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How do I make a saturated vapor vessel?

  1. Jun 29, 2009 #1
    I have been trying to create a 100% saturated steam (no N2) environment, which should not be too hard but so far I have been puzzled by my observations.

    For this I have obtained a vacuum vessel (1 ft high by 1 ft diameter cylinder) and a roughing vacuum pump.

    I have instrumented a pressure gauge and a few thermocouples and found that the system can pump down (dry) to about 1-2 Torr and hold pressure at the single digit level for several hours.

    My thoughts to obtain a saturated vessel were as follows:
    1. add sufficient liquid to the vessel ( I added 1.6 liters which is far more than the amount of vapor in such vessel could hold), I use purified DI water
    2. pump down a vacuum to below saturation pressure (around 25 Torr). Liquid will boil violently but if pump is powerful enough, it can remove vapor faster than evaporation can occur and it is possible to "briefly" stay below saturation pressure.
    3. my thinking was then to close off the valve to the pump, and let the vapor evaporate until the vacuum is filled with water vapor (note, I do not use terms as Relative humidity as I think I have achieved "pure steam" rather than water absorbed in air).
    4. my theory says that the final state should approach the saturated pressure at the temperature of the vessel (room temperature), which should be around 25 Torr.

    Now I performed this experiment today, steps 1 and 2 worked well, but to my surprise I am debating whether my thinking is flawed for step 3 and 4. The vapor does evaporate briefly but the pressure does not reach 25 Torr. For instance one of the attempts I had, the vapor pressure would stick to around 13 Torr, which has a saturation temperature of 17.2 C, at least 8 degrees below room temperature and also 9 degrees below the water temperature (measured at 26 C).

    Is my thinking that the vessel in the final state will approach saturation pressure at the rooms temperature flawed?

    Your feedback is appreciated,


  2. jcsd
  3. Jun 30, 2009 #2


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    Staff Emeritus
    Science Advisor
    Gold Member

    Do you measure the water temperature in situ ? Because the forced evaporation will of course cool down your water. There can also be a problem with the accuracy of your measurement devices. Your approach is in principle correct, I'd say.
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