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Fuel pressure change in closed fuel tank

  1. Oct 22, 2011 #1
    I am measuring pressure at the bottom of a fuel tank and temp variations are giving me a tough time. I wish to clear my head so here goes my reasoning as temp increases

    - Tank Volume increases causing - height of fuel to decrease, pressure at the bottom of tank to decrease
    - Fuel density decreases causing - volume of fuel to increase, height of fuel to increase, pressure at bottom of tank to increase
    - Fuel vaporization increases causing - volume of fuel to decrease, height of fuel to decrease, pressure of fuel to decrease, however this leads to an increased pressure on the fuel, so since I am measuring relative to ambient pressure this should show an increase in pressure, right? The mathematics involved / equations are unknown to me, a few pointers here would be great!
    - Any other effect?

    What would the net effect of a temp rise be? Currently my data shows that as temp increases, pressure decreases.

    Tank is 300ltrs, fuel is diesel.

  2. jcsd
  3. Oct 22, 2011 #2


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    Welcome to PF, Themindflayer.
    I am in no way qualified to answer your question, but I have one of my own which might help to narrow down the scope of the problem.
    What is your purpose in this measurement? Could you perhaps figure out some way to take an average so as to eliminate the thermal fluctuations? Maybe freeze-frame a measurement at an average point? Just thinking out loud...
  4. Oct 22, 2011 #3
    I need to measure the fuel volume in a tank. I am using a pressure sensor to do that. My data shows pressure variation when no fuel is being consumed (hence change in volume), after compensating for temp the pressure variation still shows enough variation to appear as a liter or two discrepancy in fuel.

    Sometimes in an entire day with no fuel consumption there is still a discrepancy of a liter of two which is boggling me.

    Sometimes in an entire day the fuel variation sums up to zero.

  5. Oct 22, 2011 #4


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    You do know, I assume, that the fuel itself will change in volume, but not mass, due to thermal effects. I seems to me that you're trying to nail down a non-existent change in the actual amount of fuel in the tank.
  6. Oct 23, 2011 #5
    Pressure equals density times height so the fuel expanding with temperature will not affect the pressure.
  7. Oct 26, 2011 #6


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    :confused: :confused:

    What in the living name of hell does height have to do with this? If you light a stick of dynamite, the pressure is a lot higher during detonation than it is before, with no change of altitude.
  8. Oct 26, 2011 #7


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    What is the sensitity of your sensor? 1 % may mean a fuel variation of 297l to 303l
    Perhaps it has some hysterisis.
    Next readings take a stick and measure the height of the fuel in the tank Take temp readings too. And then try to correlate your stick, temp and pressure sensor readings.
  9. Oct 27, 2011 #8
    How are you measuring the pressure?

    BTW if you light a stick of dynamite it burns like the wood pulp it mostly is. Blasting cap or its equal or no boom.
  10. Oct 28, 2011 #9
    Really? The depth, height of the level, of the liquid. Come on it's not that hard.
  11. Oct 28, 2011 #10


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    You might try putting a pressure sensor at the top of the tank, above the fule. Look at the difference between sensors.
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