Tuning," Pgs. 3 and 4.) 3.  SET THE SENSITIVITY At this point there will probably be no depth indicated on the   LCD.      "Probably"   because   the   SENSITIVITY   was turned all the way down when the controls were preset in an  earlier  step.    With  a  minimum  sensitivity  setting  it would    require    an    extremely    strong    return    echo    to activate  the  digital  circuitry.    This  condition  usually  can take   place   only   in   shallow   water   over   a   hard   sandy bottom. To   read   the   depth,   slowly   advance   the   SENSITIVITY knob to that point where the water depth just starts to be displayed. On the Model 550, at that point, the "No Echo" indication will disappear and the water depth will be displayed. This is called the sensitivity "threshold" point.  Once the threshold has been determined, continue to advance the SENSITIVITY   control   to   about   TWO   GRADUATIONS HIGHER THAN THE THRESHOLD POINT. This is the correct procedure for setting the SENSITIVITY   level   for   normal   depth   reading.      It   will result in the most stable and consistent operation of the unit.  and should be repeated periodically to assure that the    two-graduations-above-threshold    setting    is    being maintained.    As  your  boat  navigates  from  one  area  to another,     the     depth,     bottom     condition,     sub-surface interference,  and  other  factors  affecting  the  strength  of the return echo change.  This procedure will allow some variations   in   return   echo   characteristics,   due   to   the factors noted above, without frequent false indications. As  an  example,  the  strength  of  the  return  echo  can  be reduced by more than 50% and the correct depth still will be  displayed.    In  fact,  the  echo  can  even  disappear  for several  counts.    Yet  a  memory  circuit  in  the  depthfinder will   continue   to   cause   the   last   true   indication   to   be displayed. Also,  when  your  depthfinder  is  correctly  adjusted,  the unit's sensitivity will not be so great as to cause frequent false  depth  readings  from  echo  signals  bounced  off  of fish,  weeds,  thermoclines,  or  miscellaneous  responses from the sides of the transducer's beam pattern. If  the  No.    530  and  540  displays  occasionally  go  blank, or   on   the   model   No.   550   the   "No   Echo"   indication appears,   the   sensitivity   should   be   readjusted   upward. These symptoms can be caused by navigating into much deeper   water,   or   into   an   area   with   a   soft,   muddy   or weeded  bottom.    Additional  receiver  sensitivity  provides the proper signal strength to trigger the digital processor. Similarly,  if  the  display  frequently  indicates  an  abnormal shallower depth reading-say you enter an area known to be  about  50  ft.    deep,  where  the  display  indicates  50  ... 50 ...  49 ... 48, jumps to 17 or 23 or some other random number,  and  then  back  to  50-you  can  assume  that  the unit  is  responding  to  a  target  between  the  surface  and the bottom. A  more  frequent  cause  of  "falsing".    or  the  momentary display   of   an   abnormal   random   number   in   all   digital depthfinders,   is   subsurface   interference.      The   sonar signal    is    reflected    off    various    particulates,    marine vegitation and organisms.  air bubbles from other boats, prop-wash,  engine  exhaust,  cavitation,  or  waves  which are  always  present  in  the  upper  layer  of  any  body  of water. In  this  case  the  SENSITIVITY  setting  may  be  too  high, and   again   it   should   be   readjusted.      In   some   very infrequent    cases,    the    return    echo    signal    from    the intermediate   target   actually   may   be   stronger   than   the bottom  echo  signal.    Readjustment  of  SENSITIVITY  will have    little    affect    in    eliminating    the    display    of    an occasional random number under this condition. As  with  any  depthfinder,  the  maximum  depth  capability depends    on    an    almost    infinite    number    of    external variables  such  as  water  purity,  reflection  coefficient  of the target (bottom), transducer mounting and coupling to the  water,  presence  and  gradient  of  thermoclines  and subsurface  interference.    Under  "ideal"  conditions  these ratings    easily    can    be    exceeded.        Similarly,    under extremely    adverse    conditions    the    maximum    depth capability  may  be  reduced  to  half  of  these  ratings  or even less. 10