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Thread: Microns rise while pump is on?

  1. #21
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    Feb 2004
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    Thread has drifted from original question, so will add a coupleof tidbits.

    In own efforts, never worry about trying to pull an HVAC vacuum below 2000 microns. I don't think I've ever charged a system with temps below freezing in PNW. At 32F, it only takes about 4500 microns to boil water which is main reason for pulling vacuum to get all moisture out of system. Even if 4500 micron air or dry nitrogen pressure left in the lines one only sees a 0.5% reduction in effective compressor displacement.

    Have done some space related work, pulling a 10e-9 vacuum requires 2" dia 'hoses' (actually stainless pipes) as there is NO flow, the atoms have to bounce around to exit in approx. a straight line. At this type vacuum level a rule of thumb is 2 WEEKS before material like HVAC hoses have outgassed enough to achieve and hold that vacuum level.

    On original question, in a system with good instrumentation, never have seen the pressure DROP when pump turned OFF.
    I suspect that a very dirty pump oil could produce that effect - does your pump emit 'smoke' when you first start it? Turn off pump, cools slightly, water vapor from dirty oil condenses as pump cools and pressure drops.

  2. #22
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    Aug 2021
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    Thread Starter
    Quote Originally Posted by junkhound View Post
    Thread has drifted from original question, so will add a coupleof tidbits.

    In own efforts, never worry about trying to pull an HVAC vacuum below 2000 microns. I don't think I've ever charged a system with temps below freezing in PNW. At 32F, it only takes about 4500 microns to boil water which is main reason for pulling vacuum to get all moisture out of system. Even if 4500 micron air or dry nitrogen pressure left in the lines one only sees a 0.5% reduction in effective compressor displacement.

    Have done some space related work, pulling a 10e-9 vacuum requires 2" dia 'hoses' (actually stainless pipes) as there is NO flow, the atoms have to bounce around to exit in approx. a straight line. At this type vacuum level a rule of thumb is 2 WEEKS before material like HVAC hoses have outgassed enough to achieve and hold that vacuum level.

    On original question, in a system with good instrumentation, never have seen the pressure DROP when pump turned OFF.
    I suspect that a very dirty pump oil could produce that effect - does your pump emit 'smoke' when you first start it? Turn off pump, cools slightly, water vapor from dirty oil condenses as pump cools and pressure drops.
    Well see, its a new pump too . I just put fresh oil in it. And when you say smoke, youre not referring to the exhaust port, right? Because I thought that was normal for some vapor to come out of there.

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  4. #23
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    Aug 2021
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    Thread Starter
    Quote Originally Posted by viceman View Post
    When you say "pulling a vacuum on your gauges" , do you mean not connected to a system ?

    if so try it on a recovery tank or something with volume.
    Yes, Im not connected to a system. I just have the pump connected to my manifold and hoses.
    But I did end up trying it on a system and it still does that same thing. I close the vacuum port, boom. Microns drop. I have no clue why its showing higher when the pump is running.

  5. #24
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    Apr 2015
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    is it possible the built in micron gauge is connected to the pump side of the vacuum port valve?

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  7. #25
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    Stop pulling through a manifold and use vacuum rated core removers and vacuum rated hoses.

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  9. #26
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    Quote Originally Posted by cmdunn View Post
    is it possible the built in micron gauge is connected to the pump side of the vacuum port valve?
    It is on the Fieldpiece manifolds.

    Sent from the Okie state usin Tapatalk
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  10. #27
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    See what happens if you connect the pump to one port, and micron to the the opposite side of system or a tank on the other port.


    Sent from my iPhone using Tapatalk

  11. #28
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    Jun 2019
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    something may contract & expand from atmospheric pressure when you shut the pump down... just a thought

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