TM 1-1500-204-23-4 Table 4-1.  Airspeed Indicator Test Tolerances MS28045 MS28021 MS28046 Airspeed test 10 to 150 20 to 250 40 to 400 points (knots) (knots) (knots) ( = knots) 40 3 5 4 60 3 3 3 80 3 3 3 100 3 4 3 120 3 4 3 140 3 5 4 160 5 4 180 5 4 200 5 5 220 5 5 240 5 5 260 5 280 5 300 5 320 6 340 6 360 6 380 6 400 6 Changes  in  pressure  resulting  from  changes  in  altitude are  transmitted  quickly  to  the  inside  of  the  diaphragm and slowly through the capillary tube to the inside of the case.  This results in a pressure differential causing the diaphragm to expand or contract according to the rate of change  of  altitude.    The  motion  of  the  diaphragm  is transmitted to the pointer.  When the aircraft assumes a fixed  altitude,  the  pressure  in  the  diaphragm  and  case rapidly   equalize   and   the   pointer   returns   to   0.      An adjusting   screw,   located   in   the   lower   left   corner,   is provided for adjusting the zero indication. (2)    Testing.        The    following    paragraphs describe  test  procedures  and  replacement  criteria  for vertical speed indicators while installed on aircraft. (a)  Required  equipment.    To  test  the vertical    speed    indicator,    use    the    following    required equipment: · Pitot static system tester · Adapters and fittings · Plugs and caps (b) Vertical speed indicator test procedures.    Functionally  test  using  a  pitot  and  static tester. (c) Vertical speed indicator adjustment.    With  all  aircraft  systems  inoperative,  set vertical   velocity   indicator   pointer   to   0   (nine   o'   clock position)  by  means  of  adjusting  screw  at  lower  left  of instrument face. (d)   Replacement   criteria.        Replace indicators with dents or cracks in glass, stripped fittings, or illegible markings. g.   Altimeters.  Principles of operation and testing procedures  for  three  pointer  and  counter  drum  pointer altimeters are described in the following paragraphs. (1)  Principles  of  operation.    Altimeters,  as shown in figure 4-13, have pressure-responsive elements  (aneroids)  which  expand  or  contract  with  the pressure change of different altitudes.  The expansion or contraction    of    the    aneroid    with    pressure    changes actuates the linkage which is connected to the indicating hands     that     show     altitude.          A     bi-metal     yoke     is incorporated   around   the   aneroid   to   compensate   for temperature    effects    on    the    metals    of    the    aneroid mechanism. 4-14