MIL-DTL-17896E
4.6.8 Calibration error at room temperature.
4.6.8.1 Temperature. Room temperature may be considered to be any temperature
between 65 ºF and 80 ºF (18.3 ºC and 26.7 ºC). During this test, the temperature shall not change
more than 5 ºF (2.7 ºC) from the temperature at the start of the test. The barometer shall have been
at room temperature for at least one hour before this test is begun and shall not have been
subjected to pressure variation in excess of one inch for a period of at least 15 hours preceding the
test. The sample barometer shall be compared with the test barometer throughout the range over
which the tests are made.
4.6.8.2 Pressure. The pressure shall first be reduced from current atmospheric pressure
to the lowest design pressure (26.5 inches for type I and 28 inches for type II), then increased to
the highest design pressure (31.5 inches for the type I and 38 inches for the type II), and finally
reduced to current atmospheric pressure. Test data of barometer readings shall be taken at both
increasing and decreasing pressures. The rate of pressure change between test points shall be not
greater than one inch per minute.
4.6.8.3 Test points. The pressure shall be held approximately constant for at least
5 minutes but not more than 15 minutes before test data is taken. At least five test points shall be
taken. Test points shall be taken at approximately one inch intervals. Test points at which data
is taken shall have the same pressure value for both increasing and decreasing pressure within
0.04 inch in any one test cycle. When tested in this manner, the calibration error shall not
exceed values specified in table IV.
Table IV. Calibration error tolerances.
Indicated
Error in inches
Indicated
Error in inches
reading
mercury (±)
reading
mercury (±)
26.5 - 26.9
0.040
29.4 - 30.4
0.025
26.9 - 27.4
0.035
30.4 - 30.9
0.030
27.4 - 27.9
0.035
30.9 - 31.5
0.035
27.9 - 28.4
0.030
31.5 - 34.0
0.040
28.4 - 28.9
0.030
34.0 - 36.0
0.040
28.9 - 2.94
0.025
36.0 - 38.0
0.040
4.6.9 Change in scale or calibration error between test points. The scale or calibration
errors shall not change in magnitude by more than four percent of the pressure change between
any two test points as specified in 4.6.8.
4.6.10 Hysteresis. Differences in readings at the same pressure between data at decreasing
and increasing pressures (see 4.6.8) shall not exceed 0.04 inch.
4.6.11 Friction. If pressure is increased or decreased and is then brought to a constant value
without reversing direction while the instrument under test is free from vibration, light tapping or
vibration of the barometer shall not produce a change in indication greater than 0.04 inch.
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