TM 1-1500-204-23-4 Figure 4-11.  Airspeed Indicator Movements   of   the   capsule   caused   by   the   resulting pressure     difference     are     converted     by     mechanical linkage  to  pointer  movement  over  the  calibrated  dial. The airspeed indicator face is graduated in knots. (2)    Testing.        The    following    paragraphs describe  test  procedures  and  replacement  criteria  for airspeed indicators while installed on aircraft. (a)  Required  equipment.    To  test  the airspeed indicator, use the following required equipment: · Pitot    static    system    tester    (The tester    must    show    evidence    of current calibration) · Adapters and fittings · Plugs and caps (b) Airspeed indicator test procedures. Test using the procedures that follow: 1 Disconnect  pitot  static  lines  on airspeed indicator. 2 Plug lines to prevent contamination. 3 Connect   airspeed   indicator   to pitot static system tester. 4 Compare    the    reading    of    the installed  indicator  against  the  reading  on  the  tester  at the test points shown in table 4-1. NOTE · Tests    do    not    need    to    go higher than the rated speed of the aircraft. · Gently    tap    indicator    before reading. 5 Perform     pitot     line     leak     test paragraph 4-3c(2)(b). 6 Perform    static    line    leak    test paragraph 4-3c(2)(c). 7 Disconnect   pitot   static   system tester. 8 Connect    pitot    static    lines    to airspeed indicator. (c) Replacement criteria. Replacement criteria are as follows: · Replace   airspeed   indicator   if   it fails   to   meet   the   tolerances   in table 4-1. NOTE The   airspeed   indicator   does   not always have graduations for each knot; therefore, interpret tolerances   3,   4,   and   6   to   retain rather than to replace indicator. · Airspeed indicators which do not pass the performance requirements  shall  be  turned  in for overhaul. f.        Vertical    Speed    Indicators.        Principles    of operation  and  testing  procedures  are  described  in  the following paragraphs. (1)   Principles   of   operation.      The   vertical velocity  indicator,  as  shown  in  figure  4-12,  located  on the  flight  instrument  panel,  indicates  the  rate  of  ascent or   descent   in   feet   per   minute.      The   pointer   moves clockwise  when  indicating  ascent  and  counterclockwise when indicating descent.  The vertical velocity indicator incorporates   two   air-enclosing   chambers:   a   pressure- sensitive  diaphragm  vented  directly  to  the  static  line, and  the  air-tight  case  of  the  instrument  vented  to  the static line through a thin-walled porcelain capillary tube. 4-13