ± 0.01 Pa (0.000016 relative) |
| Source | IAPWS; Guildner, Johnson & Jones (1976) |
| Categories | ThermodynamicEngineering & Trade |
Triple point pressure of water in every pressure unit
| microbar | 6,116.57 μbar |
| barye | 6,116.57 Ba |
| millitorr | 4,587.8048 mTorr |
| pascal | 611.657 Pa |
| millimeter of water column | 62.371656 mmH₂O |
| pound per square foot | 12.774722 lb/ft² |
| centimeter of water column | 6.2371656 cmH₂O |
| hectopascal | 6.11657 hPa |
| millibar | 6.11657 mbar |
| torr | 4.5878048 Torr |
| millimeter of mercury | 4.5878041 mmHg |
| inch of water column | 2.455577 in w.g. |
| kilopascal | 0.611657 kPa |
| foot of water column | 0.20463142 ft H₂O |
| inch of mercury | 0.18062221 inHg |
| pound per square inch | 0.088713348 psi |
| meter of water column | 0.062371656 m H₂O |
| decibar | 0.0611657 dbar |
| kilogram-force per square centimeter | 0.0062371656 kgf/cm² |
| bar | 0.00611657 bar |
| atmosphere | 0.0060365852 atm |
| megapascal | 0.000611657 MPa |
| kip per square inch | 0.000088713348 ksi |
| gigapascal | 6.1165700e-07 GPa |
| million pounds per square inch | 8.8713348e-08 Mpsi |
Learning zone
At 611.657 Pa and 273.16 K the three phases of water meet. Below this pressure liquid water has no stable range at any temperature — ice sublimes straight to vapour. That is the physics behind freeze-drying, behind the fact that Mars, whose mean surface pressure is around 600 Pa, sits almost exactly on the edge of liquid water, and behind the flash-boiling of body fluids in a vacuum.
The value is measured, not defined: Guildner, Johnson and Jones determined it in 1976 and IAPWS retains 611.657 ± 0.010 Pa. It also fixes the vapour pressure end of every steam table, since IAPWS-95 is constrained to reproduce the triple point exactly.