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Air saturation level vs temperature and pressure

  1. Jul 4, 2006 #1
    Here is my problem:

    I got an open cylinder of constant volume V that is been closed at 40'C and 100% R.H. The cylinder when closed is completely sealed. Inside the cylinder there is and heater that bring temperature at 60'C. When that temperature is reached, I see condensation inside the cylinder (Based on real experiment)

    My initial tough was since temperature is increasing, there would be not condensation inside the cylinder. (Saturation level of air increase with temperature) But if the heater is turned off and temperature reach equilibrium at 15'C, some condensed water will be present inside the cylinder.

    Since I see condensation appear at 60’C it seems like I missed something. I decided to verify pressure increase inside the cylinder based on perfect gas law. Since it’s a constant volume I can say: P/T = K where K is a constant. If initial pressure is 100 Kpa @ 40’C the pressure at 60’C will 150 Kpa. Usually pressure variations have small impact on air saturation level. Since I’m working with small volume maybe the impact is more significant. I now trying to find what are the saturation levels for a specific pressure and temperature. Does someone can help me with this request?

  2. jcsd
  3. Jul 4, 2006 #2


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    Staff: Mentor

    Here is a psychrometric chart:

    No, those small pressures will have virtually no impact on the water vapor.

    What I think you are missing, though, is that condensation doesn't happen when the air reaches saturation temp/humidity, but rather when the surface reaches (goes below) the saturation (dew point) temperature. So if you increase the temperature and humidity inside, but the surface of the cylinder remains cooler, you'll get condensation.
  4. Jul 6, 2006 #3
    I would also venture that if you gave it time, the condensation would "re-evaporate" into the atmosphere of your cylinder. Remember each time a molocule of water leaves one of those droplets on the cylinder, the rest of the droplet is cooled slightly because of the excess energy the molocule stole to escape.

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