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I The influence of humidity when creating a vacuum

  1. Sep 30, 2016 #1
    Hello folks,

    I have two simple questions and some ideas about it:

    1. Why is it harder to create a vacuum and takes more time to evacuate the chamber when the air inside is humid?

    Humid air is lighter than dry air. So I have no idea why it takes longer...

    2. Why do you use nitrogen to vent the chamber?

    Is it because nitrogen is lighter than air and so at the same gas density you have more particles of a light gas than of a heavy one and because the pressure only depends on the particle noumber density (same temperature)? So I reach the pressure compensation with nitrogen faster and I use less mass. When I evacuate my chamber I have a lighter mass inside it and the amount of time to evacuate is shorter because my pump has to work less?

    I really appreciate any help and explanations. Feel free to mention details if you like! :)

    Thanks in advance guys!!!
  2. jcsd
  3. Sep 30, 2016 #2


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    Because water is difficult the pump out.
    The speed at which you can pump depends on the particular molecule (and the type of pump used); nitrogen is easy to pump which is why it is used to flush vacuum systems.
    Water is much more difficult to pump on, and generally speaking you need to use a combination of flushing with purge gas (e.g. nitrogen) and baking of the system to get rid of it.

    The difference in pump speed between molecules depends on several factors: the weight of the molecule is one factor (helium pumps slowly) also important is the tendency of some molecules -such as water- to get adsorbed on surfaces, once on the surface you can only get rid of it by raising the temperature (baking) or "hitting" it with another inert molecule (e.g. purging with nitrogen).
    Any good book in vacuum technology will have tables where the pumping speed of different molecules is tabulated.
  4. Sep 30, 2016 #3


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    Hello Karido, welcome:
    Same to you :smile: ! These questions come from a context, right ? A lab or a textbook story ? Or perhaps homework (for which we have a separate forum !!

    1. Who says that it is harder ? What chamber ?
    2. What gave you the idea to use nitrogen in the first place ? Why not hydrogen or carbon diaoxide ?
  5. Sep 30, 2016 #4
    Water vapor is frequently removed by pumps that operate with water or
    steam as a pump fluid,
    for example, water ring pumps or steam ejector
    pumps. This depends considerably on circumstances, however, because
    the economy of steam ejector pumps at low pressures is generally far
    inferior to that of rotary pumps.
    For pumping a vapor – gas mixture in which
    the vapor portion is large but the air portion is small, the vapor can be
    pumped by condensers and the permanent gases, by relatively small gas
    ballast pumps..

    If you are actually involved in generating Vacuum
    can consult....<https://www3.nd.edu/~nsl/Lectures/urls/LEYBOLD_FUNDAMENTALS.pdf> [Broken]
    agree with
    @BvU comments... above
    Last edited by a moderator: May 8, 2017
  6. Sep 30, 2016 #5
  7. Sep 30, 2016 #6
    That's what I would have thought also.
  8. Sep 30, 2016 #7
    Wow... Well you can ask questions if you start writing something about the topic... you don't have to know the surroundings. Start posting constructive stuff and stop this "separate forum" stuff! I know where to post! Thanks!

    So, to mention some details, I just want some basic knowledge about this topic because I am doing some SEM research! And Nitrogen is used in this context to vent the chamber and get faster evacuation times! Not finding too much on the fly I think it is a nice topic for a discussion! Thanks for the other answers!
  9. Sep 30, 2016 #8


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    If you frequently work with vacuum pumps etc (as you will do if you use a SEM) if might be worth getting a book about vacuum technology. I still use Chambers et al "Basic Vacuum technology" as a reference. I bought it when I took a course in vacuum technology back when I was an undergraduate in the late 90s but I think it is still available. It is quite a good "practical" book and reasonably up to date.
  10. Sep 30, 2016 #9


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    I can't dive into the background of each and every poster for each and every thread. If you had referred to your first (!) thread, that would have helped me help you better a great deal.

    In the forums you know so well, there are guidelines to make life better for all. Your original post (in this thread) looked a lot like that from a high school kid having to answer a few questions from the instructions leaflet.

    For my masters I worked with a He cryostat at a few K with a pretty good vacuum and for my PhD with the best vacuum this side of the moon. The sheer fact that I answered, even without you giving any context ('the' chamber ?) should indicate that I actually try to provide constructive help. Sorry you mistook my further inquiries as being non-constructive. They were not intended as such.
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