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How much oil do I need?

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  • How much oil do I need?

    For our experts on board. I received this question from a LinkedIn connection, so thought perhaps someone here could help. The question comes from Yash:

    "If I consider a 1 liter cylinder and need to generate 10, 25 and 70 bar of pressure within .. how much oil do I need to inject to generate the right amount of pressure?"

  • #2
    Oh boy, that's a technical question. It depends on the bulk modulus of the oil (how much it resists compression). The rule of thumb is 1% per 2000 PSI (138 bar). So then you just need to do the math to figure out 10, 25 and 70 bar.

    Compressibility of oil = 1/138 per bar = 0.00724 %/bar

    10 bar = 0.0724% x 1000 ml = 0.724 ml
    25 bar = 0.181% x 1000ml = 1.81 ml
    70 bar = 0.5068% x 1000ml = 5.068 ml

    Please note that I skipped the step of converting the percentage into a decimal. Eg, 0.181% = 0.00181

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    • #3
      Assume the bulk modulus of oil stays constant with pressure the equation is
      VolumeFinal=VolumeInitial*exp(-(PressureFinal-PressureInitial) /BulkModulusOfOil)

      It would be nice to add MathJax so I can enter good looking formulas in LaTex or MathML.
      \[VolumeFinal=VolumeInitial\cdot { e }^{ -\frac { (PressureFinal-PressureInitial) }{ BulkModulusOfOil } }\]
      Last edited by Peter Nachtwey; 02-05-2016, 11:40 PM.

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      • #4
        Great to see you made it over, Peter!

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