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Mass-Volume-Pressure relationship

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  • Mass-Volume-Pressure relationship


    I'm from Pakistan and here CNG is bening sold in units of Kg.

    I would like to know how to calculate the Kg of CNG in the cylinder manually by taking the pressure differencial and capacity of the cylinder.

  • #2
    Re: Mass-Volume-Pressure relationship

    One may ASSUME that at full pressure, the tank would be considered full, while at half pressure, the tank would be half full, but, the actual ratio is slightly different.

    As fuel is compressed to higher pressures, the drop in pressure is not directly proportional to the drop in volume. For example, at full pressure with a full tank, then dropping to half pressure, the tank is actually about 40% of actual capacity due to gas compression laws.

    Most onboard dash gauges are calibrated to show an approximate volume, but as many people can testify, most vehicles go a longer distance when the tank gauge goes from full to the 75% mark, then from half to 1/4 tank, and even faster from 1/4 to empty.



    • #3
      Re: Mass-Volume-Pressure relationship

      Hello Wasrana,

      Here are two resources that may help.

      First, the U.S. uses a conversion factor of 5.66 lb of CNG = 1 gallon equivalent gasoline/petrol. (Don't make me do the math for kg/liter). You can also convert from psi to bar easily. Go to:

      Second is a graph from our Institute for Gas Technology that shows pressure/temperature relationships.

      Good Luck,

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