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How can water pressure be measured in m

  1. May 11, 2010 #1
    How can water pressure be measured in "m"

    Hi

    I need to understand how minimum residual pressure can be measured in meters. At least I think its meters. The specific sentence in the abstract is:

    "The South African civil engineering fraternity has grown to accept 24 m as the design criterion for minimum residual pressure in water distribution systems. However, the theoretical peak demand in many systems has increased beyond the point where minimum residual pressure exceeds 24 m – at least according to hydraulic models."

    A copy of this paper can be found at
    http://www.scielo.org.za/scielo.php?pid=S1816-79502009000200007&script=sci_arttext

    I am developing material to help the South African waterworks department teach its staff along the lines of South African unit standard 254073, and while I am a skilled materials developer, I do not have a background in fluid dynamics. (It's South Africa)

    Any help would be much appreciated.
     
  2. jcsd
  3. May 11, 2010 #2
    Re: How can water pressure be measured in "m"

    You can express pressure in 'head'.
    http://en.wikipedia.org/wiki/Pressure_head

    Ie the desgin pressure is that made by a column of water 24m high. Or the difference between atmospheric (for example) and another perssure is 24m.
     
  4. May 11, 2010 #3
    Re: How can water pressure be measured in "m"

    Thanks for the reply,

    So would a way that I could explain it be that if the 'head' in a water pipe is 24m, that pressure would be enough to push that water 24m up at normal g? Could I say that a head of 10 m is low pressure, and a head of 50m is high pressure.

    another way that I could say it would be that a 24m high column of water would push down with 24m head of pressure. What effect would the diameter of the pipe have on this pressure?

    Is there a way that I can convert between head, pascal and pounds per square inch?

    Much appreciated.
     
  5. May 11, 2010 #4

    uart

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    Science Advisor

    Re: How can water pressure be measured in "m"

    While Pa is the SI unit for pressure there are various different units still in common usage. Below are a few of the most common ones and their corresponding relation to the standard atmosphere.

    1 atm (Atmosphere) = 1.013E5 Pa = 1.013 Bar = 14.7 psi = 10.3m water = 760 mm Hg

    * some of the above are approximate.
     
  6. May 11, 2010 #5
    Re: How can water pressure be measured in "m"

    You don't use pressure head to describe a stand alone pressure (its more convenient to simply use a pressure), it's used to describe a difference in pressure. This usualyl comes from experimental values.

    As such diameter of the pipe would have no effect on the head. It simply says the difference between the two pressures is enough to send water up a colum 24m.

    Just like is a swimming pool, if you have a 25m long pool the pressure at a given depth is the same as if you were in a 50m olympic pool.
     
    Last edited: May 11, 2010
  7. May 11, 2010 #6

    jack action

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    Gold Member

    Re: How can water pressure be measured in "m"

    Imagine a column of water of a given height h and of area A. What is the pressure P felt under that column?

    So:

    P = W / A where W is the weight of the column of water.

    And:

    W = m g where m is the mass of water and g is the acceleration of gravity.

    m = rho V where rho is the density of water and V the volume of water.

    V = A h

    Putting everything together:

    P = (m g) / A = (rho V g) / A

    P = rho g h

    Since rho and g can be assume constant for some specific cases, the pressure varies only with the height of the column of water, no matter what is the area of the column (hence the diameter of the pipe). This is how we can measure pressure in meter. This case was for water, but if you use the density of mercury (Hg) instead, you could measure in "mm Hg", i.e. "equivalent pressure felt under a column of mercury with a height of X millimeters".

    The same logic applies to air pressure. If we feel a 101 kpa of air pressure on the ground, it is because of the weight of the air over our head that is pushing down on us due to gravity. (Although, in this particular case, rho and g will vary with height)
     
  8. May 12, 2010 #7
    Re: How can water pressure be measured in "m"

    Thank you for your help
     
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