We all know that water boils at $1\rm\,atm$ at $100°\rm\, C$. But if we maintain the water temperature at $30°\rm\, C$ and instead let the pressure drop (in a vacuum chamber) to around $0.3\rm\, bar$, the water starts to boil. I am currently studying thermodynamics and I have seen in the thermodynamic tables of saturated water that $p_{sat}$ of the water at $30°\rm\, C$ is around $4.2469\rm\, kPa$ (around $0.04\rm\, bar$).

So I am now confused. Is water at $30°\rm\, C$ supposed to boil at $0.04\rm\, bar$ or $0.3\rm\, bar$ ?

How could I make the calculations?

What formulas should I be using for this problem?

I will be grateful for any help.

  • $\begingroup$ You read the gauge wrong. The black scale is negative inches of mercury, where zero inches of mercury corresponds to atmospheric pressure. -30 inches of mercury is full vacuum. $\endgroup$ Apr 17, 2022 at 21:58

1 Answer 1


Use a PT diagram. As you did suspect, the correct formula would be just to look at the vapour pressure of the water on $30°\rm\, C$. The correct value is then really at around $0.042455\rm\, bar$.

As of the video you sent, the water starts boiling at around 0.04 bar as well.

The screenshot of the moment it started to boil:


The indicator is at around -0.96 bar (under the atmospheric pressure), that means around 0.04 bar of the absolute pressure, as we concluded in theory. Note that the correct scale is in bar (100 kPa, red) and not in inHg (black).

  • $\begingroup$ @GabrielNieto Why do you think it boils at 0.3 atm? $\endgroup$
    – User123
    Apr 16, 2022 at 20:46
  • $\begingroup$ Please check this video youtu.be/WTVwAZ0_9p0 $\endgroup$ Apr 17, 2022 at 4:13
  • $\begingroup$ Between minute 3 and 6 $\endgroup$ Apr 17, 2022 at 4:13
  • $\begingroup$ Thank you very much for your answer. $\endgroup$ Apr 18, 2022 at 22:09
  • $\begingroup$ @GabrielNieto If you are satisfied with the answer, make sure to accept it (green check mark) so that others see this question is already answered. $\endgroup$
    – User123
    Apr 19, 2022 at 14:17

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