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Homework Help: Vacuum pressure

  1. Apr 22, 2009 #1
    This is NOT a "problem", it's a rather a question.
    Let's say I've got a vacuum chamber, and I suck the air out of it using a vacuum pump that is capable of doing -200 Kpa. Let's also say that there is a valve on this chamber. After a full vacuum has reached, I open this valve WHILE the pump is still running (thus creating a suction at the valve opening). What would be this pressure of suction? Is it -100 Kpa (100 Kpa which is atmospheric pressure, subtract 200 Kpa which is pump capability) OR is it just 0 Kpa since we are at vacuum?
  2. jcsd
  3. Apr 22, 2009 #2


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    Initially the differential across the inlet valve will be 1 atmosphere (100 KPa).
    Then the pressure in the chamber will rise so the differential will be less than 100 KPa. The amount will depend on the rate of flow in (how large the inlet valve opening is) and the rate of pumping out. The -200 KPa rating just means the pump can achieve a near vacuum when exhausting from a closed chamber to a 2 atmosphere destination - it says nothing about how much gas can be exhausted per second.
  4. Apr 23, 2009 #3
    I see. Makes sense.
    So let's say the inlet valve of 1/4" and the pump is also hooked up to a 1/4" outlet: does this mean that rate is the same thus the pressure difference remains constant?

    What if the inlet valve is hooked up to a hose which leads to a bucket of oil (thus sucking the oil into the chamber)?
    What would happen to the pressure inside?
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