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Homework Help: Heat and relative humidity

  1. Jan 16, 2008 #1
    1. The problem statement, all variables and given/known data
    If a person breathes 10 liters per minute of air at 68 degrees F and 50% relative humidity, how much water per day must the internal membranes supply to saturate the air at 98.6 degrees F? (Assume all the moisture is exhaled). I f each gram of water extracts 580 calories as it is vaporized, how much daily heat loss in kilocalories (food calories) does this represent?

    2. Relevant equations

    3. The attempt at a solution
    My instructor provided us that saturation vapor pressure at 20 degrees C is 17.3 g/m cubed and at 37 degrees C is 44.0 g/m cubed.

    I believe I should be able to figure out the calorie portion of the problem by using a formula from my book but I am unsure about the relative humidity part. Unless I am just over looking the information that is there.

    Would I need to find the number of g/m^3 for the 10 liters to start? I'm just not quite sure.

    Thanks in advance for any help.
  2. jcsd
  3. Jan 16, 2008 #2


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    Assuming that 50% relative humidity is half the amount of water as 100% relative humidity ( I don't remember the defns).
    You know the mass of water in the 10L breathed in (68deg = 20c)
    You know the mass of water in the 10L breathed out
    Work out the mass of extra water that comes from the person
    Work out how much energy it takes to evaporate this much water.

    Be careful about working in different units.
  4. Jan 16, 2008 #3
    So would the relative humidity at 68F/20C be 8.65g/m^3, being that it's 50%?
    And the relative humidity at 98.6F/37C would be 44g/m^3 since it is 100%?

    Does the question want to how much MORE water the membranes would have to produce to completely saturate the air at 37 degrees C?
  5. Jan 16, 2008 #4


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    Yes, that's my reading of it
  6. Jan 16, 2008 #5

    i have no idea how to do or set up this problem can you help me in any way?...
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