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Gas pressure and fluid question

  1. Apr 20, 2007 #1
    It has been a number of years since I studied general physics and I'm looking for help on how to solve the following question. This is not a homework question, this is me trying to satisfy my curiousity regarding an application at work.
    I have two 2L Erlenmeyer flasks, one flask (Flask A) contains 1.5L of water and has a stopper with two 1/4 ID tubes inserted into it. The first tube (Sample Tube) extends to a point near the surface of the water, but not below it and the top of this tube is sealed with a rubber septum. The second tube (Vent) extends all the way to a point near the bottom of the flask, well below the surface of the water and connects to the second stoppered flask (Flask B) which is empty. Again, in Flask B the Vent tube extends to a point near the bottom of the flask.
    A few grams of Calcium Carbide are added to Flask A and the stopper is replaced. The calcium carbide reacts with water to generate acetylene gas and as the volume of gas increases the water in Flask A is forced through the vent tube and into Flask B and the reaction continues until the water level in Flask A is is near the bottom of the vent tube but not below it.
    How would I go about determining the amount of pressure the acetylene gas in Flask A is under at a given point during this process? What is most important to me is to determine the amount of pressure required to begin moving the water up through the vent tube and into Flask B.

    Thanks for the help! :smile:
  2. jcsd
  3. Apr 20, 2007 #2
    As the pressure rises, water rises on the vent tube until it reaches the highest point in the tube. At this moment the pressure reaches its highest value.

    After this water continue to fill the vent tube and begin to descend in flask B. The pressure diminishes. The pressure equals the difference between the limit of the water in the tube and the level of the water in flask A.

    Once this difference becomes negative, the water continues to flow from A to B, even if the pressure diminishes, because of the siphon effect (see http://en.wikipedia.org/wiki/Siphon).

    I talked about pressure as height of water. You can convert this to bars, knowing that 10.7 meters is one bar, then 1 millimeter of water is [tex]9.3\,10^{-5}[/tex] bar.

    I suppose that you are aware about explosive mixes of acetylene and air and how the reaction water-carbide is exothermic.
  4. Apr 20, 2007 #3
    Thank you lpfr for your response. I am aware of the dangers of acetylene and air, this is in part the reason for my question. It is considered unsafe to be using acetylene at pressures exceeding 15 psig and although I realize very little pressure is required to move the water through the tubing and between the two flasks, I still have concerns about this method. I am basically trying to collect some hard facts about the pressure of the acetylene gas inside the flasks as I have had a few colleagues dismiss my concern with the safety of the entire method.
    We normally use acetylene contained in a compressed gas cylinder, however this presents a problem because the acetone which the C2H2 is dissolved in within the cylinder must be scrubbed out using a H2SO4 procedure which is equally undesirable.
  5. Apr 20, 2007 #4
    All the pressures that I described are valid in the case that the water can circulate easily through the tubes. They should be large enough, and the pressure should build not too fast.

    This method is not too dangerous as this is how acetylene lamps of speleologists work.. Maybe you just need to buy one of those lamps?

    The acetone avoids the acetylene to explode when the pressure exceeds some value (12 bars?)
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