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Thread: Vacuums and degassing questions and issues

  1. #1
    Associate Engineer
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    Vacuums and degassing questions and issues

    Hey guys,

    I have been running some experiments with vacuums for the purpose of degassing fluids and for building assemblies that are bathed in hydraulic fluids that require minimal gases within their assemblies.

    When I draw a vacuum on a liquid (in this case, a special type of hydraulic fluid) I can see the fluid boil, which is obviously the small amounts of trapped air within the liquid.

    However, after many minutes under a vacuum...the fluid keeps on producing bubbles.

    In addition to this, I have found that when I introduce certain materials with the fluid (such as bare aluminum and steel) I get bubbles that come directly off of the materials...and for very very long periods of time. Basically so long that I never see an end even after running the stuff under a vacuum for many hours.

    I then found this paper.

    Vacuum_Primer.pdf

    In this, it talks about various materials giving off gases when put under certain levels of a vacuum. All new to me.

    So...I think what I'm looking for is some insight and input into what you guys think is going on here.

    If I'm getting bubbles directly off of a piece of raw steel, is that from the Zinc in the steel? And if so, what is the gas? If I trapped this "gas" and then remove the vacuum, what do I have?

    And...I guess more importantly, is degassing, for the purpose of removing trapped air within a liquid, something that you expect to get a near perfect result within a minute of two, or is it expected to have a more pure liquid within a vacuum for extended periods? In other words, should a good degass process be 5 minutes, or several hours?

    Thanks for any and all input.
    Last edited by Kelly_Bramble; 09-11-2014 at 03:39 PM.

  2. #2
    Lead Engineer
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    All fluids have what is known as a minimum vapor pressure at a given temperature. At pressures below this level the fluid itself begins to boil, or "outgas". I suspect this is the issue with your vacuum process with the hydraulic fluid; and, you are at some point, in addition to air, you are evacuating the vapor state of the hydraulic fluid itself through your vacuum pump. If this is the case, after a while, you should start to see some traces of condensed hydraulic fluid at your vacuum pump discharge.

    Your hydraulic fluid manufacturer should be able to provide you with your fluid's vapor pressure data.

    As to the issue with bare metal, that one is beyond my knowledge.

  3. #3
    Associate Engineer
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    The above discussion gives very fine ideas about the heat treating process. This is a nice post giving us fine ideas, this is a great stuff above.

  4. #4
    Principle Engineer Cragyon's Avatar
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    And...I guess more importantly, is degassing, for the purpose of removing trapped air within a liquid, something that you expect to get a near perfect result within a minute of two, or is it expected to have a more pure liquid within a vacuum for extended periods? In other words, should a good degass process be 5 minutes, or several hours?


    This would depend on the viscosity of the liquid.

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