TM 55-4920-328-13 testing   are   not   connected   and   are   not   available,   any transducer of known sensitivity may be used. If the basic sensitivity   rating   is   unknown,   the   rating   and   corrected sensitivity   must   be   calculated   using   the   procedures   in steps (e) and (f).  If the engine vibration transducers are connected,  note  and  record  each  sensitivity  factor,  then proceed to step (g). e.          The     basic     sensitivity     rating     of     velocity- responsive    vibration    transducers    is    provided    in    the following formula: S = E/V where S = sensitivity rating (mv/ips) E= millivolts V = velocity (ips) f.    The  sensitivity  rating  S  is  given  in  consistent units.    If  the  input  velocity  is  provided  in  average  ips,  it may be multiplied by S to obtain average millivolts output. If the input is given in rms ips, it may be multiplied by S to obtain  rms  millivolts  output.    Interpretation  of  sensitivity rating  is  mandatory  to  avoid  confusion,  especially  since the  vibration  meter  provides  an  average  velocity  read- out,  but  is  often  calibrated  in  systems  in  which  an  rms velocity calibrator is used.  The sensitivity factor S can be assumed   to   be   correct   only   when   given   in   terms   of operation   into   a   load   impedance   equal   to   the   input Impedance of the vibration meter.  The input impedance of  the  unit  is  10k  ohms.  Therefore,  the  transducers  used in  METS  are  rated  in  terms  of  operation  into  a  10k-ohm load.  If transducers rated for load impedances other than 10k   ohms   are   used,   It   is   necessary   to   calculate   the corrected sensitivity as follows: (1)  Sample calculations when vibration transducers open- circuit voltage is known: With S1 known to be 100 mv/ips and R1 known to be 925 ohms, and using the equation R2 S2 = S     R1 + R R1 R2 corrected  sensitivity;  the  following  sample  calculation  Is provided: 10,000 S2 = 100  925 + 10,000    =100.65 m v/psi (2)      Calculation   of   vibration   transducer   open-   circuit sensitivity:   If   the   manufacturer's   specifications   do   not provide   the   open-circuit   voltage,   but   give   the   voltage across   some   resistive   load   other   than   10k   ohms,   the corrected sensitivity may be calculated as follows: (a)  Designate S3 as the sensitivity into a load other than 10,000 ohms,i.e., R3. (b)  Designate R3 as the resistive load known. (c)  Calculate open-circuitry sensitivity S1. (d)  From S1.  calculate the corrected sensitivity S2. (e)  With S3 known to be 105 mv/ips, R3 to be 50k ohms,   and   R1   to   be   750   ohms,   the   following   sample calculation is given: Si   S3  RA     + R3    or 105  (750 + 50,000) R3                   (50,000) S1= 106.5 4 millivolts per inch per second, S2  Si      R2       or 106.5     (10,000) R1 + R2 (750 + 10,000) S2 - 99 mv/psi (f)      With   the   vibration   pickups   provided   in   the engine    accessories    connected,    position    the    SELECT control  (23,  figure  1-23)  to  SENS.    (The  RANGE  control (24)   and   MODE   control   (25)   may   be   in   any   random position.) (g)  Adjust the SENS adjustment potentiometer (30) until  the  meter  reading  corresponds  to  the  sensitivity  of the vibration transducer appropriate to the channel being adjusted. (h)  Position the SELECT control (23) to CAL.  (The RANGE  control  (24)  and  MODE  control  (25)  may  be  in any random position.) (i)    Adjust  the  CAL  adjustment  potentiometer  (26) for a full-scale reading on the meter. (j)      Repeat   steps   (f)   through   (i)   for   each   of   the   other channels. (k)      If   the   system   is   not   to   be   operated   for   an extensive  period  of  time  after  adjustment  of  the  multi- channel  vibration  meter,  deenergize  the  METS  electrical system and the generator set as specified in paragraph 2- 12. 2-11.  Checkout Prior to Engine Firing. Prior to engine firing, perform the following procedures and  verify  the  specified  conditions.  Proceed  as  follows 2-52