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