MIL-DTL-21393B
4.6.3.11 Temperature cycling (see 3.6.14). The microphone shall be placed within the thermal
chamber and maintained for a period of at least 1 (one) hour or until the microphone performance
stabilizes at a temperature of 25º ± 15ºC. The chamber temperature shall then be reduced to -55ºC and
maintained at this condition for at least 1 (one) hour or until the microphone performance stabilizes. The
internal temperature of the chamber shall then be increased to +71ºC and maintained at this condition for
at least 1 (one) hour or until the microphone performance stabilizes. The internal temperature shall then
be returned to 25º ± 15ºC. The number of complete cycles shall be 5 (five). The microphone shall be
removed from the chamber and response measurements (see 3.6.1 and 3.6.2) made after stabilization at
room temperature.
4.6.3.12 Temperature extremes (see 3.6.15).
4.6.3.12.1 Low temperature. Expose the microphone unit to an ambient temperature of -62ºC for 2
hours. Remove the microphone from the cold chamber and immediately measure the response under
normal room temperature conditions.
4.6.3.12.2 High temperature. Expose the microphone unit to an ambient temperature of +71ºC for 2
hours. Remove the microphone unit from the hot chamber. Immediately measure its response under
normal room temperature conditions.
4.6.3.13 Salt fog (see 3.6.16). The microphone unit shall be subjected to the salt spray test in
accordance with method 509 of MIL-STD-810, with alternating 24-hour periods of salt fog exposure and
drying conditions for 4 (four) 24-hour periods (two wet and two dry). The microphone unit shall then be
washed off with tap water, to remove excess salt deposits, followed by removal of excess moisture. The
visual examination and response characteristics measurement shall be immediately conducted after
removal from the salt spray chamber and the above procedures.
4.7 Reliability test (see 3.6.12 and 6.2e). The microphones under test shall be subjected to a reliability
test at the contractor's plant under the following simulated service conditions.
Temperature
Normal room temperature
Altitude
Normal room
Humidity
Room ambient
4.7.1 Test details. The purpose of the reliability test is to identify deficiencies in design, materials,
components and manufacturing processes. Successful completion, as defined herein, does not validate
compliance with mean time between failure requirements. But, together with design analyses and other
testing, this test will provide confidence that the producers' designs and production processes are
adequate to commence production following first article inspection and (if specified in the contract,
see 6.2e) accept product for shipment under Group B conformance inspection.
4.7.1.1 Test quantity. A minimum of 2 and a maximum of 5 microphones shall be selected for the
reliability test, as shown in table VIII. The units selected shall be representative of "production
configuration" in design, materials, construction and performance. The configuration of each unit shall be
recorded prior to the start of testing and shall be verified at the conclusion of the testing or upon failure of
individual unit(s), by the Government inspector.
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