MIL-PRF-49082E(CR)
3.7.8 Sand and Dust. The viewer, with protective covers installed, shall
not be damaged nor suffer degradation of performance (see and 3.13.1 and
3.13.5) by exposure of the portion of the viewer external to the vehicle to the
environment of sand and dust blowing at 35 knots. Dust used in this test shall
be 97 - 99 percent by weight SiO2 of angular structure with a size distribution
as determined by weight, such that the following is met when using the U.S.
Standard Sieve Series:
a. 100
percent of this dust shall pass through a 100 mesh screen.
b.
98
percent of this dust shall pass through a 140 mesh screen.
c.
90
percent of this dust shall pass through a 200 mesh screen.
d.
75
percent of this dust shall pass through a 325 mesh screen.
NOTE:
140 mesh silica flour as produced by the Ottwa Silica Company,
Ottwa Illinois, or equal is satisfactory for use in the
performance of this test.
Test conditions shall be such that dust is introduced at a rate of 0.3 ±0.2
grams per cubic foot into an air velocity of 1750 ±250 feet per minute at a
temperature of 23 ±2OC and a relative humidity of less than 22 percent for a
period of 6 hours. At the completion of the 6 hours period, the dust feed
shall be stopped, the air velocity reduced to 300 ±200 feet per minute and the
air temperature raised to 63 ±2OC for a period of 16 hours. At the completion
of the 16 hour period, the air temperature shall be maintained at 63 ±2OC, the
dust feed shall be returned to 0.3 ±0.2 grams per cubic foot, and the air
velocity shall be returned to 1750 ±250 feet per minute for a period of 6
hours. A the completion of the 6 hours, allow the viewer to return to
standard ambient conditions. Remove accumulated dust from the viewer by
brushing, wiping or shaking taking care not to introduce additional dust into
the viewer. Dust shall not be removed by air blast or vacuuming. After
removal of accumulated dust, the controls of the viewer shall be operated and
operation verified.
3.8 Reliability. The mean time to failure (MTTF) of the viewer shall
be 1000 hours of operation minimum in a tracked vehicle simulated field
environment.
3.9 Interchangeability. The entrance housing assembly, the objective
lens assembly and the eyepiece lens assembly shall be physically
interchangeable between viewer assemblies without degradation of the
resolution (see 3.6.5), objective focus (see 3.6.8) and eyepiece focus (see
3.6.9) requirements. These interchanges shall be made without any
modification or adjustment beyond that necessary during replacement of any of
the parts or assemblies involved.
3.10 Treatment and painting. Treatment and painting shall be anti-
reflective, shall be rust resistant and as specified on applicable drawings
unless otherwise specified in the contract.
3.11 Marking. Marking shall be in accordance with applicable drawings
unless otherwise specified in the contract.
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