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Calculate water pressure in tube end when tube is compressed

  1. Jan 26, 2012 #1
    Hello!

    Background
    I have a hobby in creating an automated home, controlling lights, reading temperatures, and so on from self build sensors and actuators. For a while I have had force sensing resistors under each leg of my bed so I can messaure my own weight every day, sleeping patterns and so on. This has been working quite good for a while, but the sensors does not stand the constant pressure and fails. I need a new solution.

    Setup
    Instead of a sensor under each leg, the bed will fully stand on a plastic tube filled with water. The tube will have a liquid pressure sensor in one end and a stop in the other end.

    Problem
    I need to know the pressure on the sensor so I can buy a suitable sensor for that working range.

    Data
    • The tube will have a diameter of about 10mm.
    • The length of the tube will be around 10m to go around and under all legs of the bed.
    • The pressure in total will be around (60+70+30) = 1600N.
    • The bed stands only on the tube and the tube will be filled with water.

    Question
    In Bar, what will be the pressure applied on the sensor?
     
    Last edited: Jan 26, 2012
  2. jcsd
  3. Jan 26, 2012 #2

    xts

    User Avatar

    Forget the idea!
    You would have equip all the legs not with tubes (which get flattened under pressure), but rather with pistons with well knows area. Other problem is that such suspension for your bed is not stable - if the weight is not distributed centrally, some legs would totally collapse, while other stay lifted.
    Better think about more robust tensometers.
     
  4. Jan 26, 2012 #3
    If the tube is filled with water I think that this will not happen because water have a very low compressibility, right?
     
  5. Jan 26, 2012 #4

    xts

    User Avatar

    Water has low compressibility, but the pipe must be ellastic (not quite predictably for plastic/rubber pipes), it suffers from ageing, water loss due to leaks and vapourisation, then you have thermal effects, and lots of others absolutely unpredictable.

    Believe me - that is the worst possible tensometer you could design ;)

    You can't beat the commercially available ones - just find a more robust one or think about proper shielding. I guess your tensometer got damaged not because of long term pressure (they are designed to withstand it), but because of lateral moves of bed legs, scratching it. Just think about shielding envelope made of hardened steel.

    Just try your design to see its flaws - install such a tube without any pressure sensor and watch how it behaves as someone sits on your bed. Or as she rolls from one side to side.
     
    Last edited: Jan 26, 2012
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