AQUA METER'S EXCLUSIVE TECHNOLOGY DEPTH  ACCURACY:  All  internal  signals  necessary  for the  digital  processor,  LCD  circuitry,  and  for  establishing the  unit's  depth  accuracy  tolerance,  are  generated  from a  single,  high-stability  Reference  Oscillator.    This  same basic  design  philosophy  is  common  to  all  three  Aqua Probe  digital  models:  the  No.    530,  No.    540,  and  No. 550.    The  depth  accuracy  designed  into  the  digitals  is based  on  the  assumption  that  the  velocity  of  ultrasonic acoustical    sound    waves    in    water    is    4800    feet-per- second.  In practice, the actual speed of sound in water varies    as    a    function    of    temperature,    salinity,    and pressure  (depth).    These  variables  alone  can  contribute a  ±  5%  worst-case-error  to  the  depth  reading  if  all  the variables  are  at  the  extremes  of  their  ranges,  within  the 600    ft.        maximum    depth    capability    of    these    units. However,    the    probabilities    of    all    of    these    factors simultaneously being at the worst case extremes of their ranges,  is  very  low  and  the  assumed  velocity  usually  is accurate.    Accordingly,  the  rated  depth  accuracy  for  the three Aqua Probe digital models is given in the following chart: In the event that a No.  540 digital remote depth indicator is  operated  with  either  a  No.    530  or  No.    550  digital depthfinder  it  is  entirely  possible  that  there  may  be  a difference in depth readings between the two units.  This difference    would    be    due    to    their    relative    accuracy tolerances.      However,   both   units   generally   would   be subjected  to  the  same  input  voltage  and  environmental conditions.      They   would   tend   to   vary   in   the   same tolerance    direction,    thus,    limiting    the    difference    in readings to 1 or 2 feet (or slightly more in deep water). When   operating   your   unit,   remember,   that   any   digital depth  sounder  only  will  indicate  the  distance  from  the transducer     face     to     the     nearest     target     within     the transducer's beam pattern.  It cannot provide information on the nature of the bottom, details of structure, or other characteristics.  These are capabilities that are unique to flasher  and  recorder  types  of  depthfinders,  such  as  the Aqua  Probe  line  of  flasher  models  and  No.    390  series straight   line   recorders.      However,   they   will   provide extremely  accurate  and  reliable  measurements  of  water depth without the need to "interpret" readings. Model Nominal Accuracy Worst Case Accuracy · 13.6VDC, + 25Deg.C - 10 to + 50°C 11 to 16VDC 530 +1.4' +1.9' +0.5% - 0.6' * ±3.0% - 1.2' * 540 +1.6' +2.1' +0.5% - 0.8' * +3.0% - + 1.2' * 550 in + 0.2' + 0.5' 2' -99' range + 0.5% - 0 ** +3.0% - 0.3' ** 550 in5% +1.1' + 1.4' 4' - 499' range +0.5% - 0.9' * +3.0% - 1.2' * * Rounded off to nearest ft. ** Rounded off to nearest 1/10 ft. As an example, a model No.  550 operated in the 2-99 ft. range,  in  exactly  60  ft.    of  water,  nominally  would  read from  a  minimum  of  59.7  ft.    to  a  maximum  of  60.5  ft. Similarly,  a  model  No.  530  in  exactly  150  ft.    normally would  read  149  ft.    minimum  to  152  ft.    maximum.    In essence.  the unit is as accurate as the normal variations in the speed of sound in water will allow. INTERFERENCE Protection  against  interference  is  built  into  your  Aqua- Probe  digital  depthfinder.    An  exclusive  filter  circuit  and special   shielding   virtually   eliminate   interference   from engine,  radio,  ignition,  pump  motors  and  other  marine electrical  devices.    However,  under  certain  conditions, even these precautions may not be sufficient and like all electronic 15