TM 1-1500-204-23-4 three  pointer  and  counter  drum  pointer  altimeters  are described in the following paragraphs. (a) Three pointer altimeters.  Perform the following tests in the instrument shop.  Tests shall be performed   at   approximately   77   (±9)°F   or   25   ( ±5)°C using    a    Hass    Type    A-1,    or    equivalent    mercurial barometer.    Unless  otherwise  specified,  each  test  for performance   may   be   conducted   with   the   instrument subjected   to   vibration   to   remove   friction   effects,   and with the barometric pressure scale of the altimeter set to 29.92 inches of mercury (Hg). 1 Scale    error    test.        With    the barometric   pressure   at   31.02   inches   of   mercury,   the altimeter  shall  be  subjected  successively  to  pressures corresponding  to  the  altitudes  specified  in  table  4-2  up to  the  maximum  altitude.    The  reduction  in  pressure shall be made at a rate not in excess of 20,000 feet per minute to within approximately 300 feet of the test point. The test point shall be approached at a rate compatible with the test equipment.  The altimeter shall be kept at the pressure corresponding to each test point for at least 1   minute,   but   not   more   than   10   minutes,   before   a reading  is  taken.    The  error  at  all  test  points  must  not exceed the tolerances specified in table 4-2. 2 Hysteresis test.    The  hysteresis test shall begin not more than 15 minutes after the initial exposure of the altimeter to the pressure corresponding to  the  upper  limit  of  the  scale  error  test  prescribed  in paragraph 1; and while the altimeter is at this pressure, the  hysteresis  test  shall  commence.    Pressure  shall  be increased at a rate simulating descent in altitude at the rate of 5,000 to 20,000 feet per minute until within 3,000 feet  of  the  first  test  point  (16,000  feet).    The  test  point shall  then  be  approached  at  a  rate  of  approximately 3,000  feet  per  minute.    The  altimeter  shall  be  kept  at this  pressure  for  at  least  5  minutes,  but  not  more  than 15  minutes,  before  the  test  reading  is  taken.    After  the reading has been taken, the pressure shall be increased further, in the same manner as before, until the pressure corresponding  to  the  second  test  point  (12,000  feet)  is reached.  The altimeter shall be kept at this pressure for at least 1 minute, but not more than 10 minutes, before the  test  reading  is  taken.    After  the  reading  has  been taken,  the  pressure  shall  be  increased  further,  in  the same  manner  as  before,  until  atmospheric  pressure  is reached.    The  reading  of  the  altimeter  at  either  of  the two    test    points    shall    not    differ    by    more    than    the tolerance  specified  in  table  4-3  from  the  reading  of  the altimeter  for  the  corresponding  altitude  recorded  during the scale error test prescribed in paragraph 1. 3 After-effect test.  Not more than 5    minutes    after    completion    of    the    hysteresis    test prescribed     in     paragraph     a     the     altimeter     reading (corrected for any change in atmospheric pressure) shall not differ from the original atmospheric pressure reading by more than the tolerance specified in table 4-3. 4 Friction test.  The altimeter shall be  subjected  to  a  steady  rate  of  decrease  of  pressure approximating  750  feet  per  minute.    At  each  altitude listed in table 4-4, the change in reading of the pointers before     and     after     vibration     shall     not     exceed     the corresponding tolerance listed in table 4-4. 5 Case leak test.  The leakage of the     altimeter     case,     when     the     pressure     within     it corresponds   to   an   altitude   of   18,000   feet,   shall   not change the altimeter reading by more than the tolerance shown in table 4-3 during an interval of one minute. 6 Barometric  scale  error  test.    At constant atmospheric pressure, the barometric pressure scale shall be set at each of the pressures (falling within its range of adjustment) that are listed in table 4-2, and shall cause the pointer to indicate the equivalent altitude difference  shown  in  table  4-5  within  a  tolerance  of  25 feet. (b)   Counter   drum   pointer   altimeters. Perform   the   following   tests   in   the   instrument   shop. Tests shall be performed under the following conditions using    a    Hass    Type    A-1,    or    equivalent    mercurial barometer or manometer: · Room   temperature   77   (±9)°F   or 25 (±5)°C. · Barometric pressure 29.92 inches of mercury (Hg). · Lighting power 5.0 (±0.1) volts ac or dc or 4.0 watts maximum. · Vibrator  power  28  (±1.0)  volts  dc or 90.0 milliamps. 1 Scale     error     test. Connect barometer   and   vacuum   pump   to   altimeter.      Connect electric  lines  to  altimeter.    The  scale  error  check  (or diaphragm  error  check)  is  conducted  as  follows.    For  a period of at least 6 hours prior to testing, altimeter shall be  held  at  atmospheric  pressure.    The  barometric  dial shall be set at 29.92 inches of mercury (Hg) and remain there throughout the test.  The direction of approach in setting Change 3  4-16