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Level sensor query

Discussion in 'Electrical Engineering' started by BIGEYE, Sep 1, 2005.

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    BIGEYE Guest

    We have an application to measure the level of diesel (DERV) in a bunded
    storage tank. The height of the tank is 1.83 metres. When the level of the
    tank reaches 1.83 metres, the tank volume is 79000 litres.
    Measurement of level will be by a submersible pressure transducer. As these
    are supplied calibrated in metres H2O, how do I convert the equivalent
    height of the diesel to give a volume of 79000 litres.
  2. Guest

    Divide the height in H2O by the specifioc gravity of the liquid in the
    tank. If you assume diesel has a S/G of .84 <typical> a meter on the
    transducer would represent 1.22 meters of fuel.
  3. Height of water is a standard unit of pressure. I suspect it has nothing
    to do with the sensor being meant for use in water.
  4. Maybe you should use anti-explosive and antiflammable components. The diesel
    shouldn't vary much it density with temperature, so the gfretwell reasoning
    is precise:

    p=ro*g*h, being ro= density, g=gravity constant and h=height.
    Note that just dividing by diesel density is only valid in case you are
    considering density of water to be 1kg/dm^3. If you were to use other
    density units such as pounds/ft^^3.
    Best regards.
    Ignacio Simón Yarza
    Mech&electronics and control eng.

    BIGEYE Guest

  6. John G

    John G Guest

    Yes Height of Water is a standard of pressure but the important point
    is -- Is a water sensor suitable to use in the explosive / corrosive
    environment of diesel oil?
  7. Jerry Avins

    Jerry Avins Guest

    Is the volume proportional to height? Many fuel tanks have semicircular
    bottoms and tops. Submerging the pressure transducer to the bottom may
    be a problem. A good way is to read the pressure in the outlet pipe,
    provided that the pressure drop due to flow can be neglected.

  8. Again, nobody said it was a water sensor. It is a pressure transducer.
    "Water" only has to do with the calibration of the sensor. Of course the
    sensor you use needs to be appropriate for the environment, but the fact
    that the calibration is in units of height of water says absolutely nothing
    about what the suitable environment is.
  9. The pressure in the outlet pipe will be proportional to the height of the
    fuel. Physics says your solution suffers exactly the problem you're trying
    to solve.

    I suspect the effects of rounded tops and bottoms will be largely
    negligible, except at the almost empty and almost full levels.

    If you need an exact calculation, you can use a pressure sensor to get
    height, and use a lookup table to match the volume, or you can weigh the
  10. Jerry Avins

    Jerry Avins Guest

    What problem is that?
    The diesel-fuel tanks for my emergency generators are about five feet
    long horizontally, with substantially flat ends. The tanks are about 2.5
    feet thick, with semicircular tops and bottoms being and flat
    midsections also about 2.5 feet high, for a total height of about five
    feet. The volume is the sum of a cylinder and a box, each 2.5 x 5. The
    rounded parts are not a negligible part of the whole.

    2000 gallons is the allowable limit for indoor fuel storage, so the
    tanks are not completely filled. (They hold about 2100 gallons.) The
    rest of the reserve is buried outdoors, with heaters to allow it to flow
    in cold weather. The indoor tank allows time for the outdoor fuel to
    warm, and provides gravity feed to the turbines (similar to jet
    engines). We measure fuel on hand, both indoors and out, with a dip
    stick and conversion table.

  11. daestrom

    daestrom Guest

    Well, I assume the transducer is designed for this hazardous environment.

    To convert from meters H2O to another fluid, simply divide the reading by
    the specific gravity of the fluid. Since diesel is less dense than water, a
    full tank will not exert as much pressure on the transducer and the signal
    will indicate a height less than 1.83 meters. But when you take the
    transducer reading and divide it by the specific gravity of diesel fuel, you
    correct for this density difference.

  12. ANY pressure sensor, whether sitting on the bottom of the tank as the OP
    described, or at the outlet pipe, will provide an output proportional to
    the height of liquid in the tank. The outlet at the tank will give you the
    same reading, plus an offset for any difference in height, as the bottom of
    the tank sensor.
  13. Ooops, I got it. You're addressing the problem of getting the sensor in
    the tank. I just thought that would be done through a sealed bung or

    Been a long week.
  14. Jerry Avins

    Jerry Avins Guest

    One of us misunderstood something.

    It is easier to connect the sensor to a tee in the outlet line than to
    put it into the tank.

    If there is a pressure drop in the line due to flow, the reading will be
    low. That can be avoided with a large diameter line between the tank and
    the tee, or a separate line for the gauge in which there is no flow.

    I understand the physics of pressure head and frictional losses, but I
    don't know which problem you alluded to.

  15. Jerry Avins

    Jerry Avins Guest


    I think that's the hard way, but in case someone wants to do it, how can
    the weight/volume relation be nonlinear?

  16. Jerry Avins

    Jerry Avins Guest

    "Level" is often used loosely, as in "What is your level of
    uncertainty?" I would bet the price of a beer that the OP cares about
    how much fuel there is, not about physical height.

    Weight is linear with amount, and therefore a useful (though a bit
    difficult to measure). If head is wanted, then pressure is the better on
    both counts -- ease of measurement, and direct linearity.
    If height above the floor is wanted, then pressure is linear (with an
    offset that depends on the position of the gauge).
    There are many options. The OP asked for practical help.


  17. BIGEYE

    BIGEYE Guest

    Thanks for the info guys, but I never said the tank was non-linear. The tank
    level is linear, and the answer I was seeking was given in an early reply.
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