TM 5-3895-374-24-2 FIGURE C The  maximum  length  of  connecting  wire   between   the transmitter and the receiver is a function of wire size and receiver resistance. That portion of the total current loop resistance represented by the resistance of the connecting  wires  themselves  should  not  exceed  .,j%  of the  receiver  resistance.    For  extremely  long  runs  (over 1,000 feet), it is desirable to select receivers with higher resistances  in  order  to  keep  the  size  and  cost  of  the connecting   leads   as   low   as   possible.      In   installations where   the   connecting   run   is   no   more   than   100   feet, connecting  lead  wire  as  small  as  No.    22  Ga.    can  be used. FIGURE D Several  Series  604  transmitters  can  be  operated  with  a single  power  ply  as  depicted  above  in  Figure  D.    Be careful  to  specify  a  supply  with  sufficient  capacity.    The minimum current requirement at a  given  voltage  can  be calculated by multiplying the number of units x 20 mA.  In the   example   shown   this   would   be   4   x   20   or   80   mA minimum. PRESSURE RANGING Each  Series  604  Transmitter  is  factory  calibrated  to  the range   given   in   the   model   number   chart.      However, special  calibration  is  also  available.    If  this  is  the  case, the   transmitter   will   be   so   marked.      For   purposes   of clarification in these instructions, range is defined as that pressure  which  applied  to  the  transmitter  produces  20 milliamps of current in the loop.  Zero pressure is always assumed to be 4 milliamps. If a transmitter pressure range other than that supplied is required,  the  following  re-ranging  procedure  should  be followed: 1. With the transmitter connected to the companion receiver per  the  instructions  above,  an  accurate milliammeter     with     a     full     scale reading of approximately 30 milliamps should be inserted in series   with   the   current   Ioop.      A   controllable pressure     source     capable     of     achieving     the desired  range  should  be  connected  to  the  high pressure port of the transmitter and teed into an accurate reference pressure gage or manometer.    Be  sure  to  vent  the  low  pressure port  to  atmosphere.    The   instrument   must   be ranged  in  the  same  position  in  which  it  will  be used.  Vertical mounting recommended. 2. Apply electrical power to the system and allow it to stabilize for 10 minutes. 3. With   no   pressure   applied   to   transmitter,   use "zero" adjustment to set loop current at 4 mA. 4. Apply full range pressure and set loop current at 20 mA using "span" adjustment. 5. Relieve pressure and allow transmitter to stabilize for 2 minutes. 6. Zero  and  span  adjustments  may  be  interactive so  repeat  steps  3  through  5  until  zero  and  full range pressures consistently produce loop currents of 4 and 20 mA respectively. 7. Remove  the  milliammeter  from  the  current  loop and     proceed     with     final installation of the transmitter and receiver. (page 3-711)