Figure 8-6.-Calibration chart, gradation feeder gates. vibrators on the outside of the fine aggregate     bins,     or     both.     Vibrators     should be   wired   to   cut   off   automatically   when   the feeder stops. This eliminates excessive packing   in   the   bin. The following conditions underscore the    need    for    proper    cold    feeding: Wide    variations    in    the    moisture content    or    in    the    quantity    of    a    specific aggregate    at    the    cold    feed    may    cause    a considerable    change    in    the    temperature    of the    aggregate    leaving    the    dryer. A     sudden     increase     in     the     cold feed    can    overload    the    screens,    creating    a carry-over    of    the    fine    aggregate    into    the coarse    aggregate    hot    bins. Erratic     feeding     may     cause     some bins to overfill while starving others. This    can    result    in    the    following    problems: 1.   Layers   of   variable   grading   in   the hot     bin     gradation     unit     storage, especially in the fine bin, resulting    in    alternating    rich    and lean     batches 2.   An   overloaded   dust   collection   system 3.   A   reduced   dryer   draft The cold aggregate feeder gates should be calibrated. Most manufacturers furnish approximate calibrations for the gate     openings     of     their     equipment.     When these     are     available,     they     are     helpful     in making    the    initial    gate    setting.    But    the only     accurate     way     to     set     gates     is     by making    a    calibration    chart    for    each    gate, using   the   aggregate   to   be   used   in   the   mix. The gate opening (in inches or square    inches)    is    plotted    on    the    chart    as the    horizontal    coordinate,    and    the    pounds of    material    per    revolution    of    the    feeding mechanism    (or    pounds    per    minute)    is    the vertical coordinate. When the calibration chart    is    being    prepared,    the    gate    is    set, usually   at   25   percent   or   less   of   the   total opening,    and    the    feeder    is    started.    When the feeder is running normally, the material    is    measured    into    a    tare    container and    weighed    at    known    time    intervals    (or number of revolutions). This gives one point on the calibration chart. The operation    is    repeated    for    three    or    more gate    openings    and    the    points    connected    on the    chart    (fig.    8-6).    After    the    gates    have been calibrated and locked, minor adjustments     may     be     necessary     to     assure uniform production. When    the    gates    discharge    on    to    the belt    conveyer,    their    output    may    be    checked by    closing    all    of    the    gates    except    one, which     is     set     at     one     of     the     calibration points.    When    the    gates    cannot    be    closed completely,    it    may    be    necessary    to    stop the feeder or disconnect it if it is mechanically driven. The    plant    is    started    and    brought    to normal    operating    speed.    Then    the    plant    is stopped    and    the    material    from    a    measured section of the belt is removed and weighed,    using    care    to    remove    all    fines. The    weight    of    the    material,    divided by the length (in feet) of the belt section,    multiplied    by    the    belt    speed    (in feet    per    minute),    will    give    the    amount    of material     delivered     per     minute     from     the gate   opening.   The   material   from   other   gate openings   is   determined   in   the   same   manner and    the    gate    calibration    chart    plotted    as described above. When   variable   speed   drives   are   used   to control belt feeders, calibration is simplified. The gate opening can be estimated,   and   the   speed   of   the   belt   can   be increased or decreased to deliver the required   percentage   and   tonnage   of   aggregate. In    calculating    the    output    of    a    gate for    a    given    opening,    deduct    the    weight    of the surface moisture on the aggregate being weighed. This is very important 8-4