Fuel/Water tank level indicator design

Discussion in 'Sailboats' started by bobisnoob, Jan 7, 2013.

  1. bobisnoob
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    bobisnoob New Member

    Hi guys, I'm designing a liquid level indicator for water,fuel and waste water tanks using a tube with a pressure sensor attached to the top of the tube, so basically I'm measuring the pressure inside the tube.The problem is I can't figure out how to get the proper readings of the liquid level inside the tank when the sailing boat starts to move because the liquid inside the tube starts to change, therefore the pressure inside the tube changes as well. Any ideas to find out the proper reading of the sensor?

    Regards,
    bob
     
  2. Frosty

    Frosty Previous Member

    This system is as old as the hills and worked very well measuring fuel and water as you say.

    Thing is it was monitored intermittently when a reading was required , it was not a constant read out.
     
  3. bobisnoob
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    bobisnoob New Member

    Thank you for your reply. Do you have any idea about designing a sensor that gives me the proper reading anytime?
     
  4. Crowsnest
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    Crowsnest Junior Member

    Hi ¡¡
    The surface of the liquid in the tank, has a Surface Area center that can be taken as the point around which the whole surface swings and tilts.
    The height from the tank bottom to this point can be taken as almost constant.
    If the sensor pipe runs vertically down to almost the tank bottom, crossing through that point, heel and trim lecture errors would be minimal.

    Regards
     
  5. bobisnoob
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    bobisnoob New Member

    OK that's true when we have a regular tank shape. What about irregular shape?
     
  6. Crowsnest
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    Crowsnest Junior Member

    Well:
    I wrote the simplest, easier solution.
    As far as you are only going to measure the liquid column height, the lower pipe opening has to be placed at the "reference" zero level point inside the tank (Ideally at the vertical of the Surface minimum error point).
    The path that pipe will run along, does not mind, as hydrostatic pressure will balance tank and pipe heights.
    An irregular tank, will anyway have a minimum error surface point, that has to be found.
    With a pipe and a sensor, anyway, only hydrostatic pressure or height level can be measured (depends on the arrangement). With that data, by means of a calibration table, volume is found.
    If you want an accurate measuring, taking into account wavy surface, possible chops, etc .... there is no simple way to perform it. The question is .... Does it afford to install such a high precission device ?

    Regards
     
  7. bobisnoob
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    bobisnoob New Member

    Thank you for your quick reply Crowsnest. But for sailboat the pressure inside the tube will keep changing as the boat swings back and forth, therefore I will not get the proper reading from the calibration table. So my question basically is, how to get an accurate reading within 10% error of the actual volume reading in the tank?
    If I'm mistaken let me know.
    Regards,
    bob
     
  8. michael pierzga
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    michael pierzga Senior Member

    Once the boat is in motion you cant acuratly read tank level.

    What Pressure sensor are you using ? My Gray and Black tank pressure sensors are looking mighty old after 20 years..Time for a replacement, but I cant read any details on the sensor itself. .
     
  9. Crowsnest
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    Crowsnest Junior Member

    Hi again:
    I agree with Michael, and not, you are not mistaken.
    With the boat under sail (May I know the boat and tanks sizes ?) depending on the tank design, its hard, complex or impossible to have a good reading.

    Regards
     
  10. DGreenwood
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    DGreenwood Senior Member

    In a very general description for how this could be done. I don't know exactly what type of mechanisms you are using but consider this method.

    First convert your output from the pressure device to a digital output. Use the digital output to provide continuous data to a simple microprocessor. Write software that averages or smooths the data and output the results to a simple readout (LCD or the like)

    Also it just occurred to me that pushing the liquid column out of the dip tube would probably have an averaging effect. This would require some type of very small compressor to occasionally push it out.
     
  11. Crowsnest
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    Crowsnest Junior Member

    As an option for the electronic solution, an hydrostatic averaging device may be suitable for your problem.
    If an arrangement of input lines are connected to a single output one, pressure at that least one will be the average of the input.
    It can be performed only by piping and a single sensor.
     
  12. sean9c
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    sean9c Senior Member

    Why not just measure the flow out of the tank. You know what you started with, you know how much you've used, you know what remains
     
  13. rxcomposite
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    rxcomposite Senior Member

    If you notice, modern car fuel gauge has a delay. It takes a long time before the reading moves. This is to prevent false reading when the tank is sloshing.

    In ships where the tanks are complicated, the linear reading from the sounding pipe is referenced to a tank sounding table that was previously calculated. The list. trim, density of fuel, temperature, is factored in.

    To do it electronically, your meter should be reading voltage from the capacitor. The voltage from the capacitor discharges thru the resistor slowly acting as a damper.

    For non linear readings, you need voltage shaping RC filters or better yet, use a series of voltage IC comparators, each circuit tripping at predermined settings.
     
  14. michael pierzga
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    michael pierzga Senior Member

    A fuel counter would seem logical. Calibrate it at full tank or with a sounding rod, then start the countdown

    All the suggestions in this thread seem to be a bit to scientific and complex for simple fuel gauges. Boat systems should be simple.

    When in doubt I still use sounding rods and a table.
     

  15. Crowsnest
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    Crowsnest Junior Member

    Once again, I agree with Michael.
    The simplest, the better.

    Regards
     
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